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: July 2015 Doc Ref: 29388tn568 2 © Amec Foster Wheeler Environment & Infrastructure UK Limited 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. July 2015 Doc Ref: 29388tn568 3 © Amec Foster Wheeler Environment & Infrastructure UK Limited 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.