Annex 3: Supply Forecast

Water Resources Management Plan 2019 Annex 3: Supply Forecast January 2020 Version 2 Contents 1. Executive summary ............................................................................ 7 1.1 The components of our supply forecast ........................................ 7 1.2 Developing our supply forecast..................................................... 8 1.2.1 Artificial drought generation.................................................... 8 1.2.2 Groundwater resource assessment ....................................... 9 1.2.3 Surface water resource assessment .....................................10 1.2.4 Conjunctive use modelling ....................................................10 1.2.5 Modelling the impacts of climate change ..............................11 1.3 Deployable output ........................................................................12 1.4 Sustainability reductions ..............................................................15 1.5 Impacts of climate change ...........................................................17 1.6 Process losses.............................................................................18 1.7 Outage .........................................................................................19 2. Components of the supply forecast ...................................................20 2.1 Deployable output ........................................................................20 2.2 Bulk imports and exports .............................................................21 2.3 Climate change ............................................................................21 2.4 Sustainability reductions ..............................................................22 2.5 Process losses.............................................................................22 2.6 Outage .........................................................................................22 2.7 Other reductions in resource availability ......................................22 3. Methodology for developing the supply forecast ................................24 3.1 Introduction ..................................................................................24 3.1.1 Our risk principle ...................................................................24 3.1.2 Supply (water resource) forecast ..........................................24 3.2 Artificial weather generation ........................................................26 3.2.1 Background to our approach .................................................26 3.2.2 Our weather generator ..........................................................27 3.2.3 Weather generator overview .................................................28 3.2.4 Weather generator input data ................................................31 3.2.5 Weather generator calibration ...............................................31 3.2.6 Weather generator modelling process ..................................33 3.2.7 Sub-sampling for water resource model input .......................35 3.2.8 Disaggregation of daily rainfall and generation of PET data..40 ii Water Resources Management Plan 2019 Annex 3: Supply Forecast 3.2.9 Peer review of our approach .................................................43 3.2.10 Evaluation of an alternative, potential evapotranspiration led, approach .....................................................................................45 3.2.11 Climate modelling for WRMP24. ........................................46 3.3 Groundwater resource assessment .............................................47 3.3.1 Introduction ...........................................................................47 3.3.2 Our groundwater resource models ........................................47 3.3.3 Input time series ....................................................................48 3.3.4 Indicator borehole regression models ...................................48 3.3.5 Other constraints to deployable outputs ................................49 3.3.6 Changes to catchment scale groundwater models ................50 3.3.7 Test and Itchen groundwater model (Hampshire WRZs) ......50 3.3.8 Isle of Wight WRZ .................................................................62 3.3.9 Sussex North WRZ ...............................................................66 3.3.10 Sussex Worthing and Sussex Brighton WRZs ...................66 3.3.11 Kent Medway WRZs ..........................................................70 3.3.12 North Kent groundwater model ..........................................70 3.3.13 East Kent groundwater model (Kent Thanet WRZ) ............71 3.4 Hydrological modelling .................................................................76 3.4.1 Summary ...............................................................................76 3.4.2 Background ...........................................................................76 3.4.3 Model update ........................................................................77 3.4.4 Flow naturalisation ................................................................78 3.4.5 Rainfall-runoff modelling .......................................................78 3.4.6 Flow denaturalisation ............................................................80 3.4.7 Aggregation of flows in the Medway ......................................82 3.4.8 Generation of stochastic flow sequences ..............................82 3.5 Surface water resource assessment and conjunctive use modelling ...............................................................................................84 3.5.1 Summary ...............................................................................84 3.5.2 Background ...........................................................................84 3.5.3 Aquator models .....................................................................85 3.5.4 Aquator model updates .........................................................86 3.5.5 Overview of Aquator models and surface water resources ...86 3.5.6 Demand profile development ................................................86 3.5.7 Operational control rules .......................................................89 3.5.8 Drought triggers and drought interventions ...........................89 3.5.9 Modelling of drought measures .............................................90 iii Water Resources Management Plan 2019 Annex 3: Supply Forecast 3.5.10 Levels of service ................................................................90 3.5.11 Assessment of deployable outputs ....................................91 3.5.12 Modelling of the Eastern area ............................................92 3.5.13 Modelling of Sussex North WRZ ........................................94 3.5.14 Modelling of the Isle of Wight at Sandown .........................95 3.6 Methodology for modelling the impacts of climate change on water resources ..............................................................................................96 3.6.1 UKCP09 datasets used .........................................................96 3.6.2 Evaluation of drought indicators ............................................97 3.6.3 Sampling of climate change perturbations ............................98 3.6.4 Application to water resource models .................................101 3.6.5 Scaling of climate change impacts ......................................102 3.6.6 Future climate change modelling approach.........................103 3.7 Water quality impacts on deployable output ..............................104 4. Deployable outputs ..........................................................................109 4.1 Deployable outputs - effect of drought measures ......................109 4.1.1 Effect of supply side measures ...........................................109 4.1.2 Effect of demand restrictions ...............................................109 4.2 Western area .............................................................................112 4.2.1 Isle of Wight WRZ ...............................................................112 4.2.2 Hampshire Southampton West WRZ ..................................112 4.2.3 Hampshire Southampton East WRZ ...................................112 4.2.4 Hampshire Winchester WRZ ...............................................112 4.2.5 Hampshire Rural WRZ ........................................................113 4.2.6 Hampshire Andover WRZ ...................................................113 4.2.7 Hampshire Kingsclere WRZ ................................................113 4.2.8 Western area summary .......................................................113 4.3 Central area ...............................................................................119 4.3.1 Sussex North WRZ .............................................................119 4.3.2 Sussex Worthing WRZ ........................................................119 4.3.3 Sussex Brighton WRZ .........................................................120 4.3.4 Central area summary .........................................................121 4.4 Eastern area ..............................................................................126 4.4.1 Kent Medway West WRZ ....................................................126 4.4.2 Kent Medway East WRZ .....................................................127 4.4.3 Sussex Hastings WRZ ........................................................127 4.4.4 Kent Thanet WRZ ...............................................................127

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