Understanding the hydrological functioning of a shallow lake system within a coastal karstic aquifer in Wales, UK

Bosherston Lakes in west Wales are an internationally important set of linked freshwater lowland lakes which were created by damming tidal creeks in the 18th and 19th centuries. The lakes have an average water depth of 1–3 m, and receive surface water inflows but have an uncertain interaction with the underlying karstic Carboniferous Limestone. This paper describes the application of the Soil Water Assessment Tool (SWAT) to improve the understanding of the hydrological functioning of the lake system. The temporally variable and limited observational data were used within a two-step calibration process. The simulated surface water inflows and groundwater levels were calibrated, followed by the lake volumes (Nash–Sutcliffe (NS) coefficient ranging from 0.67 to 0.74). Finally the simulated lake volumes were validated (NS ranging from 0.56 to 0.74) and the simulated lake outflows demonstrated to be plausible. Simulations reveal that three of the four linked water bodies lose significant water to the underlying aquifer. The simulated water balance demonstrates that the catchment outputs are dominated by evapotranspiration, surface outflow from the lake system to the sea and coastal groundwater discharge, with abstraction and lake evaporation being of lesser importance. The coastal groundwater discharge originates from both leakage from the lakes and previously unrecognised larger scale groundwater flow paths in the limestone aquifer. The study has provided an improved basis for the future hydrological management of the catchment and lakes and has demonstrated the wider utility of SWAT in simulating karstic syst