Scale-classification of water table response in a low-gradient Prairie watershed

English Abstract:

While shallow subsurface stormflow has been found a major contributor to runoff generation in many temperate, moderate to high-relief catchments, its importance in low-gradient environments remains unclear. Our objective was therefore to characterize the water table-runoff relationship in low-gradient, Canadian Prairie landscapes by (1) deriving a range of metrics of water table response, and (2) inferring the presence of various runoff processes from those metrics. The focus was on the Catfish Creek Watershed (Manitoba, Canada), which drains 642 km² of forest and agricultural land. Nine different riparian areas were selected to illustrate a range of land use conditions (e.g., forest, shrub or bare land cover), upslope accumulated areas and human landscape features (e.g., dykes). Each riparian area was equipped with three, 1.5 m deep water-table wells and paired with a stilling well in the adjacent stream, recording data every 15 minutes in 2013 and 2014. For each site, hydrological events were delineated and the stream response compared with the water table response. Specifically, several metrics of event-scale water table response were computed, notably the pre-event water table depth, the maximum water table rise, the rates of water table rise and recession, the time lag from the initiation of precipitation to the maximum well response, and an hysteresis index quantifying the delay in response between the water table and the stream. Preliminary analyses show that the metrics were highly variable among sites. The metrics also suggest different event conditions leading to either saturation-excess overland flow, local subsurface flow or more widespread translatory flow as the dominant runoff process in the riparian areas. Some of the metrics portraying hysteresis or recession characteristics were markedly different between short and longer events, thus reflecting a scale-dependence effect on soil hydrology.