Multi-decadal groundwater level records, which provide information about long-term variability and trends, are relatively rare. Whilst a number of studies have sought to reconstruct river flow records, there have been few attempts to reconstruct groundwater level time-series over a number of decades. Using long rainfall and temperature records, we developed and applied a methodology to do this using a lumped conceptual model. We applied the model to six sites in the UK, in four different aquifers: Chalk, limestone, sandstone and Greensand. Acceptable models of observed monthly groundwater levels were generated at four of the sites, with maximum Nash -Sutcliffe Efficiency scores of between 0.84 and 0.93 over the calibration and evaluation periods, respectively. These four models were then used to reconstruct the monthly groundwater level time-series over approximately 60 years back to 1910. Uncertainty in the simulated levels associated with model parameters was assessed using the Generalized Likelihood Uncertainty Estimation method. Known historical droughts and wet period in the UK are clearly identifiable in the reconstructed levels, which were compared using the Standardized Groundwater Level Index. Such reconstructed records provide additional information with which to improve estimates of the frequency, severity and duration of groundwater level extremes and their spatial coherence, which for example is important for the assessment of the yield of boreholes during drought periods.

5Highlights 6  A seamless GIS-groundwater flow model is presented 7  It directly uses GIS data for groundwater flow modelling within the GIS environment 8  This easy-to-use and flexible model can be used by non-modellers 9  It is downloadable and can be used for any purpose free of charge 10  The model developed herein can be integrated into other raster GIS packages. 11 Abstract 12There are three approaches for coupling groundwater models with GISs, i.e. loose, tight, 13and seamless. In seamless coupling a model code is written into, and run from within, a 14 GIS. We implemented BGS GISGroundwater in a GIS in this way for the first time.