| Summary: | The South-East England Chalk aquifer is arguably the most significant groundwater resource in the UK. The South-East has experienced mean temperature increases of 1.0-1.7 °C since 1961, but negligible change in precipitation. The impact of this temperature change on groundwater levels is not well understood. Two of the longest groundwater records in the world are located on the Chichester Chalk Block. The Chilgrove record from the Lavant catchment dates from 1836, and the nearby Compton record dates from 1892. These records provide an unprecedented opportunity to investigate long-term trends in groundwater levels. A broad range of statistical analyses have been applied to the Chilgrove and Compton records, which indicate that annual groundwater minima approximately conform to a normal distribution and can be used as a surrogate drought indicator. The records have shown a trend of increasing severity and frequency of minima since the early 1970s. Hydrometric data for the catchment shows a consistency with UK-wide trends of increasing temperatures and potential evapotranspiration, along with increases in winter and decreases in summer precipitation. Corresponding changes in seasonal recharge have resulted in cyclic behaviour and extreme events often providing a trigger for transition between periods of high, normal and low recharge. The Lavant Conceptual Model (LCM) has been developed for modelling groundwater levels and streamflows, and has been calibrated against historic catchment records. The model is shown to closely reproduce the historic groundwater trends. A Soil Moisture Accounting Procedure (SMAP) based on solving Richards' equation has been coupled to the LCM to create a second model that performs more accurately in drought conditions. Applying UKCP09 Weather Generator emission scenario data to the LCM has indicated that projected minima will demonstrate large amounts of variability, similar to that observed in the historic data, and that distinct future trends are difficult to identify.
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