1 © Amec Foster Wheeler Environment & Infrastructure UK Limited

Technical Note: Comments on the use of groundwater and river flow models, focused on the Candover Stream simulation in the Test and Itchen Model

Background and comments from World Wildlife Fund (UK)

Scope, author and reviewer

Rob Soley at Amec Foster Wheeler was asked by Southern Water Services to compile this Technical Note in response to comments made by WWF on the use of the Test and Itchen Groundwater (and River Flow) Model to support the assessment of flow impacts on the Candover Stream (sometimes named as the Candover Brook).

Rob has led the development and use of groundwater and river flow models across many of the Chalk catchments in Southern and Eastern England. He has also worked closely with the Environment Agency and other regulators over the last 16 years to design and improve the water resource assessment and environmental flow screening approaches used in Catchment Abstraction Management Strategies, and in support of EU Habitats Directive and Water Framework Directive objectives across the UK.

The note has been reviewed by Tim Power at Amec Foster Wheeler who has carried out or managed all of the groundwater and river flow modelling for the Test and Itchen.

(Within the appendices to this report, there is reference to work carried out by Entec UK Ltd and AMEC E&I UK Ltd – both of which are predecessor companies to Amec Foster Wheeler E&I UK Ltd).

Comments from WWF (UK)

A joint letter from Hampshire and Isle of Wight Wildlife Trust (Chief Executive Debbie Tann) and WWF (Rose O’Neill) to Matthew Wright (CEO Southern Water) and James Humphreys (Environment Agency Area Manager, Solent and South Downs), dated 17 December 2014, included the sentence:

“We do not believe that the groundwater models used to assess the impacts of increased pumping from groundwater for stream augmentation are sufficiently accurate to enable reliable assessment of groundwater level, resultant stream flow and ecological responses to episodic pumping during low flow spells.”

Some parts of a note entitled ‘Use of groundwater model outputs for water resources decision making’ (written by Rob Soley in 2008 as input to an Atkins project undertaken for the Environment Agency Solent and South Downs Area) were shared in response to this sentence, and WWF were asked for their reaction and further comments at a subsequent meeting of the Augmentation Technical Working Group of the Hampshire and Isle of Wight Water Resources Steering Group.

WWF duly circulated a further paper, reference WWF-UK/CF/10/06/2015, accepting that the points made in the ‘Use of GW models’ note and conclusions drawn are fair ‘insofar as they go’, but listing the points summarised below:

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1. Groundwater models perform best when used: a. to compare prediction between runs;

b. for considering changes or impacts over larger areas;

c. for defining impacts close to measurement points; d. for defining changes in aggregate flows rather than at individual groundwater levels; and

e. for defining changes over the long term, and/or on average.

2. Conversely, groundwater models are not at their best: a. for absolute comparisons with water levels/heads;

b. for impact predictions over small areas;

c. for defining impacts further from measurement points; d. for defining impacts at specific places; and

e. for defining impacts at specific points or during specific periods.

3. Because of these limitations it is an over-statement to claim that “groundwater models are particularly good at deriving groundwater abstraction impacts” with regards to localised and specific time predictions.

4. Models (in general) which have been calibrated against some of the available data, and verified or validated against other data (not used in the calibration) can be viewed with more confidence than those where no data have been held back for verification. The Test and Itchen groundwater and river flow model does not appear to have been verified or validated in this way, as part of determining its ‘fitness for purpose’.

5. Groundwater models can and do provide insight and understanding of aquifer-stream interactions in the round, across the piece and over the long run, but impacts on particular sites, at particular times or conditions are of most concern for environmental protection, so WWF continue to be concerned by the risks of placing too much trust in model predictions. They may be ‘best available’ but they are not sufficient.

6. Stating that the degree of uncertainty associated with groundwater model predictions of flow impacts is often dwarfed by the uncertainty of the hydro-ecological consequences of those flow impacts provides ‘nothing but cold comfort’. 7. So groundwater model results are useful, but not wholly trustworthy.

Aim and contents of this note

This note is intended to address WWFs comments directly (Section 2), and (in Section 3) to introduce some further documents (provided as appendices) which:

 expand on the ‘eyes open’ use of groundwater and river flow models (wherever they exist);

 emphasise why effective groundwater abstraction management and environmental protection needs to continue to refer to them as part of the assessment tool kit; and

 provide further documentation of the update/verification and subsequent localised refinement of the Test and Itchen Groundwater Model, including in the Candover Stream catchment.

The note and associated documents are to be circulated to the ‘Augmentation Technical Working Group’ members in advance of the next meeting in July 2015. The aim is to establish a consensus understanding of the way in which model predictions can be referred to and used, so that discussion time in future can be more focussed on interpretation of the available evidence (including model outputs) to inform regulatory decisions.

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Summary response to WWF comments

In summary, Amec Foster Wheeler do not disagree strongly with many elements of the points raised by WWF, taken at face value. We welcome the recognition that groundwater models provide insight into patterns of groundwater abstraction impacts, and would certainly agree with the conclusion that ‘model results are useful but not wholly trustworthy’. We would not claim otherwise - emphasising that field measurements are also important in establishing the conceptual understanding on which the model is based, but come with their own problems and limitations, particularly when trying to characterise the impacts of groundwater abstraction on ephemeral winterbournes, upstream of flow gauging stations. The best decisions often have to be based on an ‘eyes open’ combination of field observation, model predictions and judgement – with the emphasis on how to most effectively realise environmental benefits.

We would argue, however, that groundwater and river flow models in general can provide more insight into the local and time specific patterns of abstraction impact than implied by some of the WWF comments, which could be read to suggest they can only be reliably referred to for ‘long-term’, ‘average’ impact predictions ‘across the piece’. The purpose and design intent underpinning these models is focused on understanding how patterns of groundwater abstraction impacts vary - both spatially and in time - in the context of a naturally fluctuating and regionally distributed water table, and its changing relationship with the surface water drainage network. As such, they are uniquely well placed to predict patterns of flow impacts during lower groundwater level/ river flow times, and to compare these with impacts when groundwater levels and river flows are higher - they are not just for ‘long term average’ assessment. This is valuable for hydro-ecological interpretation because it integrates impacts across the full flow range in a way which can focus on predictions during specific dry summer months (for which ecological data are available), or can be collated across longer term flow duration curve summaries, for comparison with flow screening standards. ‘What the flow would be without groundwater abstraction’ is not readily measureable. Distributed, transient groundwater and river flow models are better designed to make predictions in this regard than any other modelling or calculation approaches which assume more fixed and linear relationships between the ground and surface water systems. They are, indeed the ‘best available’ tool for this purpose because they attempt to represent real processes of groundwater flow, storage and stream interaction. Nonetheless, we agree that model predictions should never be viewed with ‘blind faith’.

We would also contend that, in the absence of better information, model predictions between reasonably calibrated data points (e.g. gauged flows) can still be said to be ‘consistent with available data’, and therefore have some value. Interpolation and extrapolation are part of any model’s purpose, given that it is not possible or feasible to measure everything, everywhere, all the time (and even if it were, this still is not ‘measuring flow impacts’). So models can provide useful insight into impacts at particular places and particular times beyond available measurements. These impact predictions should not be presented as, or expected to be, ‘exactly correct’, but neither should they be discounted. Even where model calibration compared with measured groundwater levels is not perfect (which can often be the case because of the scale contrast between regional model cell size and the local layering and fissure connections which may be influencing observation borehole records), it is often possible and reasonable to add further interpretation or translation of model outputs - to make them more locally relevant. More discussion is provided in the documents referred to from Section 3.

The need to separate the use of data for model calibration/ refinement from that used for verification/ validation is a fair point to make regarding current groundwater modelling practice in the UK generally. This is particularly understandable from the perspective of lumped surface water modelling practices, which will routinely use automated calibration techniques to establish the calibration, and then test this against subsequent flow data. The initial calibration of many regional groundwater and river flow models, including the Test and Itchen, has been based on the establishment of an agreed conceptual understanding and distribution of parameters, followed by an iterative process of manual refinement by trial and error. In terms of the representation of occasionally operated influences such as the Candover support scheme, there is usually a reasonable desire to use all of the available monitoring from its initial testing, operation and recent testing to check and refine the model simulation. After this process, the ‘fitness for purpose’ decision is taken by the project steering group (including an expert peer reviewer) – before initial predictive scenarios are run, and it is correct to say that this skips the ‘verification/ validation’ step. In more recent years, the use of automated parameter estimation tools such has PEST has become more widespread (although not yet systematically applied on the Test and Itchen) – and this does include a more formal distinction of the model ‘training’ data, from the ‘verification’ data.

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However, most of the Environment Agency’s models are subsequently updated – with new abstraction and climate data added onto the end of the existing historical simulation – and the degree to which the fit between simulated and measured data is maintained does add verification, or challenge, to the model’s calibration. A technical note summarising this update/ verification step for the Test and Itchen model is referred to from Section 3, together with a separate note describing the subsequent extension and local refinement of the model, including adjustments to improve the simulation in the Candover area. These reports should provide further confidence that the Test and Itchen Groundwater Model has been subjected to rounds of further verification scrutiny and refinement in and around the local area influenced by the Augmentation scheme. There is always potential for further refinement to improve calibration – the model remains a necessarily simplified representation of the real world, developed with a finite budget and a parsimonious desire to avoid over-parameterisation such that it does not fit all of the observed data everywhere - but significant effort has been put into making it as robust a tool for supporting regulatory decisions as possible.

Finally, whilst we accept that it may not be helpful to compare the relative levels of certainty associated with flow impact predictions, and their ecological consequences, we would maintain that it is most important to stay focused on the state of the ecology, all of the pressures influencing it, and actions to maximise the benefits for it, rather than putting too much faith or reliance on comparisons between flow impacts and flow screening thresholds by themselves. This note has discussed the level of confidence underpinning groundwater and river flow model predictions, but it is also surely reasonable to query the degree of proof and evidence supporting the generic environmental flow impact thresholds which are routinely applied as pressure screening tools by the Environment Agency and Natural England (e.g. “10% of QN95” etc.), particularly where Chalk winterbourne flow would be naturally ephemeral?

Supporting documents

Four documents are provided as appendices to expand on and illustrate the discussion summarised in Section 2:

 Using groundwater models to improve the confidence in decision making for water resources (Appendix A)

This is a more recently updated, generalised and expanded version of the 2008 paper originally shared with the Augmentation Technical Working Group. It includes extra sections and provides fuller discussion around the issues.

 Groundwater abstraction impacts on river flows: predictions from regional groundwater models (Appendix B)

An externally published paper by Rob Soley, Alison Matthews and Mike Packman (amongst others) which explains, with examples from various aquifers around England, why regional models are important for predicting the impacts of groundwater abstraction on river flows. Very often, for a variety of reasons including seasonal and spatial variations in groundwater - surface water relationships and aquifer storage which are particularly relevant in headwater tributaries, abstraction impacts are less in absolute terms during low flow periods than when flows are higher. So peak rates of abstraction for spray irrigation, public supply or stream support during dry summers may have much less immediate impact on river flows than would be expected for a surface water abstraction. Without the use of models to provide this insight, expectations of low flow recovery associated with groundwater abstraction reduction can be overblown, and anticipated ecological benefits may not be realised.

 Test and Itchen Groundwater Model Update (Appendix C)

A technical note summarising the process of simply updating (with no refinement) the Test and Itchen model, from a December 2005 to a March 2011 end date, and the review of the model fit over the updated period which helped verify and challenge the model calibration, feeding into subsequent refinements.

 Test and Itchen Groundwater Model Refinement (Appendix D)

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Appendix A Using groundwater models to improve the confidence in decision making for water resources

July 2015 Doc Ref: 29388tn568 Using groundwater models to improve the confidence in decision making for water resources Introduction – what is this paper about?

This brief note has been written by Rob Soley of Amec Foster Wheeler as an overview of the issues relevant to using groundwater models to make better water resource related decisions. It is intended for the general reference/interest of any users or stakeholders considering how to get good value from a model which has just been built, reported and handed over. It is focused on water resources in England and Wales. Other applications for diffuse or point source groundwater quality problems are also important but are not considered here.

Background – why does this matter?

Modelling can be an expensive business. The construction and refinement of a regional scale time variant numerical groundwater model based on a credible conceptual understanding which is itself built up from a comprehensive analysis of available data is a costly and time consuming undertaking. Depending on the area, issues, objectives and approach, consultancy costs may be in the range of £20k to £1M for each model. Model delivery may take 0.5 to 5 years and will typically require significant Environment Agency management and specialist input, and regular engagement and review with key stakeholders, particularly the water companies – beyond the time and expertise bought from consultants.

Over the past 10 years the Environment Agency has invested significantly in this area. A series of regionally managed programmes have sought to develop models which, now cover most parts of the major aquifers in England which have significant levels of groundwater abstraction. The Agency is now in the process of uploading these models onto the ‘National Groundwater Modelling Server’ (NGMS) so they are more accessible to other staff and stakeholders for ongoing use.

The business cases justifying these investments have generally been driven by the need to make more reliable decisions. This should help to avoid disputes concerning the acceptable location, rates and management of groundwater abstractions based on an assessment of their environmental impacts on river flows, wetland groundwater levels, and saline intrusion, as well as the risk of derogating existing abstractors’ rights. Important environmental drivers include the Habitats Directive, RAMSAR and Public Service Agreement targets for SSSIs and the establishment of Catchment Abstraction Management Strategies (CAMS) to set a sustainable balance between abstraction and the environment in support of the Water Framework Directive.

The bottom line is that the costs of relocating an abstraction or developing a new source are typically in the order of £1 to £3M per Ml/d. Pumping tests and site investigations improve local understanding of possible short term impacts but it pays to use the best available platform to predict what the impacts might be in the longer term, particularly during critical low flow conditions. Groundwater models will rapidly re-pay their investment if they are used to successfully reduce the risk of environmental damage from decisions which put abstraction in the wrong place (and associated relocation costs) whilst also maximising resource potential and source peak deployable output.

As anticipated changes in climate tend towards shorter wetter winters and longer drier summers, it will become increasingly important to optimise the management of this valuable ‘reservoir under the ground’.

However, whilst recent experience has made consultants, the Agency and key stakeholders much better at the process of building, refining and delivering regional groundwater resource models, it is still early days in their ongoing application, improvement and use in decision making.

A number of questions are typically asked when a model is deemed to be ‘fit for purpose’ with regard to the objectives it was built to address, and is handed over for ‘ongoing use’. These questions, summarised in the next section, can leave stakeholders confused about what the ‘ongoing use’ could or should be. Expectations may range unreasonably from “Now we have the promised model which will tell us everything, everywhere” to “This model cannot tell us anything beyond the issues and conclusions already reported on”. This paper explores these questions and suggests that the truth lies somewhere in the middle of this spectrum.

Good groundwater model development and the questions raised at the end

The process of groundwater model development and refinement should (and, in England & Wales, increasingly does) involve key stakeholders from the outset. If effectively managed, this helps to ensure that the model is built to address the objectives and uses which are foreseen at the start of the project. It also encourages participation in the developing understanding – capturing experience and concepts from a range of people who are familiar with the areas and the issues involved. A robust and independent analysis of the available data may confirm or challenge ideas about the ways in which the groundwater and surface water systems work and interact. The resultant conceptual understanding is simplified and built into the numerical model, the refinement of which against measured river flows and groundwater levels may force further re-thinking of the processes and parameters represented.

This experience provides those involved with some appreciation of the sensitivity of the simulation to changes in the model design which may prompt further data analysis – particularly if the sensitivities relate to the questions which the model is intended to address. More formalised sensitivity analysis, occasionally supported by controlled use of automated parameter estimation techniques, may indicate where key uncertainties for further investigation lie.

But when the deadline for decisions to be made approaches and before the money runs out, the model must be used in predictive mode to address the objectives of the project. Typically, three ‘CAMS standard’ scenarios are run to investigate the existing and potential maximum ‘in-combination’ impacts of abstraction and discharge – these are the natural, recent actual and fully licensed scenarios. Further investigation of the impacts of individual sources or of alternative groundwater support scheme operations may also be included, reported and ‘signed off’ as ‘fit for purpose’ (or not) by the project steering group and key stakeholders.

The model and project reports are handed over, hopefully with training for key Agency and some times water company staff. Before the steering and stakeholder groups are disbanded, the debate usually moves on to questions regarding the ongoing use of the model. The business case justifying the initial investment assumes that it should continue being managed as an asset which is updated and used to support decision making into the future. But there are concerns that some of the proposed applications may not be appropriate (or only appropriate with modification or refinement), that there may be an over dependence on the model results, or that future users may misinterpret output without a full appreciation of the uncertainties and caveats involved.

Some attempt is usually made to address these concerns in the final reporting by providing general health warnings and spatially specific caveats balanced by encouragement to use the model appropriately. However, decision makers who have not been involved in the model development may still find it difficult to interpret results, understand what they mean, and with what certainty conclusions can be drawn. This can put people off using the model at all, or result in predictions being discounted and ignored out of hand. A tool intended as a neutral platform for decision making may become neglected or abused.

The questions often raised in this regard, which are explored in the remaining sections of this paper, are as follows:

• What are groundwater models good at predicting?

• Why can’t we provide a simple indication of how uncertain the groundwater model prediction might be?

• How can we use the model where it doesn’t ‘fit’?

• What other tools/investigations should we be considering as part of the decision making toolkit?

• How can we use groundwater models to ‘wise-up’ other tools, and in the licensing process?

• How can we manage the ongoing review, refinement and update of the model?

• Where can we find more guidance/information on all this?

What are regional groundwater resource models good at predicting?

Each model has its own set of strengths and weaknesses depending on its intended use and how it has been built and refined, and these may vary across the model and in time. But the following general statements are often true. Groundwater abstraction impacts are better than absolute flows or heads Good regional groundwater models are the best available tools for predicting the temporal and spatial variation in the impacts of groundwater abstractions (and associated discharges) on flows and heads, particularly when used in combination with site specific monitoring or investigation data. Impact predictions are typically based on DIFFERENCES between two model runs which tend to be less sensitive to misrepresentation of concepts, processes and parameters than predictions of absolute flows or heads.

Distributed numerical models are really the only appropriate tool for unravelling the way in which a groundwater abstraction may disrupt the natural patterns of groundwater flow and interaction with rivers and wetlands in the long term – either alone, or in combination/interference with other groundwater abstractions. This is a spatially complex problem which is best dealt with through gridded calculations combining a near surface daily model representation of rainfall, evaporation runoff and recharge processes with a longer stress period model representing groundwater flow, abstraction and interaction with streams, drains and rivers. Many components of this calculation are reasonably well known – the rainfall, the elevation of the surface water courses, the rates and locations of groundwater abstraction and of surface water abstraction and discharges. Most models also have a few points where gauging stations record river flows and groundwater level observations are available, comparison against which allows concepts and parameters to be tested and refined.

Flow processes and the spatial distribution of parameters controlling them are typically much more uncertain. Although understanding and model ‘fit’ will improve through refinement, the representation can never be perfect and some features of the natural flow system may prove impossible to simulate. This may in turn have a related but less marked influence on the reliability of groundwater abstraction impact predictions.

Larger scale questions with more data will mean a better answer Models will typically provide more reliable predictions to questions over larger areas. The regional refinement process may have had relatively few observations of flow and head to calibrate against and should have been underpinned by the desire to keep the concepts and parameters involved as simple as possible (i.e. to have enough parameter variation but not too much), although it is worth noting that simple concepts may give rise to heterogeneous parameter distributions. There will always be less confidence in the representations derived from a single model cell, than in predictions averaged over many cells. Confidence also reduces where there are no observations. Thus flow impacts predicted at a well calibrated gauging station towards the bottom of a catchment will generally be more reliable than impacts predicted in the upper reaches of a headwater tributary. Even so, the groundwater model may still provide the most appropriate estimate of headwater impacts because it incorporates much more of the real system behaviour than alternative tools.

Flows are predicted more reliably than heads River flow impacts which aggregate flows over a large area should also be much more reliable than the predictions of groundwater level changes in a single model cell. Comparisons between observed and modelled heads should always be treated with caution. Beyond the scale contrast (e.g. a cell may be 500 m square compared with a 0.1m diameter borehole) and the model’s inability to simulate radial flow and drawdown close to abstractions, there will often be uncertainties around whether the observation borehole water level is representative of the groundwater system being modelled. The observed level may be strongly influenced by the depth of the screen in a layered aquifer, or by the interconnectivity of one or more key fractures to the regional flow system – features which cannot be readily modelled.

River flow predictions are better than those for wetlands

Taken in combination, the previous two points (large scale more reliable than small scale, and flows better than heads) mean that predictions of impacts on river flows will typically be more reliable and require less ‘translation’ than the assessment of impacts on wetland water levels and flows. The small scale and complexity of near surface layering and drainage management can have a large influence on the local shallow water table and this cannot always be fully incorporated directly into the groundwater model itself. . In spite of this apparent limitation, careful consideration of model behaviour, coupled with a thorough understanding of how the model works and additional calculations to help ‘translate’ the model predictions to this small scale, can yield acceptable predictions. In some cases, further ‘wetland models’ may be required in order to reach this objective.

Time period and flow condition influence how good the answer is The time period for which the prediction is required also influences how good the groundwater model answers are. One of the earliest stages in model refinement should be to check that the long term average flows simulated in rivers flowing off the model are a close match to gauged flows. And long term average groundwater abstraction impact predictions can usually be made with more confidence than estimates for shorter periods. But groundwater models are normally developed to focus on the prediction of impacts during average to low flow periods and towards extreme droughts. Calibration effort and model acceptance criteria may therefore sacrifice the representation of high flows in favour of matching lower flows, insofar as this is consistent with a credible conceptual model.

Many real world groundwater flow processes are ‘non-linear’ such that the behaviour of the aquifer and surface water system varies according to groundwater levels and rates of recharge or abstraction. It is often difficult to represent the most extreme ends of this behaviour in a groundwater model calibrated to fit more ‘typical’ conditions over a 20 or 30 year period. Most groundwater models operate on stress periods of between a week and a month, supported in some cases by near surface runoff generated on a daily average basis, so peak flood flows which occur within hours or even minutes can rarely be reliably predicted. It may also be difficult to match the most extreme droughts because of poorly understood processes (rather than stress period constraints).

The type of abstraction or climate scenario investigated also matters Finally the type of abstraction and discharge or climate change impact question being asked will influence how reliable the answer is. As stated above, groundwater models are particularly good at deriving groundwater abstraction impacts. In recent years modelling techniques have improved the representation of total catchment flows – including runoff and interflow as well as baseflow from the aquifer, and incorporating surface water abstractions and discharges. This has helped to improve confidence because modelled river flows can be compared directly with gauged data without complicated post-processing. It has also greatly improved the representation of groundwater – river support schemes which can be triggered by flow simulated in the model and provide a properly accounted for prediction of net yield under various flow conditions which it is difficult to get from any other tool.

Whilst it is clearly worth ‘bothering to use the groundwater model’ to investigate the impacts of ‘ large’ groundwater abstractions , this may not be the case with much smaller abstractions which are likely to have only very localised impacts. Some groundwater models may have excluded numerous small abstractions from the simulation. And other simpler/quicker tools exist which can be used to risk screen small influences in combination with existing licences as part of the determination process (e.g. CAMSLedgers, the Water Resources GIS etc). The Agency has yet to derive guidance as to where the abstraction threshold triggering use of a groundwater model should lie in this regard.

Just because the groundwater model may incorporate a representation of a surface water abstraction does not mean that it is the best tool to understand and investigate its impact. Some surface water abstractions have complex licence constraints such as Hands Off Flow conditions. Others have large daily maximum or instantaneous abstraction rates for irrigation purposes which can have severe but short term impacts on flows. Such licences may be more appropriately investigated in a daily flow spreadsheet or other tools – possibly supported by post-processing of the underlying groundwater model output.

Based on the principles that predictions will be more reliable close to well matched observations and within the range of historically experienced conditions, it also follows that the results of the ‘recent actual’ abstraction and discharge scenario will be more reliable than either the ‘natural’ or the ‘fully licensed’ scenarios.

The natural scenario is typically realised by switching off all abstractions, discharges and mains leakage, but leaves land use and drainage distribution as currently simulated. This provides an essential reference condition for many assessments but care is needed to ensure that the simulation is appropriate (Shepley and Streetly, 2007). It may sometimes be more appropriate, for example, to use the modelled groundwater abstraction impacts in combination with gauged flows and surface water influences in an alternative derivation of natural flows. As part of surface water licensing decisions, Hands off Flows (HOFs) may need to be determined which will be referenced to gauging station measurements. If the gauging station record is not closely simulated in the groundwater model it may be better to use modelled abstraction impacts as part of gauge naturalisation to provide the reference conditions for setting these HOFs in a CAMSLedger.

The fully licensed scenario for CAMS and Habitats Directive simulates maximum rates of abstraction with discharges held at their recent actual rates. This is inherently precautionary and also represents an extrapolation beyond historically calibrated pressures. Although models are commonly used for extrapolation, the resulting predictions will inevitably be less reliable.

Similarly, whilst a groundwater model is an obvious tool to search for and test the potential impact of new abstraction sites , the reliability of such predictions in the area local to the new sites will improve only as drilling and test pumping provide supporting information, ideally supplemented with observations of groundwater level response to pumping in the longer term. Models can and should be used extensively before new drilling takes place – they are a cheap and effective tool to help narrow down site selection options. However, very often the introduction of observed geology, groundwater levels or flows at a new site will challenge the local configuration or parameterisation of the model. On such occasions it may be appropriate to modify and refine before re-running the predictions to support decision making.

Groundwater models can also be used to investigate the impacts of climate change scenarios . This can be achieved by applying UKCIP change predictions to rainfall and potential evaporation inputs over the recent calibration period. In other examples the model simulation has been extended back in time to incorporate more severe droughts from the first half of the 20 th century. The focus of such predictions may be either environmental, or on the deployable output of groundwater abstraction sources and river support schemes (as in the Itchen modelling). Once again, models are probably the best platform for such investigations because they account for the storage and flow in groundwater system through extreme multi-year droughts in a rigorous way. Typically, in climate change assessments done to date, only the magnitude of rainfall and potential evaporation is adjusted, whilst the pattern of occurrence remains the same. It is possible to assess changes in pattern of occurrence, but this may require application of stochastic techniques which are computationally relatively time-consuming and expensive. Results should be viewed with caution because the predicted climate change parameters are themselves extremely uncertain. Furthermore potential associated land-use changes are also uncertain and have thus far typically been ignored, even though they may have a much greater impact.

Why can’t we provide a simple indication of how uncertain the groundwater model prediction might be?

In the age of ‘risk based decision making’, stakeholders may ask whether the predictions made by groundwater models can be accompanied by associated levels of uncertainty e.g. “the impact is predicted to be 8 Ml/d ± 10%”.

This is not an unreasonable request, particularly from those more used to receiving and interpreting outputs from surface water models. For many simpler lumped hydrological models the number of input and output parameters is relatively few and a systematic estimate of uncertainty can be readily and quickly achieved through automated and stochastic consideration of parameter sensitivity.

Uncertainty is a subject of hot current debate in groundwater modelling circles, as evidenced by UK Groundwater Modelling Forum workshops on ‘uncertainty’ and ‘decision making’. A considerable range of often very technical scientific papers are also available.

However, spatially & temporally distributed regional groundwater modes are often very large and may typically take 2 to 5 hours to run on computers in a commercial consultancy or Agency office. Automated parameter estimation and uncertainty analysis techniques have been developed (e.g. using ‘PEST’ or ‘UCODE’) but these require expert and careful handling to avoid misinterpretation of results.

It is important to recognise that there are several distinctive sources of uncertainty including: • Data uncertainty – have I got any? Where I have, is it right? All the time, some of the time? • Conceptual/process/structural uncertainty – is the model built with the right structure and processes to represent the agreed understanding of what’s going on well enough? • Parameter uncertainty – are the distributions and numbers reasonable? This is perhaps the most amenable type of uncertainty to formalised analysis through tools such as PEST, providing the computing power and time are available; and • Scenario uncertainty – relating to the type of question asked or the change being investigated. All of these elements are combined in the uncertainty associated with the prediction.

From previous section it should be apparent that a general statement about the uncertainty of the model as a whole and with regard to any future use would be fairly meaningless. The answer will usually be ‘it all depends…’.

At the start of most modelling projects an agreed set of ‘acceptance criteria’ or ‘calibration targets’ are often defined to indicate when the model is deemed to be ‘fit for purpose’ so that calibration can stop and prediction begin. Throughout model refinement, maps are produced summarising time series comparisons of observed and modelled flows and heads (e.g. average or Q95 modelled flow minus gauged flow, mapped at all gauging stations as colour and size scaled symbols). By the end of the project these maps may be used to illustrate general conclusions and ‘health warnings’ regarding the areas and times/flow conditions where the absolute simulation may be less reliable. In reality, however, the acceptability criteria are rarely met everywhere. And even where the ‘fits’ are poor it is often still possible to extract credible estimates of abstraction impact, or to ‘translate’ output based on systematic relationships with observations, thereby enhancing the predictive value of the model.

It is more reasonable to ask for an indication of uncertainty in relation to a specific question/scenario and a specific output predicted at a specific location. It should be possible for someone who knows the model to provide a qualitative assessment of this but more formal analysis of all potential uncertainty sources is much more difficult.

But attempts to be too precise about model uncertainty may also be misleading. Groundwater model output is typically only one component of the decision making toolkit (see later Sections). For most environmental impact questions, the decision must also be based on predicted ecological regime response to changes in groundwater and surface water levels and flows. Hydro-ecological relationships are still very poorly understood and may be much less clear-cut than relationships between the ecology and water quality, sediment load or temperature. Hence groundwater model uncertainty is often dwarfed by that associated with hydro-ecology or with the environmental economics and sustainability considerations of options being considered.

How can I use the model where it doesn’t ‘fit’?

Most people reviewing comparisons between modelled flows or heads and gauged or observed values will tend to make rapid judgements about how reliable the model is depending on the goodness of fit. Where the groundwater model fits the observed data well, and the observed data are known to be reliable, and the concepts and parameters used in the model are credible, this is reasonable. However, groundwater models often suffer a bad press in comparison with lumped hydrological models in this regard as the latter often fit better - the tendency to use a poor fit as an automatic excuse to skip or ignore the GW modelling results should be resisted.

If the model fits well elsewhere it is always worth checking the reliability of the data itself first. Some river flow gauges are known to systematically under or over read under a variety of flow conditions, and observed groundwater levels may often be significantly influenced by borehole construction and local connection to the regional aquifer. Where the data are deemed reliable, the next option to consider is whether conceptual or parameter refinement – either locally or over a larger area - might improve the fit without making it worse elsewhere. This will take time and therefore money. These are usually in short supply towards the end of the model refinement/prediction phases, but the possibilities for further refinement should always be considered if the decision warrants it (e.g. concerning a new application).

The alternative is to use the model as it is. If the possible reasons for the mis-match are understood, there are usually ways of getting some value from models even where the fit with observed data appears to be poor. Each prediction needs to be considered in it own right but some illustrative examples of techniques are listed below:

• Jan van Wonderen of Motts reviewed the match between modelled and observed Chalk heads from the East Hampshire and Chichester Chalk Block model. Many hydrographs were well matched and in almost all cases the relative variation in levels within and between years seemed credible, even if the absolute levels and/or amplitude did not match. Application of a simple ‘translation’ function resulted in very close matches in most cases where there are sufficient observations to build up a relationship. This potentially improves the predictive value of the model for extrapolation into periods or scenarios without observation. It should be noted however that caution is advised when applying such translations, especially where non-linear conditions may influence behaviour;

• If the absolute match between modelled and gauged low flows is poor and checks have been carried out to confirm that the gauge is reliable, all artificial influences have been correctly represented and flows are elsewhere well matched, it may be more appropriate to derive natural scenario flows by removing modelled flow impacts from gauged flows, rather than using the naturalised model flows directly. This can be useful for the flow duration curves in CAMSLedgers where it is important that recent actual and fully licensed scenario flows are reasonable in relation to the gauged record of actual flows which may be referenced by Hands Off Flow conditions on surface water licences;

• If the flow match is poor but the model is to be used to investigate the impacts of augmentation pumping of groundwater support schemes, it may often be necessary to ‘translate’ the operational ‘trigger’ rates of flow so that the scheme is switched on and off in the model in a realistic pattern, even if the triggers are slightly higher or lower than those actually written on the licence; and

• There is usually a need to carry out further post-processing or even further modelling in order to add small scale ‘within cell’ detail to output which needs to be compared with shallow water table observations on wetlands.

All the above approaches can add considerably to the ‘worth’ of a groundwater model, but should all also be viewed with caution. They may often be preferable to using the model output ‘raw’, but, in the case of adjustments made to river flows in particular, they may simply be compromises covering up mis-representation of processes or parameters in the model. The pursuit of ‘perfect fits’ whatever the cost in conceptual compromise should be avoided. A re-assessment of the data or of other tools alongside the groundwater model may be the best way to move the decision making forward.

What other tools/investigations should we consider as part of the decision making toolkit?

Groundwater models should be part of the decision making toolkit rather than being considered in isolation. A vast array of model output can and often is generated as part of a predictive investigation which can swamp the decision maker. A process of re-simplification and clear communication of results may be needed to ‘see the wood for the trees’. Consideration of the other components contributing to the decision can be very helpful in this regard to ‘keep your head above the water’. Eco-hydrological investigation findings and the economic sustainability analysis of different options may suggest that the subtle differences between groundwater modelled output can be safely ignored. It may, for example be relatively simple to bring about a significant improvement in wetland hydro-ecology by altering the management of the shallow ditch system draining it, even though this cannot be readily demonstrated through groundwater modelling scenarios.

Pumping tests of new sources (and associated water features surveys) or ‘signal test’ investigations of operational wells will always provide essential local understanding of real processes to challenge or confirm the groundwater model simulation. Indeed, a thorough analysis of all available hydrometric data (time series gauged flows, accretion profiles, time series groundwater levels etc.) is a vital foundation for any modelling study. And field investigations and monitoring will still be required after all the models have been built. Most decisions are usually based on the issues, investigation data and predictions for a particular site such as a reach of a river or a wetland. As the aim is often to ensure ecological health, this focus is as it should be because of the significant variability in factors influencing the ecology between sites. Indeed one of the strengths of a distributed groundwater model is that it can provide a spatially distributed picture of impacts (limited only by cell size) which can be compared more closely with ecological observation sites than would be possible otherwise. For example, river flows and scenario impacts can be produced for all GQA biology sites, rather than being limited to gauging stations.

However, given finite economic resources, decision making may also be assisted by some comparison between sites. i.e. “how severe are the abstraction impacts in this location compared to that one?” or “can we rank these sites in order of the severity of hydrological impact?” In order to compare the findings of one groundwater modelling study with another, or to compare rivers which have been modelled with those which have not, it is important that the results are reflected as closely as possible in the Environment Agency’s nationally consistent platforms. These are principally the CAMSledger spreadsheets and the Water Resources GIS, both of which are designed to represent spatially and temporally distributed groundwater abstraction impacts (e.g. from models) in a simplified way. These tools include the screening of scenario flows (recent actual, full licensed etc.) against the flow standards considered to support Good Ecological Status for the Water Framework Directive (WFD). The groundwater modelled impacts of abstraction and discharge are contextualised when uploaded into these tools where they can be prioritised for more detailed investigations in comparison with other sites.

Ultimately, decisions are made by people based on judgement and discussion using the best evidence available from a variety of sources.

Groundwater models used to ‘wise-up’ other tools and in licensing

Some examples of good practice exist where groundwater models have been used to ‘wise-up’ CAMSLedgers and the Water Resources GIS and the Agency-owned tools and procedures for doing this have most recently been summarised in a ‘CAMSLedger – Groundwater Model Alignment Report’. As the Agency has streamlined its abstraction licensing process and migrating it to National Permitting, and as groundwater models are being migrated to the NGMS, there is also a need to set out and trial the licence determination process as it looks to make more active use of the tools that are now available (the WR GIS-based ‘Application Risk Screening Tool’ and groundwater models – on or off NGMS).

The decide – monitor – review/refine/update – predict cycle

Time limited abstraction licensing, CAMS and the WFD River Basin Planning process into which these are linked, are all cyclical. In the end practitioners must accept some uncertainty, make decisions, monitor the consequences and review periodically. It is important to establish protocols for deciding when to use a groundwater model within this process, when to refine it, and when to update it. A model should not be ‘stuck on the shelf’ as a ‘finished article’ but will require investment into the future. How this all works with the NGMS still remains to be seen. Regional Agency modellers will need to retain an overview of who is using them for what purposes as the focus moves over the coming years from ‘design and build’ to ‘use, refine and update’.

Further reading and guidance

Alignment of model and CAMSLedger river flow and impact assessments, April 2013, draft report by AMEC for Anglian Region ‘In River Needs’ Programme.

Groundwater Resource Modelling: A Case Study from the UK, 2012 Geol Soc Special Publication 364 (eds Hepley, M.G, Whiteman, M.I., Humne, P.J. and Grout, M. W.

The estimation of ‘natural’ summer outflows from the Permo-Triassic Sandstone aquifer, UK, M.G. Shepley & M. Streetly, Quarterly Journal of Engineering Geology and Hydrogeology, 2007; v. 40; issue.3; p. 213-227 Procedings of the National Groundwater Modellers’ Forum workshops on stakeholders & communication, 31 January 2008 also uncertainty 16 September 2005. See website: www.groundwateruk.org/html/modelling/home.htm

Agency groundwater modellers in Regions and Nationally e.g. Paul Shaw, Arifur Rahman, Mark Grout, Rolf Farrell, Simon Gebbett etc etc

Agency’s National Groundwater Modelling and NGMS Guidance – from Mark Whiteman or Rolf Farrell at EA Leeds

RAM Framework user manuals – contact Fiona Lobley at EA Leeds

Author: Rob Soley ......

Reviewer: Tim Power ......

Copyright and Non-Disclosure Notice The contents and layout of this report are subject to copyright owned by AMEC (©AMEC Environment & Infrastructure UK Limited 2013) save to the extent that copyright has been legally assigned by us to another party or is used by AMEC under licence. To the extent that we own the copyright in this report, it may not be copied or used without our prior written agreement for any purpose other than the purpose indicated in this report. The methodology (if any) contained in this report is provided to you in confidence and must not be disclosed or copied to third parties without the prior written agreement of AMEC. Disclosure of that information may constitute an actionable breach of confidence or may otherwise prejudice our commercial interests. Any third party who obtains access to this report by any means will, in any event, be subject to the Third Party Disclaimer set out below. Third Party Disclaimer Any disclosure of this report to a third party is subject to this disclaimer. The report was prepared by AMEC at the instruction of, and for use by, our client named on the front of the report. It does not in any way constitute advice to any third party who is able to access it by any means. AMEC excludes to the fullest extent lawfully permitted all liability whatsoever for any loss or damage howsoever arising from reliance on the contents of this report. We do not however exclude our liability (if any) for personal injury or death resulting from our negligence, for fraud or any other matter in relation to which we cannot legally exclude liability.

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Appendix B Groundwater abstraction impacts on river flows: predictions from regional groundwater models

July 2015 Doc Ref: 29388tn568 Downloaded from http://sp.lyellcollection.org/ by guest on April 23, 2012

Groundwater abstraction impacts on river flows: predictions from regional groundwater models

R. W. N. SOLEY1*, A. MATTHEWS2, D. ROSS3, C. H. MAGINNESS4,5, M. PACKMAN5,6 & P. J. HULME7,8 1AMEC Environment & Infrastructure Ltd, Copper Beeches, St Kew, Bodmin, Cornwall PL30 3HB, UK 2Environment Agency, Colvedene Court, Wessex Way, Colden Common, Winchester, Hampshire SO21 1WP, UK 3Atkins Ltd, Woodcote Grove, Ashley Road, Epsom, Surrey KT18 5BW, UK 4AMEC Environment & Infrastructure Ltd, Gables House, Kenilworth Road, Leamington CV32 6JX, UK 5Present address: Pattle Delamore Partners Ltd, PDP House, 235 Broadway, Newmarket, Auckland 1149, New Zealand 6Southern Water, 249 Fairlee Road, Newport, Isle of Wight PO30 2JU, UK 7Environment Agency, Olton Court, 10 Warwick Road, Olton, Solihull B92 7HX, UK 8Present address: pjHYDRO Ltd, 26 Bodmin Avenue, Stafford ST17 0EF, UK *Corresponding author (e-mail: [email protected])

Abstract: Regional groundwater resource models are often built to improve confidence in pre- dicted groundwater abstraction impacts on river flows and groundwater levels. By explicitly repre- senting the aquifer system geometry, properties and boundaries, together with transient recharge and abstraction pressures, such models provide a robust platform to support abstraction impact assessment, alongside evidence from field data and investigations. Regulatory drivers include the European Union Habitats and Water Framework Directives and other abstraction licensing decisions. This paper presents examples of the spatial and temporal patterns of groundwater abstraction impacts predicted by several models. A variety of presentation formats are used to illus- trate the simulated flow impacts of abstractions both individually, and in combination with other surface water abstractions and discharges. Model predictions from a range of abstraction, aquifer, and river settings are often more complex than would be suggested by simpler tools and approaches. In many cases, absolute low-flow impacts are less than long-term groundwater abstrac- tion rates. The ‘real world’ hydrogeological mechanisms behind these impact patterns are dis- cussed. The paper also recommends a protocol for using regional models to assess individual licensed groundwater abstraction impacts across the full range of historic climate conditions (typi- cally, as monitored since 1970) and in the context of other operational artificial influences.

Across England and Wales the Environment Protected Areas designated under the European Agency is the environmental regulator responsible Union (EU) Habitats Directive (Council of Euro- for issuing licences to abstract water. The licence pean Communities 1992) are also the focus of a ret- determination process includes consideration of rospective ‘Review of Consents’ process that is the impacts of the proposed abstraction both indi- seeking to establish whether any existing abstrac- vidually, and also in combination with other artifi- tion licence may have the potential to adversely cial influences. The environmental receptors for affect site integrity either by itself or in association which long-term predictions need to be made with other influences. If such impacts cannot be include flows and water levels in rivers, lakes, estu- ruled out, action must be taken to restore the site aries and wetlands, together with groundwater levels and reduce future risks. Beyond these Protected and water quality. Established abstraction rights Areas the EU Water Framework Directive (Coun- must also be protected from derogation associated cil of European Communities 2000) has defined with new consents. a comprehensive network of Groundwater and

From:Shepley, M. G., Whiteman, M. I., Hulme,P.J.&Grout, M. W. (eds) 2012. Groundwater Resources Modelling: A Case Study from the UK. Geological Society, London, Special Publications, 364, 269–288. http://dx.doi.org/10.1144/ SP364.17 # The Geological Society of London 2012. Publishing disclaimer: www.geolsoc.org.uk/pub_ethics Downloaded from http://sp.lyellcollection.org/ by guest on April 23, 2012

270 R. W. N. SOLEY ET AL.

Surface Water Body receptors, which the Environ- been published considering a range of abstraction– ment Agency assesses, reports and acts on through aquifer–river relationships (e.g. Hantush 1965, and the Catchment Abstraction Management Strategy more recently Hunt 1999). Many of these have (CAMS) and River Basin Planning process. White- been built into the Environment Agency’s ‘Impact man et al. (2012) provide a wider explanation of the of Groundwater Abstraction on River Flows’ role of groundwater modelling in these regulatory (IGARF) suite of predictive tools, as summarized processes – in which water companies, other initially in Kirk & Herbert (2002), and extended abstractors and conservation organizations are also to consider neural network modelling approaches stakeholders. by Parkin et al. (2007), and object-oriented code A natural flow regime is generally taken as the enhancements in Jackson et al. (2008) and Hulme consistent reference condition from which both arti- et al. (2012). Many authors describe the ‘real ficially influenced flows and screening thresholds world’, non-linear hydrogeological mechanisms are derived and compared. In the examples pre- illustrated in this paper, which go beyond idealized sented here, this is taken as the flow that would analytical representations and may result in impacts occur in response to meteorological conditions during low-flow periods being less than such recorded since 1970, assuming recent patterns of approaches predict (e.g. Rushton 2002, summar- land use and drainage, but in the absence of any izing several previous UK aquifer studies, and abstraction or discharge pressures. Initial risk Evans 2007, considering drier climate examples). screening is therefore based on the deviation of The UK hydrological community also has a well- flows from natural, but stronger field-based evi- founded understanding of groundwater abstraction dence of ecological consequences is usually impor- impact complexities. Seasonally variable stream- tant to justify intervention, that is, the ecological flow depletion factors are described, for example, benefits associated with the predicted flow recovery by Clausen et al. (1994). However, the site-specific also need to be assessed. factors involved are such that this complexity has If restorative action involving the reduction or not been systematically built into standard UK re-distribution of licensed abstraction is required, hydrological low-flow prediction software, such as this can be very costly (typically in the range £2–7 Low Flows 2000 (Young et al. 2003). million per Ml/day, that is, per 1000 m3/day, Therefore, regionally distributed, time-variant according to the research for the Department for numerical models, mostly based on the MODFLOW the Environment, Food and Rural Affairs 2007). So code (MaDonald & Harbaugh 1988), have become it is important to understand and improve the level more important for flow impact prediction in the of confidence associated with the ‘basic’ prediction UK and elsewhere. Examples of such models of abstraction impacts on water flows and levels. described separately in this volume include the Appreciating the uncertainties surrounding associ- West Midlands–Worfe Permo-Triassic Sandstone ated ecological effects and the broader economic model (Shepley & Soley 2012), and the Wessex context of sustainable solutions is also essential. Basin Chalk model (Soley et al. 2012), both of The flow impacts of surface water abstractions which have been used to illustrate groundwater from rivers can be clearly located, measured and pre- abstraction impacts in this paper. The examples dicted. The long-term impacts of groundwater included are all based on single-model deterministic abstractions on surface water flows are much less representations, deemed to be ‘fit for purpose’ by easily defined. This has been one of the main the steering groups managing their development reasons for the development of regional groundwater (i.e. calibrated through refinement of a historical resource models and many such models have been simulation in comparison with measured flows used for groundwater abstraction impact prediction and groundwater levels and ready to be used for in recent years. This paper presents a number of abstraction impact assessment, subject to general examples from a range of settings and seeks to high- and model-specific caveats). While it is important to light common hydrogeological features and mechan- acknowledge and, if possible, quantify the predictive isms that are relevant to understanding spatial and uncertainties associated with use of such single temporal patterns of impact on surface water flows. models, this is not discussed here because the Although some of the presentation formats paper’s aim is to illustrate impact predictions across included here are innovative, the groundwater mod- a broad range of hydrogeological situations. elling techniques underpinning the predictions However, regional numerical models are expen- described are not new, and most of the impact mech- sive to develop and have their limitations. It is anisms discussed have been reported in the literature always particularly important to consider model previously. predictions alongside field evidence. A well-refined Since Theis (1941) considered the effects of a model should have built in the understanding from well pumping on flow in a nearby stream, a large field observations and investigations, in so far as number of analytical solutions to this problem have this is possible when upscaled to model cells and Downloaded from http://sp.lyellcollection.org/ by guest on April 23, 2012

GROUNDWATER ABSTRACTION IMPACTS ON FLOW 271 stress periods. However, observed responses to groundwater system. Leakage out of stream cells pumping tests often reveal features and mecha- is therefore limited by the rate of flow available in nisms that are not well represented in the regional the stream, as well as by the boundary conductance model, thereby tempering confidence in model pre- and the driving head difference between surface dictions that can otherwise be viewed with too stage and groundwater head (or bed bottom ele- much certainty. For example, Jackson et al. (2008) vation, whichever is highest). demonstrate the failure of a variety of modelling Furthermore, the Stream Package input files were techniques to simulate observed responses to the all prepared using the ‘4R code’ (Heathcote et al. large-scale testing of the Candover river support 2004), which calculates rainfall, recharge and runoff scheme in Hampshire, which are strongly influenced across the model grid on a daily basis before accu- by flow through solution enhanced fractures in the mulation within the longer MODFLOW ground- Chalk. Weber & Perry (2006) also show the value water model stress periods. Distributed runoff is of a thorough statistical analysis of hydrometric routed through the stream cell network and is com- data when trying to understand flow trends in bined with the time-variant influence of surface relation to abstraction, land use and climate press- water abstractions and discharges. Natural recharge ures in a karstic heterogeneous aquifer where cred- or interflow may also be augmented by leakage from ible parameterization of a distributed numerical water supply system pipes and household connec- model is difficult to achieve. tions, or from canals, etc. After running through MODFLOW, Stream Package flows thus incorpor- ate total catchment runoff and groundwater base- Flow impact prediction examples, formats flow components, together with combined artificial and methods influences, and can be directly compared with gauged flows during the model refinement or Figure 1 shows the areas for which abstraction calibration phase. impact examples are presented and provides an over- A full justification of the credibility of the view of the associated figure numbers, site names models used for the many examples presented here and principal aquifer settings, together with the is beyond the scope of this paper, although two are regional groundwater models used for analysis. All described in more detail elsewhere in this volume of the individual sources (Figs 2–8) are for public (Shepley & Soley 2012; Soley et al. 2012). Some water supply. Most have a baseload of relatively general comments on the process of building and constant abstraction through the year, although one refining a model to the point where it can be used can operate at much higher rates during the sum- for the prediction of abstraction impacts on flow mer and autumn periods, when flows are lower, to are, however, appropriate. help meet peak demand. The simplest examples To achieve a credible simulation of ground- are introduced first. A variety of river flow and water–surface water interaction, it is firstly impor- impact formats are presented rather than adhering tant that river stage elevations, which can usually to a standard template for all examples. However, be easily measured, are surveyed and correctly the common intention is to demonstrate how and incorporated into the Stream Package. These bound- why abstraction impacts are predicted to vary in aries define the ‘overflow levels’ for the groundwater time and spatially. system being modelled and care is needed to ensure The final example (Fig. 9, for the River Wensum that the stage elevations fall smoothly in between catchment) is different in that it illustrates the distri- known survey points, taking into account any steps bution of the combined impact of all artificial influ- associated with weirs. Stream cells often need to ences (i.e. all groundwater abstractions plus surface be extended up dry valleys – beyond the mapped water abstractions plus discharges) on total flows as location of rivers – to allow aquifer discharge accumulated down the river network. during periods of highest groundwater levels. These examples are taken from Permo-Triassic Stream conductance defines the ease with which Sandstone, Chalk and Lower Greensand aquifers head–stage gradients drive flows between the built into six groundwater models across four of groundwater system in the model and surface the Environment Agency’s Regions (Southern, water in the stream cell. This may be initially para- Anglian, Midlands and South West – Fig. 1). In meterized according to an understanding of the all cases the river flow impact formats presented length and width of the channel and permeability are based on the output of the MODFLOW Stream of the stream bed or other near-surface materials Package (Prudic 1989). This is distinguished from that are not explicitly represented as groundwater other similar MODFLOW boundary packages (i.e. model layers. However, measured patterns of down- the Drain Package and the River Package) in that stream flow accretion or loss along streams collated flow accumulation downstream is accounted for, for several models (including those covering the as well as interaction with the underlying Chalk of southern England described in Soley Downloaded from http://sp.lyellcollection.org/ by guest on April 23, 2012

272 R. W. N. SOLEY ET AL.

Fig. 1. The location and summary characteristics of the groundwater abstraction impact examples presented. Downloaded from http://sp.lyellcollection.org/ by guest on April 23, 2012

GROUNDWATER ABSTRACTION IMPACTS ON FLOW 273

Fig. 2. Chalk groundwater overflowing from the Havant and Bedhampton springs sourceworks. Abstraction of the captured spring water for public supply is metered and some of the excess, overflowing to Langstone Harbour, is also gauged. et al. 2012) suggest that rates of groundwater dis- observed relationships between surface and charge into streams (when groundwater levels are groundwater levels down the river reaches being high) are often much greater than rates of leakage considered should be reasonably reflected in the through the stream bed back to the aquifer during model, as should patterns of abstraction-related drier periods. In order to achieve a credible rep- drawdown, at least over the longer-term timescales, resentation of this behaviour, the bed elevations which should be more reliably represented than built into the MODFLOW Stream Package are early time pumping responses. often brought closer to the stream stage during The adequacy of the match between simulated refinement so that leakage rates are limited to and measured flows and heads is judged against match measured losses as soon as heads fall below acceptance criteria defined by the group of Environ- the bed bottom. ment Agency staff, other stakeholders and expert The refinement process needs to make use of all peer reviewers overseeing the work. In headwater reliable measurements of river flows. These include areas where the lengths of gaining streams can continuous time-series from gauging stations, and vary seasonally with the condition of the ground- spot flow measurement accretion profiles. Surface water system, it is particularly important to ensure water abstractions and discharges should be associ- that the timing and extent of no-flow reaches are ated with stepwise flow losses and gains, respect- adequately represented because no further flow ively, on the accretion profiles. Care is needed to reduction due to abstraction is possible. ensure that the simulated time-series for these Having established a historic simulation that is surface water artificial influences receive as much considered ‘fit for purpose’ in representing the quality assurance attention as the groundwater aquifer system and its observed response to recharge abstractions that are more typically the focus of and abstraction stresses (i.e. including understand- groundwater modelling studies. ing from large-scale pumping tests, where avail- Close fits between simulated and measured able), the 4R–MODFLOW models were used for groundwater levels are typically more difficult to the abstraction impact predictions presented here. achieve than with flows. This may be because obser- This typically involved running two ‘what if’ scen- vation boreholes only ‘sample’ a small and some- arios through the agreed model that differed only times unrepresentative part of the aquifer, or be- with respect to the individual abstractions or com- cause the more radial or even turbulent flows close bined influence(s) being considered. Changes in to an abstraction well are poorly represented across modelled stream cell total outflows or in ground- a much larger regional model cell. Nonetheless, the water–surface water exchanges were processed in Downloaded from http://sp.lyellcollection.org/ by guest on April 23, 2012

274 R. W. N. SOLEY ET AL.

Fig. 3. The location of the Glandford groundwater abstraction boreholes next to the River Glavern and the associated long-term average spatial distribution of reductions in groundwater to surface water flow predicted at MODFLOW stream cells by the Yare and North Norfolk groundwater model. a variety of ways to produce the formats presented in significant part of this is captured and abstracted Figures 3–8. for public supply. Figure 2 is a photograph of part of the source works showing some of the excess overflow not put into supply. The rates of abstrac- Individual groundwater abstraction impact tion are measured so that the locations and size of predictions flow reductions can be clearly identified, just as in the case of a surface water abstraction from a Havant and Bedhampton spring capture river. This in turn means that minimum residual impacts, East Hampshire flow conditions can be imposed on spring source abstraction licences as a practical way of constrain- The simplest form of ‘groundwater’ abstraction is to ing impacts on flows downstream. capture spring flows. In East Hampshire the Havant The East Hampshire and Chichester Chalk and Bedhampton springs are located close to the model represents these spring sources simply – as coast (Fig. 1). They represent one of the largest a series of surface water abstractions from stream and lowest elevation outflows from the Chalk, drain- cells that are configured to simulate the spring dis- ing recharge from the unconfined aquifer to the charges from the groundwater system (Entec UK north via karstic flow pathways beneath a shallow 2008). Unlike pumped borehole or shaft sources London Clay filled syncline (Fig. 6). Approximately that have the potential to draw on groundwater 100 Ml/day of groundwater would naturally flow storage, there is no possibility that these spring from the springs into Langstone Harbour, but a abstraction impacts could be re-distributed spatially Downloaded from http://sp.lyellcollection.org/ by guest on April 23, 2012

GROUNDWATER ABSTRACTION IMPACTS ON FLOW 275 or with time – they can be seen and measured as simulation (1970–2005). They are clearly greatest they occur. on the River Glavern, adjacent to the abstraction, There is, however, some uncertainty regarding reducing both to the north and the south, with smaller the rates of residual groundwater outflows to the impacts on a tributary flowing in from the east, and Harbour, which are more difficult to measure and on water courses marginal to the coast on all the may include less visually obvious areas of more stream cells. This pattern is probably close to that diffuse discharge. The regional model has been which would be predicted by analytical tools such helpful in constraining this uncertainty because it as IGARF (Environment Agency 2004). Consider- incorporates consistently calculated estimates of ation of time series flow output from the two runs recharge inputs to the whole aquifer based on (such as presented in Fig. 4 for the Ashwood exam- meteorological data and has been calibrated against ple) suggests that this average impact pattern does all of the measured outflows from the system. It has not vary much with time or flow conditions. Model therefore improved confidence in the quantification stream flow impacts add together to represent c. of flows not captured for abstraction. It has also 96% of the water abstracted from Glandford. helped to characterize the more complex impacts Interrogation of model flow budget output (dis- of other Chalk borehole sources on outflows from cussed in more detail for the Bocking example, the springs and other river receptors, as described Fig. 7) indicates that reductions in groundwater further below, for example, for the Worlds End outflows to boundaries representing the North Sea abstraction (Fig. 6). amount to a further 2% of the abstraction, with the remaining 2% accounted for as differences in storage change over the whole period of simulation (because Glandford borehole abstraction impacts the starting heads for both runs were the same). on the River Glavern, North Norfolk Figure 3 also includes a simply mapped indi- cation of the distribution of stress associated with The Glandford boreholes abstract from the Chalk all the licensed groundwater abstractions. These next to the perennially flowing lower reaches of circles are centred on each abstraction point and the River Glavern, near the North Norfolk coast have an area which, if multiplied by the annual (Fig. 3). They are represented as groundwater average recharge (estimated for the aquifer within abstractions in the Yare and North Norfolk regional this catchment), is equivalent to the licensed rate model, which has a regular 200 m grid and multiple of abstraction. This is a convenient and consistent layers simulating flow and storage within the Chalk format used by the Environment Agency across and overlying superficial deposits. The construction England and Wales that has been included on and calibration of this part of the model incorporat- most of the other maps in this paper. ing historic records of abstraction and discharges is In some unconfined aquifer examples such as described in Entec UK (2006). Glandford, which Figure 3 shows to be the largest As part of Review of Consents investigations abstraction in an area of otherwise more sparsely focusing on Protected Areas along the coast mar- spread pressures, the intersection of such equivalent gins, a ‘fully licensed’ scenario has been run recharge circles with surface water subcatchments through the model to predict the impact of all of may provide a credible indication of the distribution the abstractions (from groundwater and surface of impacts on river reaches draining these areas. water) operating at their maximum consented rates These boreholes are located in a relatively high (rather than their actual quantities), in combination transmissivity valley next to a perennially flowing with treated discharge returns. Several variants on river, with relatively good connection modelled this ‘baseline’ scenario have also been run to con- between the river and the aquifer through permeable sider the impacts associated with individual abstrac- drift. There are no other nearby abstractions to pull tions including Glandford – by switching them off groundwater levels down below the river bed. The and calculating the difference in groundwater to model suggests that the Glandford boreholes inter- stream boundary flows between the two runs. In cept water that would otherwise discharge to the Figure 3 (as for subsequent examples), the reduction river, rather than inducing leakage from it, without in flow to each stream cell has been divided by the much seasonal variation in drawdown or storage. long-term average abstraction rate for the source In the examples that follow, however, ground- being investigated (the Glandford licence averages water models built to incorporate more complex to c. 4.5 Ml/day), with the results mapped as a per- (but reasonably well defined) conceptual features centage of this change in modelled abstraction. This suggest more complicated temporal and spatial pat- makes it easier to compare the results from sources terns of flow impact than would be predicted by the with different abstraction rates. use of analytical approaches or the simple intersec- For this example, impacts are shown as long- tion of equivalent recharge circles with surface term averages for the whole period of the model water catchments. Downloaded from http://sp.lyellcollection.org/ by guest on April 23, 2012

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Fig. 4. The location of the Ashwood boreholes and simulated time series abstraction impacts predicted by the West Midlands and Worfe groundwater model. (a) Ashwood shaft location within the Kidderminster gauged surface water catchment of the River Stour; (b) time series of modelled river flows and Ashwood flow difference abstraction impacts simulated at the Kidderminster gauge, assuming fully licensed operation.

Ashwood borehole abstraction impacts (Fig. 4a). The overlapping equivalent recharge on the Smestow Brook and River Stour, circles shown on this figure indicate that the Permo- Triassic Sandstone aquifer from which Ashwood West Midlands pumps is subject to much higher levels of abstrac- The Ashwood source is also located next to a tion stress than the Chalk around Glandford perennial reach of the Smestow Brook – the main discussed previously. Drawdown is spread more tributary to the River Stour upstream of the Kidder- regionally because of layering, which reduces verti- minster gauging station in the West Midlands cal hydraulic connectivity across formations, and Downloaded from http://sp.lyellcollection.org/ by guest on April 23, 2012

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Fig. 5. The location of the Leckford and Clarendon boreholes close to the River Bourne with illustrations of the spatial variation of their associated abstraction impacts under contrasting flow conditions, as predicted by the Wessex Basin groundwater model. Groundwater to surface water flow impacts for both Leckford and Clarendon(′) sources for stress periods when modelled flows at the downstream reference gauge are (a/a′) around Q30 (moderately high flows); and (b/b′) around Q95 (low flows); (c)Ml/day measured flow in the River Bourne from repeated longitudinal spot flow gauging surveys across a range of flow conditions. because of interference between wells. A separate treated sewage discharge from the Wolverhampton paper within this publication (Shepley & Soley conurbation. 2012) explains that a large proportion of the low The hydraulic gradient between the rivers and flows in the Smestow Brook is not natural discharge the underlying aquifer changes seasonally as the from the aquifer (as to the River Glavern), but rather relatively high rates of groundwater abstraction Downloaded from http://sp.lyellcollection.org/ by guest on April 23, 2012

278 R. W. N. SOLEY ET AL.

Fig. 6. The location of the Worlds End boreholes close to the River Wallington with illustrations of the spatial variation of abstraction impacts across the flow regime predicted by the East Hampshire and Chichester Chalk groundwater model. Groundwater to surface water flow impact maps for stress periods when modelled flows gauged on the River Meon are (a) greater than Q10 (high flows); and (b) less than Q90 (low flows); (c)Ml/day impacts of Worlds End abstraction modelled across the flow duration curve for the three main receptors: Havant & Bedhampton Springs + River Meon + River Wallington.

within the valleys draw the water table down below than the Chalk (typically in the range 1–2%). During the stage level, inducing leakage from surface water low-flow periods groundwater levels may be below to ground. The rate of leakage is limited by the the stage of some reaches of the main river, tribu- hydraulic characteristics of the bed materials – it taries or springs whereas groundwater becomes does not continue to increase linearly as the under- effluent to these watercourses during the winter as lying water table continues to fall and some of the the water table rises in response to recharge. abstracted water is drawn from storage. The sand- These features are well represented within the stone has a specific yield of c. 10% – much higher time-variant distributed West Midlands and Worfe Downloaded from http://sp.lyellcollection.org/ by guest on April 23, 2012

GROUNDWATER ABSTRACTION IMPACTS ON FLOW 279 regional MODFLOW groundwater model. This This river flow pattern is essentially natural. The model is extended to include the secondary Carbon- longitudinal elevation profile of the Bourne reveals iferous areas draining onto the Permo-Triassic it to represent a relatively high-level drain on the Sandstone, as well as the influence of surface groundwater system in comparison with the more water discharges and leakage from household con- deeply incised River Avon to the west, and the nections and canals using the 4R near-surface pro- River Test to the east. Its middle reaches are also cesses code (Heathcote et al. 2004). Total flows ‘under-drained’ by a particularly transmissive ‘hard- simulated at Kidderminster compare reasonably ground’ horizon within the Chalk – the Whitway well with the gauged record (typically within 20% Rock – which lies within a shallow eastwardly plun- of low flows). ging syncline. This feature is largely responsible for Figure 4b shows time series flows simulated draining water away from the upper reaches of the from a pair of model runs intended to investigate Bourne towards the Avon and tributaries of the the individual impact of the Ashwood abstraction Test, and accretion only picks up downstream of compared with a baseline which includes all other its re-appearance at outcrop in the lower reaches. sources in operation as well. The difference Groundwater levels and catchments fluctuate mark- between the total stream cell flows is plotted edly between seasons with the whole river only against the right hand axis and represents the recov- flowing when groundwater levels are high and ery predicted after switching Ashwood off. Over a fluxes exceed the ‘carrying capacity’ of the aquifer period of 10 years there is a long-term flow recovery (particularly the Whitway Rock flow horizon). that flattens out at c. 18 Ml/day – close to the Leckford is located next to the upper to middle abstraction rate (18.2 Ml/day). However, even reaches of the Bourne, whereas Clarendon lies though the abstraction rate is fairly constant, there close to its perennial confluence with the Avon. are significant annual and medium-term variations Two stream cell maps of groundwater to river flow apparent in the impact time series which are super- reductions are presented for each of these sources imposed onto the longer-term recovery. These in Figure 5. Separate model runs were carried out impact variations closely follow simulated ground- for each source and in all cases they have been water levels, because of the ‘real world’ factors mapped as percentages of the licensed abstraction described above (hydraulic layering, high rates of rate being investigated so that they can be compared abstraction from other nearby sources, high against a common key. These differ from the long- storage, leakage rates limited by bed materials). term average map for Glandford (Fig. 3) in that They are explored in greater detail in Shepley & the groundwater model stress periods sampled to Soley (2012 – see particularly figs 6 & 7 therein). calculate the impacts were selected to represent Therefore, during droughts with low groundwater different percentile exceedance conditions accord- levels, flow impacts are predicted to fall to 14 Ml/ ing to flows simulated at the Laverstock gauging day – c. 80% of the rate of abstraction – whereas, station. Thus, Figures 5a and a′ represent the impacts when groundwater levels (and baseflows) are associated with Leckford and Clarendon, respect- highest, simulated impacts also rise to c. 26 Ml/day. ively, for stress periods when flows at Clarendon were c. Q30 (actually Q29.5 to Q30.5), whereas Figures 5b and b′ show impacts averaged from Leckford and Clarendon borehole abstraction stress periods around the low-flow Q95 statistic. impacts on the Rivers Bourne, Avon and Test, Leckford impacts (Fig. 5a, b) vary significantly Wessex Basin according to the flow condition – both in their spatial distribution, and in the total impact Both Leckford (Fig. 5a, b) and Clarendon (Fig. 5a′, summed across the river network – in a manner b′) public water supply sources are near the River that reflects the natural variations in groundwater– Bourne in Wiltshire and abstract from the Chalk river flow relationships described previously. At as simulated in the Wessex Basin regional model Q95 there can be no impact on the middle reaches (further details described in Soley et al. 2012). of the river because they do not flow naturally. Repeated spot flow gauging surveys demonstrate Overall impacts amount to much less than the the ‘winterbourne’ characteristics of this river average rate of abstraction, and these are mostly (Fig. 5c). Flow gains from the Chalk in its upper apparent along the River Avon, together with a reaches are lost downstream. Some artificial support short reach next to the source itself and a few is provided by discharge from a small sewage treat- stream cells along the lower river. A greater percen- ment works, but its middle reaches only flow for tage of the abstracted water is accounted for at Q30 c. 30% of the time. There is strong flow accretion with more impacts upstream and downstream along in the lower reaches such that these flow perennially the Bourne, plus along longer reaches of the Avon, at the Laverstock gauging station just upstream of and on some of the tributaries to the Test in the the confluence with the River Avon. east. When considering the equivalent maps for Downloaded from http://sp.lyellcollection.org/ by guest on April 23, 2012

280 R. W. N. SOLEY ET AL.

Fig. 7. The location of the Bocking borehole which abstracts from Chalk confined by the London Clay with groundwater to surface water flow impact maps predicted by the Essex model for stress periods when modelled flows are (a) greater than Q10 (high flows); and (b) less than Q90 (low flows); (c) mAOD (metres Above Ordnance Datum) observed and modelled confined Chalk groundwater levels 190 m north west of the Bocking abstraction; (d)Ml/day difference in whole groundwater model water balance between the two Bocking individual impact assessment runs: (Bocking ON at constant 4 Ml/day run) MINUS (Bocking OFF run). higher flow stress periods (e.g. around Q10 – not Impacts mapped for Clarendon (Fig. 5a′,b′) are, shown here), the total impacts are considerably however, much less seasonally variable. As for the higher than the average abstraction rate – to a Glandford example, groundwater levels are locally greater extent than described for Ashwood from constrained by flow to the perennial river reaches Figure 4, and for reasons reflecting the natural of the Bourne and the Avon and these are where catchment behaviour rather than the artificial draw- the impacts of abstraction are apparent both at down associated with abstraction. Q95 and at Q30, together with minor impacts on a Downloaded from http://sp.lyellcollection.org/ by guest on April 23, 2012

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Fig. 8. (a) The location of the conjunctive river intake and wellfield which abstracts from the Lower Greensand in the confined Hardham Basin; (b) the impacts on stream outflows associated with a 14 Ml/day long-term average wellfield abstraction at a constant rate, and with a summer peak of 18 Ml/day; and (c) water balance impacts on storage and MODFLOW Drain, Stream and Evapotranspiration Package flows. Downloaded from http://sp.lyellcollection.org/ by guest on April 23, 2012

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Fig. 9. 4R and MODFLOW stream cell maps for the River Wensum catchment showing in-combination total-flow impacts of fully licensed groundwater and surface water abstractions, and discharges under Q95 flow conditions, in comparison with a natural model run in which all of these influences are switched off. (a) Impacts mapped as Ml/day combining all upstream influences; (b) combined impacts mapped as a percentage of the natural QN95 flow for each stream cell.

tributary to the Test further east. A very similar periods of low flow (Entec UK 2008). As previously pattern is maintained across the whole flow range described, the aquifer block to the north of the from Q1 to Q99. Palaeogene filled syncline drains continually to the Havant and Bedhampton springs to the east, and Worlds End borehole abstraction impacts on also to the River Meon further west, so groundwater levels fall below the bed of the River Wallington. the Rivers Wallington and Meon, and on During the winter recharge raises groundwater Havant and Bedhampton springs, East levels until they ‘overflow’ into this river and Hampshire other intermittent winterbournes. The Worlds End source abstracts from the con- The River Wallington, next to which the Worlds fined Chalk just at the edge of the syncline, so inter- End source is located (Fig. 6), is also not an effective action with the river is locally restricted by the drain on the Chalk groundwater system during low-permeability London Clay. The elevations of Downloaded from http://sp.lyellcollection.org/ by guest on April 23, 2012

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switch individual abstraction up to repeated FULL LICENSED rates (with 'warm start' heads)

"the impact of repeated RECENT individual Full ACTUAL scenario as Licensed Abstraction the BASELINE for minus = in the long term, in individual impacts the context of (with 'warm start' everything else at heads) Recent Actual rates"

switch individual abstraction OFF (with 'warm start' heads)

Fig. 10. Recommended groundwater model runs, starting heads and post-processing protocol to predict the long-term impacts of an individual abstraction at fully licensed rates. The prediction context is that all other influences are repeated at recent actual rates, across the long-term historic variation in historic climate.

these various rivers and springs and their connec- for example between rapidly rising and falling limbs tivity with the aquifer are all reasonably well con- of the hydrograph. This is why the flow impact strained by survey measurements and have been variation calculated in this way becomes more built into the East Hampshire and Chichester irregular at higher flows, whereas the variation is Chalk regional model stream cells. smoother when groundwater levels and flows are This East Hampshire 4R–MODFLOW model lower. provides a reasonable simulation of flow to the The maps and flow duration impacts reveal a River Wallington, the springs and the lower reaches complex pattern of impact specific to each receptor. of the River Meon. It has been used to map the pre- During lowest flow periods, impacts on the River dicted impacts of the Worlds End source for high- Wallington are close to zero, but these rise to flow stress periods (greater than Q10) in Figure 6a, c. 11 Ml/day when flows are highest. The reverse and for low flows (less than Q90) in Figure 6b. is true for impacts on the Havant and Bedhampton Analysis of the modelled flow time series for each Springs, which represents the lowest drainage of the three main receptors has also allowed the vari- point from the aquifer – impacts are predicted ation in impacts for all flow percentiles to be plotted to be greatest (c. 2Ml/day) at low flows, but fall in Figure 6c. This plot adds together the modelled to c. 0.5 Ml/day above Q10. A further 2 Ml/day reductions in flow for each of the receptors. The of the abstraction from Worlds End is apparent as flow reductions are calculated as the average differ- a roughly constant loss of flow from the River ence between modelled flows for stress periods Meon across the flow regime. When added together, selected according to the flow duration statistics predicted impacts on these three receptors account modelled on the River Meon. As the model is run for most of the c. 10.2 Ml/day Worlds End abstrac- with three stress periods per month, there are a tion in the long-term. Combined impacts at lowest total of 1260 modelled flows sampled over the flows are only 60% of this long-term average – 1970–2004 simulation period, that is, 12 or 13 for largely because of the poor connection of the each of the 100 percentiles plotted. The relationship River Wallington locally with the aquifer and the between modelled groundwater levels and river regional flow system, which only discharges into flows will vary between the selected stress periods, this ‘winterbourne’ at high groundwater levels. Downloaded from http://sp.lyellcollection.org/ by guest on April 23, 2012

284 R. W. N. SOLEY ET AL.

Bocking confined abstraction flow and account for most of this water by the end of the groundwater level impacts, Essex run, but only after 40 years. Throughout this time storage changes associated with the simulated shifts The Bocking source abstracts from a much more in groundwater levels are gradually reducing. A deeply confined Chalk borehole in Essex, over very small change in groundwater outflow to the 10 km south of the edge of the aquifer subcrop coastal general head boundaries further south is also (Fig. 7a–c). The historic calibration run of the predicted. Essex MODFLOW model simulates the large draw- downs observed only 190 m from the abstraction reasonably well considering that the model has a Hardham basin conjunctive abstraction regular 200 by 200 m grid (Fig. 7c). Groundwater scheme, Sussex levels confined by the low permeability London Clay and Lower London Cenozoic deposits have The final ‘individual source’ example considers the fallen and risen by over 20 m in response to Hardham wellfield, which abstracts from the Folk- pumping from Bocking and other boreholes in the estone Beds aquifer – the upper formation of the Braintree area. These are very poorly connected to Lower Greensand – where it is confined by the the overlying river network south of the Chalk, Gault Clay (Fig. 8a), as well as by alluvial and estu- where it flows through shallow superficial gravel arine superficial deposits. These low-permeability aquifers, so the absolute differences between mod- aquicludes limit interaction with the overlying shal- elled and observed confined groundwater levels lower water table and river system and groundwater have little bearing on the credibility of the simulated levels within the Hardham Basin fall and rise by up flow impacts. to 9 m in response to variable rates of abstraction. This historic simulation was used as the baseline The Hardham 4R–MODFLOW model was com- for a rapid prediction of the possible long-term missioned by the water company to investigate the impacts of Bocking abstraction on the river network. impacts of wellfield abstraction on three wetlands Two new runs were carried out and compared – one comprising the Arun Valley Special Protection with Bocking switched off after 2 years, and a Area as part of the Habitats Directive Review of second with abstraction raised to the fully licensed Consents process (Atkins 2009). rate of 4 Ml/day (again after 2 years). Simulated The wellfield is operated in conjunction with a groundwater levels for these two scenarios in the surface water intake on the River Rother and this Chalk close to the abstraction are included in combination of sources provides flexibility to vary Figure 7c. Following an initially rapid period of the seasonal profile of abstraction. Beyond the drawdown, or recovery, they continue to diverge primary purpose of wetland impact characterization, over a period of some 40 years, despite the low the water company has developed its model and storage associated with the confined Chalk. For carried out initial runs to consider the river flow much of this time, however, groundwater levels pre- implications of ‘resting’ the wellfield during winter dicted for the Bocking fully licensed run are within periods and maximizing groundwater use in the the silty/sandy Lower London Cenozoic deposits, summer when supply demands are higher and low which have a higher specific yield. The influence river flows constrain rates of surface water abstrac- of changing abstraction rates for other sources is tion. Preliminary findings are described below. still apparent in heads predicted through both runs. Figure 8b shows simulated stream cell flows and Figures 7a and b show the spatial distribution of flow reductions from 1995 to 2002 associated with reduction in flow to streams from stress periods two abstraction profiles, both at an annual average representing high flows (above Q10) and low flows of 14 Ml/day. The first profile has constant abstrac- (less than Q90) respectively. Impacts are apparent tion at 14 Ml/day and the second abstracts at where the most deeply incised rivers closest to the 12 Ml/day from January to June, 18 Ml/day from source flow off the Chalk outcrop and are consider- July to September and 14 Ml/day from October to ably greater for higher flow periods, as has been December. The predicted stream flow impacts of noted elsewhere. However, when calculated as an these two abstraction profiles are virtually indistin- average over the whole model run period, these guishable. This suggests that there may be flexibility river flow impacts only equate to c. 2.4 Ml/day – to abstract from the wellfield at higher rates during much less than the average rate of abstraction. periods of low flow and peak demand, without Differences in the whole model water balance increasing impacts on the river. components are plotted as time series in Figure 7d. Figure 8b indicates that the average stream flow The immediate change in overall groundwater impact from these preliminary runs associated with abstraction (4 Ml/day) is immediately evident and both abstraction profiles is c. 7Ml/day, falling to remains constant thereafter. Long-term shifts in c. 4.4 Ml/day when river flows are lower. This flows leaking from streams and flowing to them includes impacts on ditches over flood plain and Downloaded from http://sp.lyellcollection.org/ by guest on April 23, 2012

GROUNDWATER ABSTRACTION IMPACTS ON FLOW 285 wetland areas close to the main river channels that Often, however, the models predict that low-flow are also modelled as part of the routed stream cell impacts are less in absolute terms than when ground- network, but represents only 50% of the ground- water levels are higher. This may be related to natural water abstraction rate in the long term. factors such as the increased length of overflowing Flow balance information for the central area of river reaches (see also Hulme et al. 2012) and the model corresponding to the Hardham Basin springs as groundwater levels rise and, for the helps to account for the rest of the abstracted Chalk in particular, the enhanced speed of flow water (Fig. 8c). Seasonal variations in storage are response associated with the saturation of more on the order of 2–3 Ml/day, but longer-term transmissive shallow fissure systems (e.g. Leckford changes (1987–2006) are only 0.3 Ml/day within and Worlds End). These two examples also demon- this central area. More significant in summer strate the value of incorporating time-variant months is a reduction in evapotranspiration from recharge and accurate elevations for stream and riparian areas. This flux is simulated using the spring boundaries in regional distributed models. MODFLOW Evapotranspiration Package, such that The existence of regionally transmissive hardground shallow water table drawdown can result in a reduc- horizons within the Chalk often leads to significant tion in evaporation losses. The change in these differences between topographic and groundwater losses is c. 1.2 Ml/day in the long term, increasing catchment areas. As a general rule, flow impacts to 3.8 Ml/day in summer. Reductions in ground- will be greatest where and when natural discharge water discharge to MODFLOW Drain Package from the aquifer is highest. In particularly complex boundaries that are located on the unconfined aquifer examples such as Worlds End, the relationships away from the flood plain amount to 3.7 Ml/day in between flow and impact may vary according to the long term, increasing to 4.4 Ml/day in summer. the receptor (i.e. low level spring impacts are greatest These drain cell flow reductions would probably add in the summer, whereas higher-level winterbourne to river flow impacts in winter, but might be partly reaches are impacted most when heads are high). associated with reduced evapotranspiration in Hydraulic layering within the aquifer, confin- summer. Average increases in lateral groundwater ing aquicludes and low-permeability stream bed inflows to this central flow balance area account materials may all make it harder for boreholes to for the remainder of the abstraction. induce leakage from overlying streams, such that Further model refinement and predictive runs are water is drawn from storage and groundwater planned to understand how changes in all these levels fall during the summer. This may buffer MODFLOW boundary outputs from the whole impacts on surface flows, particularly if the aqui- model might combine to indicate total-flow impacts fer has a higher specific yield (e.g. the Lower Green- on the River Arun. sand at Hardham and Permo-Triassic Sandstone at Ashwood). Abstraction impacts from deeply con- fined sources such as Bocking may take up to 40 Summary of the ‘real world’ hydrogeological years to develop in rivers draining the outcrop of mechanisms highlighted by modelled the aquifer, but can be accounted for in long-term abstraction impact predictions modelling predictions. Layering can also cause drawdown to spread The examples presented are all based on regional over a wider area than might be anticipated based modelling predictions but do illustrate and reflect on a more homogeneous conceptual understanding a number of credible ‘real world’ mechanisms that of an aquifer, resulting in more diffuse flow influence the spatial and temporal patterns of impacts across the river network. The local pattern groundwater abstraction impacts on river flows. In of abstraction from other groundwater sources can the case of spring capture sources (such as at be important as well. At Ashwood and Bocking Havant and Bedhampton), there is no uncertainty drawdown pressures from other boreholes interfere in the timing or location of impacts, especially if to pull heads down below the stream bed, changing both the abstracted water and the residual flow are the baseline context within which the individual gauged. Some borehole sources located next to source impacts are assessed. rivers that gain baseflow from the unconfined Through careful site selection and operational aquifer all year round may also be associated with management, these hydrogeological mechanisms relatively simple (spatially and temporally constant) can be exploited to minimize the low-flow impacts patterns of predicted impact (e.g. Glandford and of abstraction while continuing to meet peak Clarendon). Such sources intercept groundwater summer demands (as at Hardham). The Hardham flowing to rivers rather than inducing leakage, and example also illustrates the importance of consider- they do not need to create their own drawdown ing how riparian evapotranspiration losses may be catchments in the manner of sources further from reduced by drawdown in floodplain and wetland the discharge boundary. areas where the water table is shallow. Some of Downloaded from http://sp.lyellcollection.org/ by guest on April 23, 2012

286 R. W. N. SOLEY ET AL. the abstracted water would otherwise be evaporated zero). The surface water abstraction for Norwich from floodplain vegetation or ditches during stands out as a large proportion of natural low summer low-flow periods. flows, but most tributary headwater reaches also have proportionally high impacts. Their predicted Groundwater abstraction impacts in flow reductions are very small, but so are their natural flows. combination with surface water abstractions and discharges, Wensum Recommended modelling protocol for catchment example individual groundwater abstraction impact Beyond investigating the impacts of individual prediction and for assessment combined groundwater abstractions, regional models that with surface water influences account for accumulated baseflow, runoff, surface water abstractions and discharges are ideal plat- Groundwater models are an important part of the forms to assess the impact of all these influences ‘toolkit’ for predicting the long-term impacts of acting in combination, in the context of predicted abstractions individually and in combination with natural flows. Example maps from the River other flow influences, but evidence from field inves- Wensum catchment in Norfolk are shown in tigations is also important. Often pumping tests or Figure 9 based on differences in the total outflow ‘signal tests’ (where abstraction rates from an oper- for each MODFLOW stream cell, rather than the ational source are varied systematically) highlight groundwater–surface water exchanges that have patterns of shorter term impact that are helpful in been mapped in previous figures. understanding where the water may be derived Stream cell flows from a model run with no arti- from in the longer term. Recent Habitats Directive ficial influences (a ‘natural’ run) have been sub- investigations into the impacts of the Chitterne tracted from flows for a run with fully licensed source (on Salisbury Plain, Fig. 1, just to the west groundwater and surface water abstractions, and of the area mapped in Fig. 5). provide one discharges. The results are mapped as ‘fully licensed example of the value that can be extracted from a in-combination’ impacts (in Ml/day). In Figure 9a thorough analysis of intensive monitoring associ- the Q95 flow condition map is based on statistically ated with such tests. The understanding gained sampled flows simulated by the Yare and North will be incorporated into the final report of the Norfolk model through the entire run. Similar Wessex Basin model to be published by the formats can be prepared for any time-specific Environment Agency during 2010. periods. The map also indicates the location and Some time may be required to refine the parame- size of the artificial influences causing these terization of the groundwater model local to the impacts. It demonstrates that, while groundwater source being investigated if the initial impact pre- abstraction is responsible for lower than natural dictions do not reflect such field data, in so far as flows across much of the river system, by far the this is achievable on a rectilinear regional grid. If greatest reduction in absolute flow rate is associated possible, and good investigation data exist for com- with a large surface water abstraction (supplying parison, the stress periods for the groundwater Norwich) in the lower reaches of the Wensum. model can be temporarily reduced to daily in order There is also one tributary reach where the influence to compare and improve its simulated test draw- of a sewage treatment works discharge is predicted down response, before switching back to the to raise flows above natural. weekly, 10 day or monthly periods more typically Such total-flow impact predictions are often used for longer-term calibration and prediction. more useful for studies considering the potential However, a ‘perfect model’ is never achievable ecological consequences of water management and all stakeholders involved in reviewing model options than the investigation of individual abstrac- results should be prepared to balance them along- tions discussed previously. They can be used to side field evidence in making decisions. target ecological sampling regimes and interpret Once a ‘fit for purpose’ model has been devel- biological monitoring data based on degrees of oped, the Environment Agency will typically flow impact. In this regard it is also helpful to put require three ‘in-combination’ abstraction and dis- the impact in the context of the natural flow charge scenarios to be run: natural (as a reference expected at the location. for flow impact screening); recent actual (annually Figure 9b is an example for the same River repeated cycles of recent influences – to predict Wensum and tributaries, and presents the flow how these would impact flows based on longer-term impacts from Figure 9a as a percentage of the meteorological variations); and fully licensed (a natural low-flow QN95 for each stream cell (so similarly long-term but ‘worst case’ prediction to long as the natural flow is not negligibly small or use for further licensing assessments). Downloaded from http://sp.lyellcollection.org/ by guest on April 23, 2012

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Of these three, it is recommended that the recent with some notable exceptions, impacts are likely to actual scenario provides the most appropriate base- be greater during higher flow periods when they are line context for the prediction of individual abstrac- less ‘noticeable’. tion source impacts (Fig. 10). Shepley & Taylor Groundwater will therefore continue to play a (2003) describe the use of this scenario in detail. It vital role in supporting peak rates of summer is based on current patterns of influence that are abstraction into the future – buffering the predicted often well measured and are within the historic cali- river flow impacts of climate change and abstrac- bration range of the model. Both the natural and tion in a manner that would only otherwise be poss- fully licensed scenarios are more hypothetical and ible through increased demands on surface water subject to greater uncertainty. In order to character- reservoirs. ize the long-term fully licensed impacts of an indi- vidual abstraction, Figure 10 shows that two Many thanks go to my fellow authors for their contributions variations are required from the recent actual base- (Alison, Don, Clare, Mike and Paul), to the Environment Agency hydrogeologists who managed the development line: one with it switched off, and a second with it of their models (S. Fletcher, M. Shepley, M. Grout, U. Buss, turned up to fully licensed rates. If the conceptual P. Shaw, J. Grundy and G. Bryan), to Southern Water and numerical model development, in comparison Services for the use of their Hardham Basin model, to with field data, suggests that alternative parameter- AMEC Environment & Infrastructure for encouraging izations could give equivalent results, the sensitivity engagement with the UK Groundwater Modellers Forum, of the impact predictions to such uncertainties and to K. Rushton for insisting on a rigorous description should be investigated. of ‘where the water came from’ without descending into If time permits, all of these model runs should ‘model speak’. be allowed to reach a dynamic equilibrium before the main prediction period begins. One of the most References severe droughts experienced in the UK was in 1976. The occurrence of this notable low-flow ATKINS. 2009. Arun Valley SPA Sustainability Study. period early in the historical record of climate and Hardham Basin Groundwater Model – Model Con- struction and Calibration. A report for Southern flows used in groundwater modelling (typically Water Services Ltd. starting in 1970) makes it important to check that COUNCIL OF EUROPEAN COMMUNITIES. 1992. any long-term shifts in water balance components Directive on the Conservation of Natural Habitats associated with inconsistent starting heads have and of Wild Fauna and Flora (92/43/EEC). been worked through. If necessary (as in the example COUNCIL OF EUROPEAN COMMUNITIES. 2000. for Bocking), this might require the scenarios to be Directive on Establishing a Framework for Community run through twice, with the second run using starting Action in the Field of Water Policy (2000/60/EC). heads from the end of the first run. Shepley & Clausen, B., Young,A.R.&Gustard, A. 1994. Model- Streetly (2007) describe this approach in relation ling the impact of groundwater abstractions on low river flows. In: FRIEND: Flow Regimes from Inter- to achieving an appropriate natural scenario run. national Experimental and Network Data (Proceed- ings of the Braunschweig Conference, October 1993). Publication no. 221. IAHS, Wallingford. Conclusions DEPARTMENT FOR THE ENVIRONMENT, FOOD AND RURAL AFFAIRS. 2007. Preliminary Cost Regional groundwater models facilitate predictions Effectiveness Analysis of the Water Framework Direc- of long-term abstraction impacts on rivers, which tive, Chapter 4.2, Preliminary Cost Effectiveness take account of ‘real world’ three-dimensional, time- Analysis (pCEA) of Water Resources Measures to variant hydrogeological mechanisms and thereby go meet WFD Environmental Objectives. World Wide Web Address: http://www.wfdcrp.co.uk/pdf%5C beyond what is possible with linear analytical appro- Chapter%204.2%20Water%20resources.pdf aches. Many of these mechanisms have been summar- ENTEC UK. 2006. Phase 3 Project Record for the Yare ized and illustrated through the individual source and North Norfolk Groundwater Resource Model. examples presented. By using the MODFLOW North Norfolk Reporting Area. Volume 1 Characteris- Stream Package and a code like 4R to incorporate ation of Catchment Behaviour. Contractor Report for runoff, interflow and the influences of surface the Environment Agency of England and Wales, water abstractions and discharges, in-combination Anglian Region. impact maps and time series can also be developed ENTEC UK. 2008. East Hampshire and Chichester Chalk and placed in their natural flow context. Numerical Modelling Project. Phase 2B Predictive Scenarios and Abstraction Impact Assessment. Con- While some patterns of impact are relatively tractor Report for the Environment Agency of simple, modelling predictions often suggest that England and Wales, Southern Region. absolute (Ml/day) low-flow reductions during ENVIRONMENT AGENCY. 2004. IGARF1 v4 User recession periods are less than when groundwater Manual. Environment Agency of England and Wales levels are rising or high. As a broad generalization, Report NC/00/28. Downloaded from http://sp.lyellcollection.org/ by guest on April 23, 2012

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Evans, R. 2007. The Impact of Groundwater Use on Aus- Groundwater Development. Geological Society, tralia’s Rivers. Technical Report by Sinclair Knight London, Special Publications, 193, 199–210. Merz. Land and Water Australia. Shepley,M.G.&Soley, R. W. N. 2012. The use of Hantush, M. S. 1965. Wells near streams with semi- groundwater levels and numerical models for the pervious beds. Journal of Geophysical Research, 70, management of a layered, moderate-diffusivity 2829–2838. aquifer. In: Shepley, M. G., Whiteman, M. I., Heathcote, J. A., Lewis,R.T.&Soley, R. W. N. 2004. Hulme,P.J.&Grout, M. W. (eds) Groundwater Rainfall routing to runoff and recharge for regional Resources Modelling: A Case Study from the UK. groundwater resource models. Quarterly Journal of Geological Society, London, Special Publications, Engineering Geology and Hydrogeology, 37, 113–130. 364, 303–318. Hulme, P. J., Jackson, C. R., Atkins, J. K., Hughes, Shepley,M.G.&Streetly, M. 2007. The estimation of A. G., Mansour, M. M., Seymour,K.J.&Wilson, ‘natural’ summer outflows from the Permo-Triassic K. J. 2012. A rapid model for estimating the depletion Sandstone aquifer, United Kingdom. Quarterly in river flows due to groundwater abstraction. In: Journal of Engineering Geology and Hydrogeology, Shepley, M. G., Whiteman, M. I., Hulme,P.J.& 40, 213–227. Grout, M. W. (eds) Groundwater Resources Model- Shepley,M.&Taylor, A. 2003. Exploration of aquifer ling: A Case Study from the UK. Geological Society, management options using a groundwater model. London, Special Publications, 364, 289–302. Water and Environment Journal, 17, 176–180. Hunt, B. 1999. Unsteady stream depletion from ground- Soley, R., Power, T., Mortimore, R., Shaw, P., water pumping. Groundwater, 37, 98–102. Dottridge, J., Bryan,G.&Colley, I. 2012. Model- Jackson, C. R., Mansour, M. M., Hughes,A.G.& ling the hydrogeology and managed aquifer system of Hulme, P. J. 2008. Numerical Modelling of the the Chalk across southern England. In: Shepley, Impact of Groundwater Abstraction on River Flows. M. G., Whiteman, M. I., Hulme,P.J.&Grout,M. Environment Agency Science Report SC030233/SR1 W. (eds) Groundwater Resources Modelling: A Case from British Geological Survey Report, OR/08/017. Study from the UK. Geological Society, London, Kirk,S.&Herbert, A. W. 2002. Assessing the impact of Special Publications, 364, 129–154. groundwater abstractions on river flows. In: Hiscock, Theis, C. V. 1941. The effect of a well on the flow of a K. M., Rivett,M.O.&Davison, R. M. (eds) Sustain- nearby stream. American Geophysical Union Trans- able Groundwater Development. Geological Society, actions, 22 534–738. London, Special Publications, 193, 211–233. Weber,K.A.&Perry, R. G. 2006. Groundwater abstrac- McDonald,M.G.&Harbaugh, A. W. 1988. A Modular tion impacts on spring flow and base flow in the Hills- Three Dimensional Finite Difference Ground-water borough River Basin, Florida, USA. Hydrogeology Flow Model. US Geological Survey Techniques of Journal, 14, 1252–1264. Water Resources Investigations Report, 06-A1. Whiteman, M. I., Seymour, K. J., van Wonderen, J. J., Parkin, G., Birkinshaw, S., Younger, P. L., Rao,Z.& Maginness, C. H., Hulme, P. J., Grout,M.W.& Kirk, S. 2007. A numerical modelling and neural Farrell, R. P. 2012. Start, development and status network approach to estimate the impact of ground- of the regulator-led national groundwater resources water abstractions on river flows. Journal of Hydrol- modelling programme in England and Wales. In: ogy, 339, 15–28. Shepley, M. G., Whiteman, M. I., Hulme,P.J.& Prudic, D. E. 1989. Documentation of a Computer Pro- Grout, M. W. (eds) Groundwater Resources Model- gram to Simulate Stream–aquifer Relations using a ling: A Case Study from the UK. Geological Society, Modular Finite Difference Groundwater Flow Model. Special Publications, 364, 19–37. US Geological Survey Open File Report, 88–729. Young, A. R., Grew,R.&Holmes, M. G. R. 2003. Low Rushton, K. R. 2002. Will reductions in groundwater Flows 2000: a national water resources assessment and abstractions improve river flows? In: Hiscock, K. M., decision support tool. Water Science and Technology, Rivett,M.O.&Davison, R. M. (eds) Sustainable 48, 119–125. C1 © Amec Foster Wheeler Environment & Infrastructure UK Limited

Appendix C Test and Itchen groundwater model update

July 2015 Doc Ref: 29388tn568 Technical Note 1

Test and Itchen Groundwater Model Update

1. Introduction

1.1.1 This technical note summarises the procedure followed in updating the Test and Itchen Groundwater Model with new data for the period January 2006 to the end of March 2011. The original groundwater model covered the period January 1965 to December 31 2002. 1.1.2 The Test and Itchen groundwater model comprises a distributed runoff and recharge model combined with a separate groundwater flow model. The recharge model is developed in the Routing of Rainfall to Runoff and Recharge (4R) code (Environment Agency). The groundwater flow model is developed in MODFLOW-96 (MF), a finite difference groundwater flow code developed by the USGS. 1.1.3 An initial update to the model (MF Run 69) was undertaken in 2006 extending the model input and output data for the period January 2003 to December 2005 with some data, including rainfall and some groundwater abstractions extending until 31 August 2006. 1.1.4 The current update (MF Run 85) further extends the simulation period from January 2006 until 31 March 2011. 1.1.5 The model update was undertaken in a sequence of steps to ensure backward consistency with existing models during the simulation period where the input data are unchanged (1965-2005). This involved first updating the 4R recharge and runoff model and subsequently applying the new recharge and runoff distribution to the updated groundwater flow model. 1.1.6 As part of this process an intermediate recharge and runoff model simulation (4R Run 89) and groundwater flow simulation (MF Run 84) were conducted. The purpose of these runs were to ensure that the most recent updated model run (4R and MF Run 69) could be reproduced after the appropriate input files had been extended and new data appended. 1.1.7 The initial update to 4R (Run 89) was conducted with new historic surface water discharge and abstraction data (for 2006-2011) and an appended potential evapotranspiration file (for 2006-2011). All other input data were left unchanged. MF Run 84 included the new recharge and stream files generated by 4R Run 89 in addition to an updated well file (for 2006-2011). 1.1.8 Surface water flows from MF Run 69 were approximately reproduced by MF Run 84, however, a number of minor differences were identified:

• Minor rounding errors created small differences between the existing and updated surface water abstraction, discharge and groundwater abstraction input files of MF Run 69 and MF Run 84. The MF Run 69 export for surface water abstractions and discharges appear to have been created in a fixed width format such that decimals of

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flows were cut off at different intervals depending upon the magnitude of the flow. This resulted in minor differences (<0.05%) in flows between MF Run 69 and MF Run 84.

• The rainfall grids for 4R input between 2003 and 2005 had been further updated in October 2006 following production of the original grids for MF Run 69. This also led to minor differences in simulated flows. For the 2006-2011 model update, the most recent set of existing rainfall grids were used for the period 2003-2005. New rainfall grids were created for the period 2006-2011 from updated rain gauge data.

• Some abstraction wells within the MF Run 69 (e.g. Chilbolton, Thruxton, and Veolia Sources 1, 2, 3 and TG) had previously been allocated returns as per recent actual cycled profiles for the period 2003 - 2005. Actual returns data for that time period have since become available and are incorporated into the updated model. This created further minor differences in the well file and output between MF Run 69 and MF Run 84.

2. Model Data Update

2.1.1 This section describes the procedure followed in updating individual datasets, in addition to any assumptions made or corrections applied to existing data sets.

2.2 Rainfall 2.2.1 Daily rain gauge data used in the previous update (4R Run 69) covered the period 1970 until 2005 (47 no. gauges). Additional rain gauge data (44 No. gauges) were received from the Environment Agency which comprised an updated dataset covering the period 2006- March 2011 2.2.2 The received rainfall data were checked for accumulations, both those labelled explicitly in the data quality remarks, or which were suspected (e.g. large rainfall amounts following gaps in data or equally apportioned rainfall over two or more days). Apportioned data were not incorporated into the updated model. In particular, large amounts of apportioned data were removed from the Bishops Canning, Bishops Waltham, Chineham, Larkhill and New Milton gauges. 2.2.3 Anomalous high rainfall values (>100mm/day) were removed from the series at Andover WTW, Marlborough and Ogbourne gauges. 2.2.4 Additional data for the Frogham rain gauge were obtained from the Wessex Basin Groundwater flow model 4R input data. 2.2.5 Based upon the updated data, new daily rainfall grids were generated as input to 4R (4R Run 89) covering the period 01/09/2006 to 31/03/2011. 2.2.6 Dummy rain gauges, used to prevent application of a blanking value at model boundaries during contouring were maintained at the same locations as previously (Frogham, Butser,

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Technical Note 3

Broad Hinton and Camberly) where updated rainfall data were available. If such data were absent, for example at Butser for which no data are available after March 2009, data were taken from the nearest alternative gauge to the appropriate corner of the model boundary.

2.3 Potential Evapotranspiration 2.3.1 New MOSES data for potential evapotranspiration (PE) were supplied by the Environment Agency for the period Jan 1965 – Aug 2011. This data differed from previous MOSES data used in earlier model runs throughout the entire model period. It has been assumed that the most recently supplied data incorporates corrections made by the Met Office and thus supersedes the existing dataset. 2.3.2 A new PE file was created for the runoff-recharge simulation (pe90.prn) that incorporates the corrected data. This was included in the updated recharge simulation (4R Run 90) and the updated 1965-2011 groundwater model (MF Run 85).

2.4 Groundwater Abstractions 2.4.1 Public Water Supply Abstraction wells were updated with returns data to 2011. Where a single return volume was supplied for multiple wells for a given licence (e.g. Easton, Otterbourne), the abstraction was split based upon previously established relationships used in earlier model iterations. 2.4.2 A missing value for the abstraction at Easton in March 2005 was also replaced with new corrected returns data. 2.4.3 An individual return for the Portsmouth Water source at Lower Upham was not received. In the updated model run it has been assumed that the aggregate abstraction return for the Portsmouth Water source at Northbrook includes Lower Upham, the abstraction is therefore simulated entirely at the Northbrook well between 2006-2011. 2.4.4 All private abstraction wells for which returns were not available, including fish farms and water cress beds, were updated to 2011 based upon the monthly contemporary profile cycles previously adopted. 2.4.5 Abstractions for Wessex Water sources and the Cholderton and District Water source at Thruxton were updated from returns used as input to the Wessex Basin groundwater flow model. 2.4.6 Abstraction licences at Faberstown PWS, West Tytherly and Hoe PS have been revoked, or not used and abstractions between 2006 and 2011 were fixed at zero.

2.5 Surface Water Abstractions and Discharges 2.5.1 Updated data were received for abstractions at Testwood and Otterbourne covering the period 2005-2011. 2.5.2 Monthly returns for the surface water abstraction at Gaters Mill (2005-2011) were converted to a daily rate before being summed per stress period and applied.

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2.5.3 Reliability problems with flow gauging equipment at Conagar Bridge Gauging Station (GS) have meant that flow data were not available from November 2010. Additional flow data to February 2011 have been calculated based upon the previously established relationship between flow at Conagar Bridge GS and Broadlands GS. This relationship has also been used to infill other gaps in the Conagar Bridge data (e.g. in early June 2008). The Conagar Bridge gauging data are also an input as a surface water abstraction in the runoff-recharge and Groundwater flow model in order to model the Great Test- Little Test split. 2.5.4 Surface water abstractions at Broadlands and Nursling Fish Farms were extended using the same steady monthly rate applied from 1965-2005 in previous model update (4R Run 69).

2.6 Surface Water Flows 2.6.1 Updates to the flow gauging data used were based upon the original model dataset which covered the period 1965-2002. New data were supplied for 19 of the 21 flow gauging stations. Data were not received for the Kimbridge gauging station. Flows at Laverstock gauging station were obtained from gauging data compiled for the Wessex Basin groundwater flow model for the period 2006-2011. 2.6.2 The updated data set gives flow gauging coverage from 2006 – 2011. However, Conagar Bridge and Romsey only have frequent data until late 2010. Gauged flows at Kimbridge were previously estimated (1965-2002) based upon a linear relationship with flow at Broadlands GS (0.936 of flow), this relationship has also been continued to the extent of the available Broadlands data (February 2011).

2.7 Other Model Corrections and Assumptions 2.7.1 The 4R catchments used in the previous model update (4R Run 69) were found to contain some minor errors, including some cells within the active groundwater flow model boundary for which recharge was not calculated correctly. In addition, not all cells which routed run-off into the active groundwater flow model boundary from external areas were included in the 4R simulations. In the updated model (4RRun 90), these errors have been corrected and a new catchment arrangement has been incorporated. 2.7.2 The 4R geology file (geol25.csv) used for the previous update run has since been superseded by a newer version (geol65.csv) developed during the drought simulation runs. The updated 2006-2011 model incorporates this newer geology arrangement. 2.7.3 The MF basic package file (te85.bas) and output control file (te85.oc) were both extended to include new stress periods up to and including December 2016 (1248 total stress periods) should any future model update be required. However, for the current update only 1110 stress periods are simulated. 2.7.4 Veolia Water sources (1, 2, 3, and TG) were relabelled in the model input files to comply with new conditions of use for returns data between 2006 and 2011. 2.7.5 The surface water discharges of Water Cress beds and fish farms were not calculated correctly in the previous update run; daily flows were multiplied to full monthly figures

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rather than approximately half monthly stress periods. This error was corrected in the updated model (4R Run 90/MF Run 85).

3. Updated Model Results

3.1.1 A comparison of long term average observed flows against modelled flows for the updated model run is presented in Table 3.1. 3.1.2 Calibration plots, flow duration curves and time series plots are presented in Figures 1- 26. The model achieves a similar level of calibration to previous updates, except for 2007 and 2008. 3.1.3 The severity of the summer rainfall in 2007 which is seen in a response in observed flow but not modelled flow, has led to problems in simulating flows on the Wessex Basin Model. Previous investigations of the Wessex Basin Model on behalf of Wessex Water were unable to improve the recharge model for summer 2007 without impacting upon calibration for the remainder of the model time period. 3.1.4 In the Test and Itchen Model, the flow calibration is also underestimated in winter 2007 and summer 2008. A comparison of the modelled output with that of the Wessex Basin Model, which overlaps on the catchment of the Test, indicates the same problem does not occur. 3.1.5 We have conducted a preliminary investigation to identify the cause of the underestimated modeled flows. However, we were unable to identify any errors with the Surface Water Abstractions and Discharges or Groundwater Abstractions compared to those supplied. 3.1.6 A comparison of the 4R model input to the Test Catchment and the Hampshire Avon Catchment suggests that the rainfall input to the Test groundwater catchment (from the Test and Itchen Model) is approximately 10% less (normalised to area) than that to the Avon groundwater catchment (from the Wessex Basin Model) in 2007 and 2008 (Figure 27) 3.1.7 The difference in recharge may be exacerbated by difference in specific yield between the models. For the Test and Itchen Model, specific yield is ~2.4%, whilst for the Wessex Basin Model, specific yield is 1.5%. As a result, there is less water being recharged to a greater storage aquifer and therefore a more subdued aquifer response is modelled.

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Table 3.1 Modelled and Actual Flows at River Gauge Locations

Gauge Name Comparison Figures Gauged Average Model Average Modelled as % of Gauged Period (Ml/day) (Ml/day) (diff (%))

Bransbury Jan 1995-2011 1, 4, 7 89 98 110% Bourne Mar 1993-2011 1, 4, 6 77 82 107% Chilbolton Total Jan 1989-2011 1, 4, 8 489 474 97% Fullerton Total Jun 1975-2011 1, 4, 9 161 178 111% Bossington Mar 1993-2011 1, 4, 10 46 45 97% Laverstock 1970-2011 1, 17 66 54 82% Broadlands 1970-2011 2, 4, 11 917 1035 113% Test Back Carrier Jan 1986-2010 2, 15 12 8 72% Romsey Nov 1977-2010 2 27 11 42% Conager Bridge Jan 1982-2010 2, 14 217 216 100% Testwood May 1987-2011 2, 4, 13 667 757 114% Ower Sep 1976-2011 2, 16 81 73 89% Borough Bridge Oct 1970-2011 3, 5, 20 49 53 108% Sewards Bridge Jul 1970-2011 3, 5, 19 57 52 92% Drove Total Jun 1975-2011 3, 5, 18 143 157 110% Easton Mar 1984-2011 3, 5, 21 370 362 98% Allbrook & Highbridge 1970-2011 3, 5, 22 469 472 101% Riverside Park May 1982-2011 3, 5, 23 496 522 105% Dunbridge Oct 1974-2011 4, 12 75 80 107% Frogmill Sep 1972 - 2011 26 41 39 95% Caker at Alton Oct 1991-2011 5, 24 8 8 111% Wey at Kings Pond Nov 1991-2011 5, 25 9 5 54%

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23/03/2012

Figure 1 Model Run 85

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Figure 2 Model Run 85

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Figure 3 Model Run 85

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Figure 4 Flow Duration Curves for Gauges on the Test and Tributaries, and the River Bourne Model Run 85

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Figure 5 Flow Duration Curves for Gauges on the Itchen and Tributaries Model Run 85

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Figure 6 Model Run 85

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Figure 7 Model Run 85

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Figure 8 Model Run 85

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Figure 9 Model Run 85

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Figure 10 Model Run 85

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Figure 11 Model Run 85

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Figure 12 Model Run 85

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Figure 13 Model Run 85

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Figure 14 Model Run 85

Conager Bridge (Av Flow Jan 1982-2010= 217.3 Ml/d)

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Figure 15 Model Run 85

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Figure 16 Model Run 85

Ower (Av Flow Sep 1976-2011= 81.1 Ml/d)

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Figure 17 Model Run 85

Laverstock (Av Flow 1970-2011= 66.3 Ml/d) Modelled Flows - Laverstock (Av Flow 1970-2002= 54.2 Ml/d)

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Figure 18 Model Run 85

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Figure 19 Model Run 85

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Figure 20 Model Run 85

Borough Bridge (Av Flow Oct 1970-2011= 48.8 Ml/d)

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Figure 21 Model Run 85

Easton (Av Flow Mar 1984-2011= 370.5 Ml/d) Modelled Flows - Easton (Av Flow Mar 1984-2011= 362 Ml/d)

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Figure 22 Model Run 85

Allbrook & Highbridge (Av Flow 1970-2011= 468.9 Ml/d)

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Figure 23 Model Run 85

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Figure 24 Model Run 85

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Figure 25 Model Run 85

Wey at Kings Pond (Av Flow Nov 1991-2011= 9.5 Ml/d)

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Figure 26 Model Run 85

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Appendix D Test and Itchen groundwater model refinement

July 2015 Doc Ref: 29388tn568 Technical Note 1

Test and Itchen Groundwater Model Refinement

1. Introduction

AMEC has been commissioned to carry out improvements by the Environment Agency under the Groundwater Resource Modelling, Geological Investigations & Hydrogeology / Hydrology Services Framework Contract (22344). Prior to this refinement, the model has been updated with support from Southern Water so that it runs from 1965 to March 2011 (Technical Note 29388N074, October 2011). The rationale behind the refinements is presented and discussed in Technical Note (32522tn23i1) issued in September 2012. This is included on the DVD which accompanies this Technical Note. This technical note describes the changes to the model during refinement and briefly summarises the results of the model output. Figures in this note are taken from layers in the ‘ModelMap’ GIS and the spreadsheets, which are available electronically alongside this report.

2. Refinement Changes

2.1 Introduction This section summarises the changes to the groundwater model during the extension and refinement processes. The modelled area was extended to the north and east (Figure 2.1) to include the Upper Greensand Formation in the east and a significant portion of the confined Chalk, beneath the London Clay, in the north. Changes were therefore made to include aquifer parameter in the new extension area and existing parameters were updated and refined. The following sections will discuss in further detail the changes made to the model.

2.2 Summary of Runlog In total 27 runs were completed during the refinement process, 4 of which were dedicated to the (pseudo) replication the Candover Augmentation Pump Test of September 2011 (an exact representation was not possible as the model period end in March 2011). Table 2.1 describes the changes made during each run.

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Technical Note 2

The initial runs consisted of changes to the active modelled area and stream cells, which will be described in Sections 2.3 and 2.4. Further refinement runs involved changes to the transmissivity and storage of the aquifer, which are described in Section 2.5. Model flows and groundwater elevations were compared with gauged flow data and observed borehole data. Certain gauges and spot flow locations were particularly difficult to model well, such as Upper Clatford, Rookesbury, Whitchurch and the flow gauges at Alton. Ten refinement runs were used to refine aquifer and stream bed properties with the aim of improving flows at these locations. Existing Artificial Influences were updated using either confirmed WRGIS profiles or data provided by the Agency. Due to the extension of the modelled area, new surface water abstractions and discharges were included, discussed further in Section 2.7.

Table 2.1 Summary of MODFLOW Runs during Model Extension and Refinement

Run Number Changes Comments/ Outcomes

93 Extended Model Area Error trapping

Updates to Model Geology and T zones in East Model Extension 94 Error trapping Area

Updated (smoothed) Stream cells, extended IBOUND, updated BCF, First Working Refinement 96 new abstractions and discharges in extension area Run

Updated the Non-Artesian Cressbeds for 14 licences (d035 - from Updated Cressbed 97 EA) Abstractions

Updated 27 Cressbed Abstractions to be on a repeated 12 month Updated Cressbed 98 cycle from 1970 - 2016 Abstractions

Updated Streamfile (locations, layers, routing etc.) & Model Geology Streampack and Geology 100 (Reading Beds included in 'Chalk' not 'London Clay') in the northern Refinement extension area

100b Extended gauge output from 51 to 53 locations

To improve flows at Refinement of T zones (u/s of Whitchurch, Rookesbury/Upper 101 Rookesbury & Upper Clatford, West of Alton) & Model Geology Clatford, and Alton gauges

Refinement of T zones (northern anticline, Rookesbury/Upper To improve flows at 103 Clatford, west of Alton) & S zones (Alluvium near Alton, Middle Chalk Rookesbury & Upper near Alton - similar to Mole model) Clatford, and Alton gauges

Refinement of S zones (increased the Sy in West 'Middle Chalk'), and To improve flows at 105 T zones (Rookesbury/Upper Clatford, Whitchurch and near Alton), Rookesbury & Upper Turned off Abstraction at Bishops Green PWS as failed to converge Clatford, and Alton gauges

106 Alre & Candover Augmentation OFF A & C

Alre & Candover Augmentation same as September 2011 Test 107 A & C period

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Technical Note 3

Table 2.1 (continued) Summary of MODFLOW Runs during Model Extension and Refinement

Run Number Changes Comments/ Outcomes

109 Refinement of T zones (Trimmed the high T zone under the Bourne, To improve flows at Trimmed the low T antlicline, Added the 'Stockbridge Fissure Zone' in Rookesbury & Upper west, removed the very high T zone west of Alton),and S zones & Clatford, and Alton gauges values (Extended 'Middle chalk' zone west and increased the Sy from 0.017 to 0.024),

Refined the streanfile (added new stream cells, decreased the stream bed thickness for a few cells near Rookesbury/Upper Clatford)

110 Refinement of T zones & values (Extended low T anticline west , To improve flows at Extended Stockbridge fissure zone to Bourne, decreased T of zone Rookesbury & Upper 5, and FACX of zone 13 - more like Mole west of Alton) Clatford, and Alton gauges

111 Refinement of T zones & values (Bourne T zones returned to Run To Improve flows on Bourne 105, Added High T at valley near Alton, Extended low T Anticline and Alton west, T zones 11 & 13 Increased T - more like Mole)

112 Updated abstractions (turned PWS Bishops Green back on) Run to check if there are dry cells near Bishops Green

113 Updated abstractions (Boxalls PWS modelled as a half fraction) Boxalls PWS close to boundary

114 Refinement of T zones & values (Anticline low T 'wider', changed T Increased Stream Bed zones near Alton, decreased T zone 11). Bottom on Upper Test and Wey, Tongham PWS also Changes to abstractions (Tongham PWS modelled as half-fraction). close to boundary

Changes to streamfile (decreased stream bed thickness on Upper Test and Wey - now 10 cm, used to be 1 m)

115 Refinement of T zones & values (Decreased T zone 11, changed T Final Changes to improve zones near Alton, moved anticline south) Alton gauges

116 Alre & Candover Augmentation OFF A & C

117 Alre & Candover Augmentation same as September 2011 Test A & C period

118 Refinement of T zones along the Candover (Now T zone 1 - higher T), decreased the T in the confined northern extension area (now 200, was 360 in Run115).

119 Refinement of T zones and values: Zone 7 and 11. Thinned and Dry cell at Bishops Green increased the T along the northern anticline axis, decreased the T in GWAB the confined northern area further, increased the T of the lower T chalk (zone 7).

120 Refinement of T zone near anticline (N of Medstead Mound and S of Final aquifer parameter run Basingstoke).

Updated wells near model boundary (now Round Copse, Shalford Farm, Hyde End, Bishops Green modelled as half abstraction, Woolhampton off as outside boundary).

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Technical Note 4

Table 2.1 (continued) Summary of MODFLOW Runs during Model Extension and Refinement

Run Number Changes Comments/ Outcomes

121 Final updates to artificial influences.

122 FL Run Dry cell at Overton Mill

No dry cells. Final updates FL Run – Refined Overton Mill Abstraction (now split across two 123 for AI left to do (scenario cells) info)

124 HISTORIC

125 NATURAL

126 RA

127 FL

128 AUGMENTATION ON (as per Agency defined rules)

2.3 Changes to the Basic ‘BAS’ file The active model area was increased northwards, to include confined Chalk beneath the London Clay, and eastwards to include the Upper Greensand formation. The modelled area increased from 2707 km2 to 3191 km2. These changes were made in the first refinement run and improved in the second and third runs. Figure 2.1 displays the changes in active modelled area.

Figure 2.1 Active Model Area and Stream Cells Pre- and Post- Refinement

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Technical Note 5

2.4 Changes to the Stream ‘STR’ File The stream network was increased to include streams and rivers in the new extension area (Figure 2.1). Northward and eastward flowing stream networks were refined; stream cells were added, moved or removed using Kennet and Mole stream cells as a guide. No changes were made to stream cells on the main Test, Itchen and Bourne rivers and their tributaries. The surface water routing network was refined, with corrections made in the new-extension area and to streams flowing north and east off the active model area. No changes were made to the routing in the bulk of the original model area. Stream bed conductance was assigned to new stream cells and corrected for deleted stream cells. The conductance was increased to 10 000 m2/d for a stream cell on the northern tributary upstream of Rookesbury spot flow location, to account for spring-fed ponds. Stream bed conductance for streams and rivers on the London Clay were set to zero, but elsewhere in the extension area the conductance was as high as 5000 m2/d. Stream bed thicknesses have been changed throughout the model refinement. Where the Chalk is confined by the London Clay the stream bed thickness has been set to ‘zero’ to ensure no flow loss from the streams. The stream bed thickness has also been reduced on the upstream sections of the River Test, Pillhill Brook and the Anton. Originally the stream bed thickness was 1 m but this was reduced to 0.1 m in refinement Run 114. Stream bed thicknesses were also reduced to 0.1 m on the River Wey and its tributaries during this run. The stream cell elevations were also ‘smoothed’ using fixed known elevations at locations along the river network. The smoothing reduces the fall in elevation between cells along a distance between two fixed points so the drop in stage is more consistent from one cell to another and ensures there are not any unintentional upward steps in elevations in a downstream direction.

2.5 Changes to the Block-Centred Flow ‘BCF’ File There were four storage zones pre-refinement, this has been increased to six to account for the extension area. The specific yield for the bulk of the model has not changed, with the exception of the area near Alton (Figure 2.2). The transmissivity across the model has been a major source of refinement, with many runs reshaping the zones to account for anticlines, synclines and fissure zones and changing the transmissivity values to account for this (Figure 2.3). The Mole Groundwater model was used as a guide for values in the eastern extension area, and the Wessex Groundwater model was used for refinement near the Bourne. Figure 2.4 displays the transmissivity (at starting heads) before and after the model refinement. Table 2.2 contains the hydraulic parameters for each transmissivity zone at the end of model refinement.

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Technical Note 6

Table 2.2 Transmissivity Zone Hydraulic Parameter Properties

Depth from Maximum Approx Equivalent min WL to KBASE FACX Hydraulic Minimum KMAX Zone Inflection Conductivity Transmissivity Number Zone Name Point m2/d (assuming (m) Unless Single Layer Lower than m/d m/d Saturated Active m/d Base of Cell depth = 80 m)

1 Very High T chalk (river enhanced) 50 5.0 1.5 80.0 3775 16

2 Higher T chalk (dry valleys/tributaries/synclines) 35 5.0 1.5 80.0 2650 16

3 High T chalk (WT above Stockbridge Rock fissure zone) 24 4.0 1.3 75.0 1520 19

4 Mod/High T chalk (WT below Stockbridge Rock fissure zone) 15 2.0 2.3 70.0 666 35

5 Mod/Low T chalk 12 1.0 1.5 40.0 188 40

6 Low T chalk (WT close to chalk base) 10 1.0 1.5 40.0 155 40

7 Lower T chalk (anticlinal axes) 5 1.0 1.0 25.0 93 25

8 Lowest T chalk (undeveloped/confined) 5 0.5 1.0 0.5 40 1

9 Upper Greensand 60 6.0 1.0 6.0 480 1

10 Extremely High T Chalk (synclinal axes) 55 10.0 3.0 100.0 5615 10

11 Extended London Clay Area to north 5 1.3 1.0 1.3 100 1

12 Extended Upper Greensand Area to east 60 6.0 1.0 6.0 480 1

13 Bromshall Lane STW 10 2.5 1.0 2.5 200 1

14 V low T 5 0.5 1.0 0.5 40 1

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Technical Note 7

Figure 2.2 Specific Yield Values Pre- and Post- Refinement

Figure 2.3 Transmissivity Zones Pre- and Post- Refinement

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Technical Note 8

Figure 2.4 Starting Transmissivity Pre- and Post- Refinement

2.6 Changes to Abstractions and Discharges Artificial Influences were updated with new information from Water Resources GIS June 2012 and information supplied by the Agency. The Agency checked and approved the final changes to the collated artificial influences which were then used in the model. Five new groundwater abstractions, one new surface water abstraction and five new surface water discharges were included in the model (Table 2.3).

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Technical Note 9

Table 2.3 New Modelled Abstractions and Discharges

Artificial Influence Label Licence Number Use Code Use Holder

GWAB Bishops Green 28/39/22/0033 W-PWS-330 PWS THAMES WATER UTILITIES LTD.

GWAB Hyde End 28/39/22/0394 ENRE450 Augmentation ENVIRONMENT AGENCY

GWAB Round Copse 28/39/22/0394 ENRE450 Augmentation ENVIRONMENT AGENCY

GWAB Shalford Farm 28/39/22/0394 ENRE450 Augmentation ENVIRONMENT AGENCY

GWAB Woolhampton 28/39/22/0394 ENRE450 Augmentation ENVIRONMENT AGENCY

SWDIS Kingsclere CSSC.2340 - STW THAMES WATER UTILITIES LTD.

SWDIS Sherborne St John CTCR.1340 - STW THAMES WATER UTILITIES LTD.

SWDIS Basingstoke CTCR.0875 - STW THAMES WATER UTILITIES LTD.

SWDIS Crondall CSSC.2450 - STW THAMES WATER UTILITIES LTD.

SWDIS Alton CTCR.1268 - STW THAMES WATER UTILITIES LTD.

Shadwell Springs (Springs at Shadwell Copse Hawkley SWAB modelled as a SWAB) 10/41/436202 W-PWS-160 PWS SOUTH EAST WATER PLC.

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Technical Note 10

2.7 Alre and Candover Pump Test 2011 Atkins, on behalf of Southern Water, conducted a pump test of the Alre and Candover boreholes during September, October and November 20111. The abstractions were modelled for a similar climatic period during June to August 1976 (as the model period does not extend to the exact test period). The modelled drawdown was compared to the observed drawdown recorded by Atkins. These results were used to refine the model in the area around the Candover. The flows along the Candover also received a large portion of refinement effort to improve the modelled flows with observed spot flows. This is discussed further in Section 3.3.

3. Results

3.1 Flows Table 3.1 displays a summary of modelled flow outputs of the pre-model refinement run (Run 90) and the post-model refinement historic run (Run 124). For the majority of the gauges the flows between the two runs have not changed significantly in terms of average flow. The gauges of Drove Lane, Caker at Alton and Allbrook & Highbridge have improved as a result of the refinement. Gauges in the extended model area are well modelled; the exception is ‘Alton upstream of High Street’, which underestimates the observed flows. Spot flow locations of Upper Clatford and Whitchurch overestimate and underestimate the observed flows respectively. These three locations have formed a significant part of the refinement runs but have proved difficult to accurately represent simultaneously within the range of acceptable parameter values. Figure 3.1 displays the average error and the Q10 error of modelled flows compared to gauged flow from the period January 1970 to March 2011.

1 ATKINS Document Number 5099146/70/DG/119 ‘Itchen Implementation NEP Scheme’

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Technical Note 11

Table 3.1 Modelled and Observed Flow Comparison Information

Flow Gauge Easting Northing Gauged Run 124 Run 90 Run 124 Run 124 Run 124 % Comparison Period Gauged Run 124 Gauged Run 124 Run 124 Q10 Qualitative Average Model Model minus Average of Gauged Q10 Modelled Average Average % of Error Comment on (Ml/day) Average Average Gauge Error (difference Q10 for flows for flows Gauged Refinement (Ml/d) (Ml/d) (difference (%)) less less flows (Ml/day)) than Q10 than Q10 less than Q10

Bransbury 442000 142000 89.3 103.5 98.9 14.2 0.16 116% Jan 1995 - Mar 2011 156 205 76 85 112% 0.122 Improved

Conager Bridge 435500 115750 217.3 216.7 216.7 -0.5 0.00 100% Jan 1982 - Nov 2010 262 259 210 210 100% 0.000 Negligible Change

Sewards Bridge 457250 132000 56.9 54.0 52.5 -2.8 -0.05 95% Jul 1970 - Mar 2011 91 96 49 46 95% -0.053 Improved

Borough Bridge 456750 132250 48.8 55.4 52.9 6.6 0.14 114% Oct 1970 - Mar 2011 76 101 43 46 108% 0.080 Improved

Easton 451000 132250 370.5 360.3 361.8 -10.1 -0.03 97% Mar 1984 - Mar 2011 524 542 339 325 96% -0.040 Improved

Bourne 444000 146250 76.7 96.6 83.6 19.9 0.26 126% Mar 1993 - Mar 2011 162 195 58 78 133% 0.334 Deteriorated

Chilbolton Total 438500 139250 489.2 464.2 476.9 -25.0 -0.05 95% Jan 1989 - Mar 2011 774 766 436 404 93% -0.073 Negligible Change

Fullerton Total 437750 139000 161.3 169.1 178.8 7.8 0.05 105% Jun 1975 - Mar 2011 239 286 146 147 101% 0.009 Negligible Change

Bossington 433250 131250 46.0 45.4 44.0 -0.6 -0.01 99% Mar 1993 - Mar 2011 96 100 35 35 102% 0.017 Negligible Change

Testwood 435250 115250 666.6 700.4 757.9 33.8 0.05 105% May 1987 - Mar 2011 1325 1373 550 569 103% 0.034 Negligible Change

Ower 432750 117250 81.1 72.8 72.7 -8.3 -0.10 90% Sep 1976 - Jan 2011 203 173 58 54 92% -0.076 Negligible Change

Drove Total 457250 132500 142.6 138.6 157.2 -4.0 -0.03 97% Jun 1975 - Mar 2011 190 190 133 129 97% -0.031 Improved

Allbrook & Highbridge 446000 121000 471.6 475.0 476.4 3.4 0.01 101% Jan 1970 - Mar 2011 695 754 430 425 99% -0.013 Negligible Change

Broadlands 435250 118750 963.4 1023.1 1056.9 59.7 0.06 106% Jan 1970 - Feb 2011 1476 1722 860 899 105% 0.045 Negligible Change

Laverstock 415500 130250 67.2 63.3 56.9 -3.8 -0.06 94% Jan 1970 - Mar 2011 126 129 51 50 99% -0.010 Negligible Change

Kimbridge 435000 123250 858.5 1039.3 1048.5 180.8 0.21 121% Jan 1970 - Feb 2011 1285 1691 773 921 119% 0.192 Negligible Change

Test Back Carrier 436250 115750 11.8 8.6 8.6 -3.2 -0.27 73% Jan 1986 - Feb 2011 23 23 5 6 111% 0.113 Negligible Change

Romsey 436000 121000 27.3 11.6 11.6 -15.7 -0.58 42% Nov 1977 - Sep 2010 64 28 20 9 44% -0.559 Negligible Change

Riverside Park 444250 115250 496.2 522.8 520.5 26.6 0.05 105% May 1982 - Mar 2011 777 842 438 461 105% 0.051 Negligible Change

Caker at Alton 472750 138250 7.6 6.2 8.6 -1.4 -0.19 81% Oct 1991 - Mar 2011 22 13 4 3 75% -0.254 Deteriorated

Wey at Kings Pond 472250 139500 9.5 7.6 4.9 -1.9 -0.20 80% Nov 1991 - Mar 2011 23 19 7 5 70% -0.299 Improved

Frog Mill 452000 114750 41.2 39.6 38.7 -1.6 -0.04 96% Sep 1972 - Mar 2011 78 77 33 33 98% -0.024 Negligible Change

Sheepbridge * 471900 165300 189.4 150.7 N/A -38.7 -0.20 80% Jan 1970 - Dec 2007 338 301 160 122 76% -0.241 -

Lodge Farm * 473300 152100 35.1 36.0 35.7 0.8 0.02 102% Feb 1975 - Dec 2007 56 62 31 31 99% -0.005 Negligible Change

Holdshot Farm * 473900 160100 68.0 64.4 N/A -3.6 -0.05 95% May 2006 - Dec 2007 143 144 57 49 86% -0.138 -

Bramshill * 475500 159100 67.1 61.6 N/A -5.5 -0.08 92% Oct 1972 - Dec 2007 134 129 54 49 91% -0.092 -

Alton U/S High Street * 471700 139500 6.8 2.1 N/A -4.7 -0.70 30% Nov 1992 - Dec 2007 18 6 4 0 6% -0.937 -

Farnham * 483700 146300 66.4 76.4 N/A 9.9 0.15 115% Apr 1989 - Dec 2007 134 145 52 62 120% 0.196 -

Brimpton * 456750 164750 113.2 110.7 82.2 -2.5 -0.02 98% Jan 1970 - Dec 2010 266 275 83 80 96% -0.039 Improved

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Technical Note 12

Table 3.1 (continued) Modelled and Observed Flow Comparison Information

Flow Gauge Easting Northing Gauged Run 124 Run 90 Run 124 Run 124 Run 124 % Comparison Period Gauged Run 124 Gauged Run 124 Run 124 Q10 Qualitative Average Model Model minus Average of Gauged Q10 Modelled Average Average % of Error Comment on (Ml/day) Average Average Gauge Error (difference Q10 for flows for flows Gauged Refinement (Ml/d) (Ml/d) (difference (%)) less less flows (Ml/day)) than Q10 than Q10 less than Q10

Dunbridge ˟˟ 431750 126250 74.8 76.9 76.2 2.1 0.03 103% Oct 1974 - Mar 2011 136 152 64 64 101% 0.005 Negligible Change

Polhampton ˟˟ 452400 150500 21.1 18.5 9.9 -2.6 -0.12 88% Sep 2006 - Mar 2010 41 39 18 15 84% -0.158 Improved

Rookesbury ˟˟ 435640 144740 58.6 64.7 31.3 6.1 0.10 110% Nov 2006 - Mar 2011 88 106 54 58 106% 0.062 Improved

Upper Clatford ˟˟ 435200 144000 55.5 83.2 90.6 27.8 0.50 150% Sep 2006 - Mar 2011 92 129 49 76 154% 0.542 Improved

Whitchurch ˟˟ 446360 147860 123.9 82.1 86.6 -41.8 -0.34 66% Sep 2006 - Mar 2011 176 138 115 75 65% -0.351 Negligible Change (* Flow gauge not previously modelled, ˟˟ Spot Flows, N/A denotes not in active modelled area of Run 90)

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Technical Note 13

Figure 3.1 Modelled Flow Errors at Gauges for Historical Run 124 for the Period January 1970 – March 2011

3.2 Heads Groundwater elevations were compared with observed elevations at 61 locations across the model area. The majority of modelled elevations in the bulk of the model have not changed during the refinement when compared with the pre-refinement Run 90 and the observed data. Table 3.2 summarises the modelled elevations compared with the observed, pre- and post- refinement. Any changes to modelled and observed ‘fit’ have been small and most locations show an improvement in their comparison with averages. Figure 3.2 displays the errors of the modelled and observed groundwater elevations. The Amplitude Error was calculated from the maximum minus average (‘amplitude’) values for the modelled post-refinement Historic run (Run 124) minus the observed values. It can be seen that most borehole ranges are represented well, as only three boreholes have an error greater than 5m. The averages calculated for the model are based on 2 values per month for the comparison period. The observed data is simply an arithmetic mean of all observations and are as such potentially skewed by irregular data frequencies. Average comparisons are therefore indicative only.

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Technical Note 14

Table 3.2 Modelled and Observed Groundwater Elevation Comparison Information

Average Average Average Pre- Post- Observed Refinement Qualitative Comment on Refinement Name Easting Northing Groundwater Groundwater Period of Record Final Calibration and Groundwater

Elevation Elevation Refinement Change

Elevation Refinement (mAOD) (Run90) - (Run124) Average Observed minus Modelled (m) Post - Average Observed minus Modelled (m) Pre

Robley Cottage 451350 154950 100 103 103 3.1 3.8 Jan 1970 - May 2003 Satisfactory - Similar

Bailiffs Cottage 456390 154780 97 100 99 3.4 2.1 Jan 1970 - Mar 2011 Good - Improved

Woodside, Vernham 433120 156450 106 118 120 12.4 14.1 Jan 1970 - Mar 2011 Dean Satisfactory - Similar

Forton, Longparish 441800 143580 53 51 51 -2.1 -2.1 Jan 1970 - Mar 2011 Satisfactory - Similar

Upper Cranbourne 448710 142100 67 67 67 -0.1 -0.2 Jan 1970 - Jun 2003 Farm Good - Similar

OB 13, Foxhill Farm 456950 139900 85 85 83 -0.3 -1.9 Dec 1973 - Mar 2011 Satisfactory - Deteriorated

Folly Farm 441460 133190 89 52 52 -37.1 -37.0 Jan 1970 - Mar 2011 Satisfactory - Similar

Kings Somborne 435750 130750 48 31 31 -16.7 -17.0 Nov 1986 - Mar 2011 Good - Improved

Crawley 443540 135270 31 61 59 29.3 27.3 Nov 1986 - Mar 2011 Good - Improved

Lopcombe Corner 425000 135440 55 71 71 16.0 15.7 May 1994 - Mar 2011 Satisfactory - Similar

Pitton Lodge, Pitton 420720 130750 64 54 54 -9.2 -9.4 Jan 1970 - Mar 2011 Satisfactory - Similar

Anmery Cottages 441580 126090 62 39 39 -22.9 -22.0 Jan 1970 - Mar 2011 Satisfactory - Similar

British Pipeline 453880 136370 47 70 69 22.7 22.0 Nov 1986 - Mar 2011 Agency Satisfactory - Improved

Hursley Crossroads 442830 124840 38 35 35 -3.0 -2.9 Feb 1987 - Apr 2003 Good - Similar

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Technical Note 15

Table 3.2 (continued) Modelled and Observed Groundwater Elevation Comparison Information

Average Average Average Pre- Post- Observed Refinement Qualitative Comment on Refinement Name Easting Northing Groundwater Groundwater Period of Record Final Calibration and Groundwater

Elevation Elevation Refinement Change

Elevation Refinement (mAOD) (Run90) - (Run124) Average Observed minus Modelled (m) Post - Average Observed minus Modelled (m) Pre

Clanville Lodge Gate 432220 149020 83 79 79 -3.3 -3.7 Nov 1986 - Jun 2003 Good - Similar

Leonards Grave 429680 134880 48 46 46 -1.5 -1.2 Jul 1996 - Mar 2011 Satisfactory - Similar

Windrush Cottages 424600 127110 39 42 41 2.7 2.1 Jun 1993 - Apr 2010 Satisfactory - Improved

Walnut Cottage 456390 154780 95 97 96 2.0 1.2 Mar 1979 - Mar 2011 Good - Improved

Rotherfield 469500 131400 91 104 103 13.1 11.3 Apr 1987 - Apr 2010 Poor- Deteriorated

West Meon 465060 126550 82 92 91 9.5 8.9 Sep 1986 - Mar 2011 Satisfactory - Similar

OB 12, Lanham Lane 459230 135370 76 77 76 1.0 -0.1 Jan 1974 - Mar 2011 Good - Improved

OB 4, Preston 460720 141930 94 97 93 2.5 -0.9 Jan 1975 - Mar 2011 Candover Satisfactory - Improved

OB 6, Powells Farm 465890 141090 100 105 100 5.6 0.7 Jan 1974 - May 2003 Satisfactory - Improved

OB 15, Itchen House 453950 132980 48 47 47 -0.5 -0.7 Mar 1977 - Mar 2011 Satisfactory - Deteriorated

Twyford Moors 447950 123610 22 22 22 -0.1 -0.1 Dec 1988 - Mar 2011 Satisfactory - Similar

Crabwood 443440 129210 91 66 58 -24.3 -32.0 Jul 1986 - Mar 2011 Poor- Deteriorated

Longwood Warren 453570 125920 73 69 63 -4.1 -9.4 Apr 1980 - Oct 2008 House Satisfactory - Deteriorated

Snoddington Down 425360 145080 89 85 83 -4.2 -6.4 Jan 1970 - Mar 2011 Farm Satisfactory - Similar

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Technical Note 16

Table 3.2 (continued) Modelled and Observed Groundwater Elevation Comparison Information

Average Average Average Pre- Post- Observed Refinement Qualitative Comment on Refinement Name Easting Northing Groundwater Groundwater Period of Record Final Calibration and Groundwater

Elevation Elevation Refinement Change

Elevation Refinement (mAOD) (Run90) - (Run124) Average Observed minus Modelled (m) Post - Average Observed minus Modelled (m) Pre

Longhedge Farm 414500 134100 53 53 53 0.7 0.6 Feb 1992 - May 2003 Cottage Satisfactory - Similar

Queen Manor Farm 418300 138700 33 68 69 35.4 36.0 Jan 1992 - Mar 2011 (FW) Satisfactory - Similar

Drewetts Barn 416200 151800 93 94 93 1.6 0.8 Feb 1992 - May 2003 Satisfactory - Similar

BEECH BARNS 468700 138520 105 108 104 2.2 -1.5 Jan 1970 - Oct 2009 Satisfactory - Improved

BENTWORTH 469010 140710 101 107 104 5.8 3.1 Apr 1987 - Oct 2010 LODGE Good - Improved

BOSSINGTON 433350 131050 30 29 29 -0.1 -0.1 Jan 1970 - Mar 1995 HOUSE Satisfactory - Similar

CHAWTON HOUSE 470900 137010 106 107 105 1.1 -1.3 Jan 1970 - Feb 2011 Good - Improved

COMBE MANOR 436850 160850 147 123 131 -23.5 -16.0 Jan 1970 - Nov 2003 Poor - Improved

CRUCK COTTAGE, 471160 135330 109 107 105 -1.9 -4.2 Jan 1970 - Oct 2009 FARRIN Poor- Deteriorated *

CURZON ST FARM 439950 158050 112 114 120 1.7 8.1 Jan 1970 - Mar 1995 FACCO Poor- Deteriorated

EAST TISTED 470580 132920 96 106 104 10.3 8.2 Jan 1970 - Oct 2009 STATION Satisfactory - Improved

FARRINGDON 470480 134910 102 107 104 4.9 2.5 Jan 1970 - Mar 2011 STATION Good - Improved

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Technical Note 17

Table 3.2 (continued) Modelled and Observed Groundwater Elevation Comparison Information

Average Average Average Pre- Post- Observed Refinement Qualitative Comment on Refinement Name Easting Northing Groundwater Groundwater Period of Record Final Calibration and Groundwater

Elevation Elevation Refinement Change

Elevation Refinement (mAOD) (Run90) - (Run124) Average Observed minus Modelled (m) Post - Average Observed minus Modelled (m) Pre

Satisfactory - Not Previously FIELD BARN FARM 451940 157310 102 999 103 897.0 0.8 Feb 1972 - Mar 2011 Modelled

GRANGE FARM 434250 161250 148 129 135 -18.3 -12.0 Jan 1970 - Nov 1997 BUTTERME Limited Data

GREEN FARM, 470460 125600 134 106 105 -27.6 -28.0 Jan 1970 - May 2000 FROXFIELD Poor - Improved

GROCERY SHOP, 437150 160900 146 123 130 -22.8 -15.0 Jan 1970 - Oct 1976 COMBE Limited Data

HACKWOOD FARM 467000 149800 88 85 84 -3.0 -4.0 Jan 1970 - Mar 2011 Poor - Similar

Hannington Farm 453880 155330 110 105 104 -5.5 -6.4 Feb 1995 - Apr 1997 Limited Data

HEBBERDENS, 469150 124750 88 100 98 11.6 10.5 Jan 1970 - Mar 2000 LOWER BOR Satisfactory - Improved

HIGHCLERE STUD 444950 157250 134 121 122 -13.8 -12.0 Jan 1970 - Mar 2011 FARM Satisfactory - Similar

JANE AUSTEN'S 470820 137590 105 107 104 2.0 -0.7 Jan 1970 - Mar 2011 HOUSE Good - Improved

LONGWOOD 453510 126000 72 69 63 -3.1 -8.8 Apr 2008 - Oct 2008 WARREN HSE Limited Data

PITHALL UPPER 457900 155410 96 98 96 2.0 -0.2 Jan 1970 - Nov 2003 WOOTTO Limited Data - Improved

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Technical Note 18

Table 3.2 (continued) Modelled and Observed Groundwater Elevation Comparison Information

Average Average Average Pre- Post- minus

Observed Refinement Qualitative Comment on Refinement Name Easting Northing Groundwater Groundwater Period of Record Final Calibration and Groundwater

Elevation Elevation Refinement Change

Elevation Refinement (mAOD) (Run90) - (Run124) Modelled (m) Post - Average Observed minus Modelled (m) Average Observed Pre

PITTHALL FARM 456830 156130 98 101 99 2.6 0.6 Jan 1970 - Apr 1900 BAUGHU Satisfactory - Improved

POLHAMPTON 451350 154950 99 103 103 3.5 4.2 Jan 1970 - Mar 2011 LODGE COT Satisfactory - Similar *

RAGMORE 468230 150030 84 83 82 -0.7 -1.7 Jan 1970 - Mar 2011 COTTAGES Satisfactory - Improved *

SELBORNE 472810 132000 107 107 105 0.6 -1.9 Apr 1987 - Mar 2011 Poor - Similar

SHALDEN MANOR 469500 141900 126 112 112 -13.7 -14.0 Jan 1970 - Mar 2011 Satisfactory - Improved

TILE BARN FARM 470971 147802 88 93 91 4.8 2.5 Oct 1971 - Mar 2011 Good - Improved

Satisfactory - Not Previously WERGS FARM 447770 158380 111 999 110 888.2 -0.5 Jan 1970 - Jun 1995 Modelled *

WINDRUSH COTTS. 424600 127100 39 41 41 2.6 2.1 Jan 2003 - Apr 2010 DEAN Poor - Similar

WOODGARSTON 458450 155050 93 97 94 3.3 1.0 Jan 1970 - Mar 2011 FARM Good - Improved

WRIGHTS FARM 436850 161450 153 124 132 -29.5 -21.0 Jan 1970 - Mar 1994 COMBE Poor - Similar

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Technical Note 19

Figure 3.2 Comparison between Modelled Historical Run 124 and Observed Groundwater Elevations

3.3 Alre and Candover Results As a result of the Alre and Candover pump test (Section 2.7) changes were made to both the zones used to characterise the hydraulic conductivity and the transmissivity of the zones. The groundwater elevations in observation boreholes near the Alre and Candover Augmentation scheme have improved (see Table 3.2) due to the refinement of modelling the pump test. Stream flow down the Candover was also an important step in the refinement. Additional spot- flow data at five locations along the Candover was supplied by the Agency to aid refinement of modelled flows. The modelled output along the Candover has significantly improved (as has the Winterbourne Signature in general), with modelled flows now predicting similar flows to the observed spot-flows. Figure 3.3 displays the modelled flows and groundwater elevations for Run 115 (during refinement) and Run 124 (Historic run) with the observed spot flow and the modelled stream elevation at Totford Road Bridge (SU5700437981). The modelled flows in the upper reaches have decreased as a result of the refinement, and as a result are more representative of the measured spot flows at along the Candover.

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Technical Note 20

Figure 3.3 Modelled Flow and Groundwater Elevations at Totford Road Bridge (SU5700437981 on the Candover) for Run 115 (during Refinement) and Historical Run 124

3.4 Flow Duration Curves Flow duration curves were plotted using modelled stress-period flow output and stress-period observed data from the start of 1970 to the end of March 2011. Two gauges, Borough Bridge and Allbrook & Highbridge, are represented in Figure 3.4. Flow duration curves were also produced for Sewards Bridge, Easton and Drove Lane; all are available electronically on the accompanying DVD.

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Technical Note 21

Figure 3.4 Flow Duration Curves for Allbrook & Highbridge (Left) and Borough Bridge Gauging Stations (Right)

3.5 Accretion Profiles Accretion profiles for the River Test and the River Itchen (including the Alre) are displayed in Figures 3.5 and 3.6 for the refined historical run (Run 124).

Figure 3.5 River Test Gauged Flow Accretion at Various Dates and Modelled Flow Profiles for the End of October 1993 (SP 602), End of November 1997 (SP 970) and End of December 2000 (SP 864)

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Technical Note 22

Figure 3.6 River Itchen-Alre Gauged Flow Accretion at Various Dates and Modelled Flow Profiles for the End of October 1993 (SP 602), End of November 1997 (SP 970) and End of December 2000 (SP 864)

3.6 Flow Maps and Modelled Drawdown Model Flow Maps was run using the Historic (Run 124), Recent Actual (Run 126), Fully Licensed (Run 127) and Natural (Run 125) outputs. Figure 3.7 displays the impact between fully licensed minus recent actual and recent actual minus natural flows. Figure 3.8 displays the modelled Recent Actual (Run 126) flow compliance. Figure 3.9 displays the drawdown of the groundwater table for Fully Licensed minus Recent Actual and Recent Actual minus Natural scenarios.

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Technical Note 23

Figure 3.7 Modelled Flow Impacts at Q95 for Fully Licensed Run 127 Minus Recent Actual Run 126 (left) and Recent Actual Run 126 Minus Natural Run 125 (all runs Augmentation is Off)

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Technical Note 24

Figure 3.8 Modelled flow compliance for Recent Actual Run 126

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Technical Note 25

Figure 3.9 Modelled Groundwater Drawdown for End of October 1993 (Stress Period 692)

3.7 Groundwater Body Recharge Estimates Annual average recharge estimates to the groundwater model area were estimated from the 4R Recharge Model for the period starting January 1970 to the end of December 2010. Figure 3.10 displays the 4R Catchments and model geology in relation to the Water Resources GIS Groundwater Bodies for the Test and Itchen. It should be noted that these are similar but not coincident. Infiltration recharge (‘spg’), bypass recharge (‘byp’), runoff recharge (‘ror’), interflow (‘int’) and rapid runoff (‘rap’) were calculated from the recharge model. Total recharge can then be estimated by calculating the sum of infiltration recharge, bypass recharge and runoff recharge. Total available water was calculated as the total of bypass recharge, infiltration recharge, interflow and rapid runoff. These values are available as grids in the ModelMap should 1970- 2010 averages for different spatial areas be required by the user. Table 3.3 displays the calculated annual recharge for each year to the 4R catchments for the Test and Itchen. The recharge volumes have been factored to correct for areas of impermeable geology on which recharge is not generated (e.g. the River Dun catchment) such the resultant recharge (in mm/a) is representative for the outcrop aquifer. Should the user require annual recharge totals for different catchments, then 4R would need to be re-run with the different catchments defined in the relevant 4R input file. Recharge is higher in the Itchen Catchment than the Test Catchment as the rainfall is higher in the Itchen than the Test and the PE lower.

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Technical Note 26

Figure 3.10 WRGIS Groundwater Bodies for the Test and Itchen Chalk and the Recharge Model (4R) Catchments and Geology

Table 3.3 Calculated Recharge from 4R Recharge Model for the Test and Itchen 4R Catchments

Year Test Recharge (mm/a) Itchen Recharge (mm/a)

1970 328 357

1971 322 332

1972 395 406

1973 143 150

1974 537 612

1975 321 407

1976 299 377

1977 428 481

1978 323 374

1979 410 491

1980 337 354

1981 378 447

1982 450 519

1983 289 331

1984 384 476

1985 293 318

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Technical Note 27

Table 3.3 (continued) Calculated Recharge from 4R Recharge Model for the Test and Itchen 4R Catchments

Year Test Recharge (mm/a) Itchen Recharge (mm/a)

1986 379 447

1987 315 398

1988 277 308

1989 269 306

1990 339 379

1991 270 302

1992 333 334

1993 425 448

1994 450 496

1995 441 459

1996 286 356

1997 232 256

1998 415 471

1999 354 401

2000 602 696

2001 473 569

2002 484 508

2003 337 408

2004 304 371

2005 181 225

2006 314 377

2007 371 430

2008 380 404

2009 363 425

2010 298 385

LTA (1970-2010) 354 405

LTA (1990-2010) 364 414

LTA (1970-2005) 370 417

3.8 Candover Augmentation Scenario (Run 128) The final run (Run 128) modelled augmentation from the Candover boreholes, which discharge to the Candover Stream when when the flows dropped below 198 Ml/d at the Allbrook &

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Technical Note 28

Highbridge gauging station, using the recent actual scenario (Run 126) as a basis. Triggers for the augmentation and the abstractions rate are in Table 3.4 (as supplied by the Agency). The longest duration the Augmentation was modelled to be in operation was from September 1973 to March 1974, which is displayed in Figure 3.11 at Borough Bridge and Allbrook & Highbridge.

Table 3.4 Candover Scheme Augmentation Triggers

Values in Ml/day Axford Bradley Wield Otterbourne SWABS

Month On Trigger Off Trigger Max Step Max Step Max Step Demand HOF

January 220 250 7 2.6 10 3.7 10 3.7 22.9 198

February 220 250 7 2.6 10 3.7 10 3.7 21.5 198

March 220 250 7 2.6 10 3.7 10 3.7 22.0 198

April 220 250 7 2.6 10 3.7 10 3.7 22.6 198

May 220 250 5.2 2.6 7.4 3.7 7.4 3.7 24.4 198

June 220 250 5.2 2.6 7.4 3.7 7.4 3.7 26.0 198

July 220 250 5.2 2.6 7.4 3.7 7.4 3.7 27.9 198

August 220 250 5.2 2.6 7.4 3.7 7.4 3.7 27.4 198

September 220 250 7 2.6 10 3.7 10 3.7 26.9 198

October 220 250 7 2.6 10 3.7 10 3.7 21.2 198

November 220 250 7 2.6 10 3.7 10 3.7 22.6 198

December 220 250 7 2.6 10 3.7 10 3.7 24.3 198

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Technical Note 29

Figure 3.11 Modelled Flows at Borough Bridge and Allbrook & Highbridge and Modelled Augmentation

4. NGMS Upload

The groundwater model is currently being prepared for upload to NGMS. The upload will be reported under a separate technical note.

5. Conclusions

The Test & Itchen groundwater model has been extended to include a portion of the confined Chalk to the north (beneath the London Clay) and to the east to include the Upper Greensand Formation. Artificial influences, abstractions and discharges, have been updated using profiles from the Agency and WRGIS (June 2012 version). Aquifer parameters (Transmissivity and Specific Yield) have been updated and refined using the Mole and Wessex groundwater models as a guide. The stream network has been increased to include the new area, and the stream elevations have been smoothed to reduce the ‘fall’ between cells with un-fixed elevations. Improvements have been made to modelled flows with focus on the Rivers Itchen and Test and their tributaries as well as the flows at Alton. The flows along the Candover have been

© AMEC Environment & Infrastructure UK Limited 24 May 2013 R:\Projects\32522 Test and Itchen GW Model Refinement\Docs\Technical Note\n058i1.doc

Technical Note 30

important to represent well due to the Candover Augmentation Scheme. The pump test conducted by Atkins was used as a guide to improve the models representation of drawdown. The model is now ready to be used for further scenarios in ‘Standalone MS-DOS’ mode and will soon (Summer 2013) be handed over to the Agency as an NGMS compatible executable.

6. Appendix A

This section details what can be found electronically:

• Timeseries flows for Historical Run (Run124), compared with gauges and spot flows, and pre-refinement run (Run 90)

- T&IFlowsRun124(Run90).xls;

• Timeseries groundwater elevations for Historical Run (Run 124), compared with observed elevations in boreholes

- hedo124.xls;

• Timeseries flows for Historical Run (Run 124) along the Candover compared with Candover spot-flows and Run 115 (during refinement)

- Candover Charts Run124v2.xls;

• Flow Duration Curves for Borough Bridge, Easton, Drove and Allbrook & Highbridge gauging stations

- Borough Bridge Stress Period Scenarios His124Nat125RA126FL127On128Off.xls

- Easton Stress Period Scenarios His124Nat125RA126FL127On128Off.xls

- Drove Stress Period Scenarios His124Nat125RA126FL127On128Off.xls

- Sewards Stress Period Scenarios His124Nat125RA126FL127On128Off.xls

- Allbrook & Highbridge Stress Period Scenarios His124Nat125RA126FL127On128Off.xls;

• Augmentation plots for the Alre and Candover Augmentation Run (Run 128) at Borough Bridge and Allbrook & Highbridge

- BoroughBridge_AugmentationScenarioPlots.xls

- Allbrook&Highbridge_AugmentationScenarioPlots.xls;

• Accretion profiles for the main rivers in the model for the Historical Run (Run 124) (Test, Itchen & Alre, Bourne, Bourne Rivulet, Wallop, Dun, Cheriton, Candover, Dever and Anton) for the end of three stress periods 692 (end of October 1993),

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790 (end of November 1997) and 864 (end of December 2000) [accretion profiles also extracted using ‘str tim’ for the above].

- AccProf_124v2.xls The GIS Model Map contains:

• Boundaries of the Test & Itchen model, pre- and post- refinement, and also the Mole and Kennet groundwater models;

• Boundaries of the WRGis Groundwater Bodies for the Test and Itchen;

• Boundaries of the recharge (4R) model catchments;

• Recharge grids of Total Available Water and Total Recharge for the period 1970 to the end of 2010 (from Run 122);

• Model Flow Maps for the Natural (Run 125), Recent Actual (Run 126) and Fully Licensed (Run 127) impacts;

• Flow gauge locations and groundwater observation borehole locations, including comparisons compared to the observed data;

• Modelled locations of abstractions and discharges (both groundwater and surface water);

• Locations of accretion profiles (main rivers in the model);

• Drawdown plots;

• Model properties including;

- mains leakage;

- stream properties (stream cells, stream routing, stream bed bottom and bed top elevation);

- storage properties (specific yield zones); - transmissivity properties (transmissivity zones, starting transmissivity);

- model geology (for the 4R recharge model);

• Ordnance Survey basemap for the grid square ‘SU’; and

• Geology (solid 250K and 625K solid and drift geology).

© AMEC Environment & Infrastructure UK Limited 24 May 2013 R:\Projects\32522 Test and Itchen GW Model Refinement\Docs\Technical Note\n058i1.doc