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The development of the Vale of St Albans groundwater model made use of two techniques not commonly used in UK regional modelling studies: parameter estimation (using PEST) and representation of dual storage. Parameter estimation techniques can significantly improve model calibration and can be used to inform model development. Because parameter estimation typically involves running a groundwater model many hundreds of times, using a steady-state version of the model can help to keep run times short. Care must be taken to ensure that the parameter distributions produced through these techniques are conceptually defensible, and they must be checked against the conceptual model and modified where necessary. Simulation of extreme conditions such as floods and droughts can be problematic for Chalk aquifers. Previous studies have shown that the Chalk can be resistant to drought, with higher flows recorded in rivers than can be accounted for from releases from fissure storage alone. An approach has been developed that simulates a weak hydraulic connection between fissures and a proportion of matrix storage within the Chalk, providing a delayed yield of significant additional volumes of water during droughts. The application of this approach results in a significant improvement in the simulation of extreme events.

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