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Using regional statistics of daily rainfall, a simple water-balance model was employed to generate runoff sequences with which to simulate the yieldktorage behaviour of reservoin in south-east England, in north-west England. and North Wales. Similarly sequences of recharge to an unconfined aquifer in eastern England were the basis of deriving its yield/ storage behaviour. Then, taking scenarios of the year 2030 rainfall and evaporation, provided by the University of East Anglia's Climatic Research Unit, reductionsin yieldwere calculatedto be5-15YobeIow present-day values. For direct supply reSeNOin, greater percentage reductions in yield were found to apply to the south-east region, as compared to the north-west. The results from the aquifer example are interpreted on a novel basis which allows an immediate comparison with the surface reservoir examples.Coastal sea-water intrusion was modelled for three common geological conditions (i) the Grimsby Chalk (confined), (ii) the Brighton Chalk (unconfined), and (iii) the Otter Valley Sandstone (unconfined). In all three cases the effect of a possible 0.6 m rise in mean sea level was s h o w to have only a marginal effect on sustainable yields, which reduced by about 1.5%Estuarine fresh-salt water interfaces are important (0 the abstraction regime of freshwater intakes in the lower reaches ofriven. The effect ofa 0.6-m sea-level rise on the saline interfacc location at high tide was evaluated by hydrodynamic computational models. Only a minor inland shift of the interface was found, less than 800 m in the Thames tideway and less than 500 m in the Lune estuary. The Severn estuary is exceptional in having its saline interface move 3.5 km landwards for the Same 0.6-m rise in mean sea level.

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Bias is a difference between model and reality. Bias can be introduced at any stage of the modelling process during a site characterisation or performance assessment programme. It is desirable to understand such bias so as to be able to optimally design and interpret a site characterisation programme. The objective of this study was to examine the source and effect of bias due to the assumptions modellers have to make because reality cannot be fully characterised in the prediction of ground-water fluxes. A well-defined synthetic “reality” was therefore constructed for this study. A limited subset of these data were independently interpreted and used to compute groundwater fluxes across specified boundaries in a cross section. The modelling results were compared to the “true” solutions derived using the full dataset. This study clarified and identified the large number of assumptions and judgements which have to be made when modelling a limited site characterisation dataset. It is concluded that bias is introduced at each modelling stage, and that it is not necessarily detectable by the modellers even if multiple runs with varied parameter values are undertaken.

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