Spatial organisation of groundwater dynamics and streamflow response from different hydropedological units in a montane catchment

Groundwater dynamics play an important role in runoff generation and hydrologic connectivity between hillslopes and streams. We monitored a network of 14 shallow groundwater (GW) wells in a 3.2 km2 experimental catchment in the Scottish Highlands. Wells were placed in three contrasting landscape units with different hydropedological characteristics and different topographic position relative to the stream network, encompassing a catena sequence from free draining podzols on steeper hillslopes to increasingly thick peats (histosols) in the valley bottom riparian zone. GW dynamics were characterised by statistical analyses of water table fluctuations, estimation of variabilities in lag times and hysteresis response in relation to streamflow. The three landscape units had distinct storage-discharge relationships and threshold responses with a certain GW level above which lateral flow dominates. Steeper hillslopes with freely draining podzols were characterised by GW fluctuations of around 150 cm in the underlying drift. GW usually showed peak response up to several hours after stream flow. During persistent wet periods the water table remained in the soil profile for short spells and connected shallow flow paths in the near surface horizons to the lower hillslopes. In the peaty gleys in the lower foot slopes, GW was characterised by a water table generally within 20 cm of the soil surface, though at some locations this could fall to 50 cm in extreme dry periods. GW responses were usually a few hours prior to the stream responses. In riparian peats, the water table was also usually less than 20 cm deep and responded several hours before the stream. These riparian peat soils remain at, or very near saturation with near-continuous GW-surface water connectivity. In contrast, the steeper slopes remain disconnected for prolonged periods and need large recharge events to overcome storage thresholds. Groundwater responses vary seasonally, and landscape controls on the spatial organisation of GW dynamics are strongest at low flows and in small events. During wettest periods, limited storage and extensive saturation weaken such controls. This study demonstrated that montane catchments can have highly dynamic GW stores which are important in generating both storm flows and baseflows.