Connecting large-scale atmospheric circulation , river flow and 1 groundwater levels in a chalk catchment in southern England 2

17 Groundwater is an important water resource and globally it represents the largest distributed 18 store of freshwater. In southern England, groundwater is a major source for public water 19 supply, and many aquifers have recently experienced both extreme low and high groundwater 20 levels. In this paper, we use observations of precipitation, river discharge and groundwater 21 levels (1964 to 2010) and an atmospheric reanalysis to explore the large-scale climate 22 patterns preceding the nine highest and lowest March river discharge and groundwater levels 23 in the chalk catchment of the River Lambourn (Berkshire Downs, southern England). Peak 24 monthly precipitation is shown to occur from October to January, while the highest river 25 discharge and groundwater levels are found from February to April. For high discharge / 26 groundwater levels, composite anomaly patterns of the mean sea level pressure show a 27 stronger than average pressure gradient across the North Atlantic Ocean, with enhanced water 28 vapour transport across southern England. For the lowest discharge / groundwater levels, a 29 blocking high pressure system is found across the British Isles deflecting storms and 30 precipitation to the north. Significantly, the intra-composite variability suggests that different 31 sequences of atmospheric states may lead to high and low discharge / groundwater events. 32