Spatio-temporal variability of hydrological connectivity through interflow

Interflow is lateral subsurface flow in hillslopes during and after precipitation events moving above a restrictive layer of lower hydraulic conductivity soil and rock. Interflow thus becomes more important in steeper hillslopes with a high contrast between the hydraulic conductivities of the layers that impede vertical water movement. However, the travel distance of a water parcel downslope in a perched water body is limited due to potential percolation of water through the impeding layer. This potential travel distance of interflow can be described with the concept of "Downslope Travel Distance" (DTD) that applies to temporary, perched groundwater in hillslopes. The determination of this downslope travel distance in catchments is possible with available topographic and subsurface data. Yet, how this interflow connects to the catchment outlet is poorly understood and depends on the spatio-temporal extension and contraction of the stream network. This presentation introduces the concept of DTD and employs calculations based on published data from various catchments and landscapes. In these catchments, DTDs ranged from about just one meter to over several hundred meters. Yet, the DTDs on must hillslopes with data are less than 50 m and less than 30% of the hillslope length showing that most shallow perched water percolates through the impeding layer before contributing to valley water or streamflow via interflow. In a subsequent step, we illustrate the spatial and temporal variability of the area connecting to the catchment outlet via interflow and thus contributing to discharge in different catchments. While soil properties and topographic characteristics generally remain stable over short periods, the wetted stream network undergoes notable changes both in the short and long term. Consequently, the pronounced variability of the area connecting to the catchment outlet via interflow is observable and characteristic for individual catchments. Lastly, we emphasize the present significant constraints of experimental studies and data concerning hillslopes in different landscapes, underscoring the necessity for revisiting research on runoff generation at the hillslope scale.