Dynamics of soil moisture, subsurface flow and runoff in a small alpine basin

摘要

This work focuses on the dynamics of the rainfall-runoff processes in a small alpine catchment (3.3 ha), localized in the central-eastern Italian Alps, where topographic relief and shallow subsurface moisture control the storage and timing of runoff on the landscape. We examined the temporal variations of soil water content, groundwater and streamflow response to rainfall input for a number of flood events; for each flood, the time lag between storm centroid and peak of soil moisture, streamflow and water table level is reported. Data reveal a systematic temporal organisation of the different responses: soil moisture peaks at almost the same time as runoff while the basin-averaged groundwater peak is delayed. This behaviour leads to a constant anticlockwise hysteretic relationship between stream discharge and water table level at the rainstorm scale. Moreover, the piezometric response across the catchment tends to be more delayed with the increasing distance from the stream; thus, groundwater temporal variations in the riparian zone are similar to runoff dynamics, but different from the ones that are more remote from the stream. Such observed dynamics support the point that runoff response in this basin is controlled by the quick reaction of the near-stream riparian zone, which is generally close to saturation. Groundwater response on the hillslope lags behind and is controlled by the formation of transient saturation at the bedrock-soil interface. Finally, a threshold behaviour between soil moisture and streamflow suggests that the antecedent moisture conditions control the size of the saturation zone and hence the runoff rate.