Deconvoluting the hydrologic response of a small till catchment: Spatial variability of groundwater level and quality in relation to streamflow

Hydrograph separations in humid temperate regions have demonstrated repeatedly that catchment stormflow is dominated by pre-event water. In areas where transient saturated lateral flow dominates, streamflow and groundwater responses are expected to be strongly related. Few previous studies have had observations distributed in both space and time to determine which portions of the catchment participate in the hydrologic response. This study utilized a spatially distributed network of electronic groundwater level sensors to study groundwater table dynamics in a forested 12.9 ha headwater catchment at Sleepers River, VT, and to test the similarity between the hydrodynamic and hydrochemical responses of groundwater and streamflow. The groundwater table response in the hydrodynamic contributing area would be closely related to the stormflow hydrograph. Conversely, groundwater chemistry in the hydrochemical source area would be similar to that of the stream water (assuming conservative mixing).; The timing of groundwater response relative to streamflow during events was strongly related to antecedent conditions, as described by the depth to groundwater before events. In general, groundwater response lagged streamflow when the pre-event groundwater table was deeper than ∼0.7 meters, but coincided with streamflow when the water table was closer to the soil surface. Groundwater levels between events were strongly related to soil stratigraphy, and particularly the interface depth between the mineral soil and underlying glacial till, which occurred at ∼0.7 meters below ground surface. Streamflow chemistry during spring melt was similar to groundwater in the upper, convergent portions of the catchment, but generally dissimilar in the lower, planar portion of the catchment. Topographic analysis in combination with streamflow quality suggested that shallow flowpaths were more prevalent in the lower portions, relative to in the upper portions.; Results suggest that the extent of the hydrodynamic contributing area is directly controlled by the spatial distribution of pre-event groundwater levels in relation to the soil/till interface. Variations in landform affected streamflow chemistry by controlling prevalent flow paths. These findings suggest that future research efforts should focus on capturing the interactions between water inputs, storage deficit, soil stratigraphy, and landform to improve our capability of simulating the hydrologic and hydrochemical response of catchments.