Update of the Blithe Hydrology 07 August 2017 Mott MacDonald 22 Station Road Cambridge CB1 2JD United Kingdom T +44 (0)1223 463500 F +44 (0)1223 461007 mottmac.com Update of the Blithe 377769 01A B Hydrology http://pims01/pims/llisapi.dll?func=ll&objaction=overview&objid=2118528682 Mott MacDonald 07 August 2017 Mott MacDonald Limited. Registered in England and Wales no. 1243967. Registered office: Mott MacDonald House, 8-10 Sydenham Road, Croydon CR0 2EE, United Kingdom Mott MacDonald | Update of the Blithe Hydrology Issue and Revision Record Revision Date Originator Checker Approver Description A July 2017 Christopher David Ocio Peter Ede Section extracted from DO analysis Rhodes and drought scenarios report B August Christopher David Ocio Peter Ede Including comments from SSW 2017 Rhodes Document reference: 377769 | 01A | B Information class: Standard This document is issued for the party which commissioned it and for specific purposes connected with the above- captioned project only. It should not be relied upon by any other party or used for any other purpose. We accept no responsibility for the consequences of this document being relied upon by any other party, or being used for any other purpose, or containing any error or omission which is due to an error or omission in data supplied to us by other parties. 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Under no circumstances may this report or any extr act or summar y ther eof be used in connection with any public or pri vate sec urities offering i ncluding any rel ated memorandum or prospectus for any securities offering or stock exchange listing or announcement. 377769 | 01A | B | 07 August 2017 http://pims01/pims/llisapi.dll?func=ll&objaction=overview&objid=2118528682 Mott MacDonald | Update of the Blithe Hydrology Contents 1 Introduction 1 2 Modelling approach 2 2.1 Requirements 2 2.2 HYSIM 2 3 Available input data 4 3.1 Requirements 4 3.2 Rainfall 4 3.3 Potential Evapotranspiration 7 3.4 Hydrometry 7 3.5 Artificial influences 10 4 Calibration-validation process 12 4.1 Approach 12 4.2 Model building 12 4.3 Calibration 13 4.4 Validation 18 5 Simulation 22 6 Summary 24 377769 | 01A | B | 07 August 2017 http://pims01/pims/llisapi.dll?func=ll&objaction=overview&objid=2118528682 Mott MacDonald | Update of the Blithe Hydrology 1 1 Introduction Blithfield Reservoir, supported during dry periods by the River Blithe Pumpback, is a key component of the SSW supply system. The annual licence of the reservoir is 29,200Ml (equivalent to an average of 80Ml/d) and the recent average abstraction is around 60Ml/d. Drought conditions and demand saving measures are established and are based on stored volume relative to established control curves. Thus, the Level of Service of the whole system is derived from the simulation of this reservoir. Given this, it is essential that simulated inflow series to the reservoir and flow series downstream at the River Blithe Pumpback intake are as accurate as possible. This report describes the methodology followed and the results obtained in this hydrology update. The adopted period of simulation is 1882-2014. The report is structured as follows: ● Modelling approach ● Available input data ● Calibration-validation process ● Simulation ● Summary The conjunctive deployable output of the SSW supply system has been modelled with water resources allocation models since WRMP04. The inflow series was largely based on a single HYSIM model set up by earlier work in 1997. This HYSIM model was calibrated to the naturalised Upper Blithe flow sequence at Blithfield Reservoir and also applied to the Lower Blithe at the location of the pumpback by decomposition. The HYSIM model was progressively refined and extended in time for the Company’s WRMP09 and WRMP14 submissions. During the WRMP14 study work it became apparent that this modelling approach had some shortfalls. In particular, calibration to new flow gauging data on the Upper and Lower Blithe was poor and the model tended to overpredict the low flows observed during the 2011/12 drought. An alternative approach using a HYSIM model developed by Severn Trent Water produced an even higher system yield, and therefore, the WRMP14 submission was made using a hybrid flow series to determine baseline deployable output and the sensitivity of the DO to climate change using the STWL model. As a consequence, for WRMP19, SSW needed a simulated flow series based on a robust calibration across the entire historic record. They required that the impact of significant upstream groundwater abstractions and discharges were explicitly modelled, and that both the reservoir and downstream catchments were separately simulated and calibrated. In addition to improving confidence in the baseline assessment of deployable output, this aimed to allow the simulation of series to study the impact of climate change and other scenarios. 377769 | 01A | B | 07 August 2017 http://pims01/pims/llisapi.dll?func=ll&objaction=overview&objid=2118528682 Mott MacDonald | Update of the Blithe Hydrology 2 2 Modelling approach 2.1 Requirements Assessment of the yield or Deployable Output of a water resources system requires a long period of data (particularly river flows for a reservoir system) to allow the assessment to cover a wide range of climatic conditions. Flow data is rarely available for the length of period required. So models are used to simulate flows over longer periods, taking advantage of the fact that rainfall records are usually much more extensive than flow records. The model selected for this purpose was HYSIM (Hydrological Simulation model). 2.2 HYSIM HYSIM is a lumped and conceptual rainfall-runoff model which was developed by Ron Manley (Water Resource Associates Ltd.) in the 1970s. It comprises a linked set of storages. The capacity of each storage, the maximum rate of transfer between them and equations which govern the transfer processes are all defined by time invariant parameters. Figure 5 shows a flow chart of the inputs, storage and processes that are defined in HYSIM. The key data inputs are rainfall and potential evapotranspiration (PET). The model also incorporates artificial influences (surface water abstractions and discharges, and groundwater abstractions), but in this study there are no sewage treatment works discharges, and abstractions are relatively minor. Although HYSIM has since been refined and further developed, it remains a similar program now compared to the one used at the time of the creation of the original simulations. One of the advantages of HYSIM is its automated optimisation procedures which facilitate rapid calibration. Although HYSIM has the option for inclusion of snowmelt it was decided not to include this element in this study. This was partly because of the lack of readily available temperature data sets but more particularly the fact that including snowmelt would have little or no effect on simulated low flows.