Dynamic Downslope Travel Distance Modeling: Interflow Modeling from Bottom of Slope Upwards

Abstract:

Downslope travel distance concepts for interflow moving over a leaky restricting layer allow dynamic interflow modeling starting from the bottom of the slope and modeling only the active contributing area as opposed to the normal approach that models the entire slope from the ridgetop no flow boundary. In watersheds featuring deep groundwater and a low conductivity layer laying under permeable topsoil, interflow contributing areas expand and shrink based on the thickness of the perched layer, the topographic slope, and the ratio of hydraulic conductivity of topsoil to that of the impending layer. In this work, we present the development and application of two dynamic interflow models that implemented a mobile boundary condition to track flow from each of contributing cells starting from the edge of stream reaches extending upslope and constrained by downslope travel distance. Both analytical models are based on Boussinesq assumptions with percolation while one is continuous and the other is event-based. The continuous interflow model incorporates a two layer soil moisture accounting water balance analysis, pedotransfer function, percolation, and evaporation routines. The event rain based analytical solution generates interflow time series based on dynamically updated distribution function of downslope travel distances of contributing cells. We applied both modeling approaches in an intensively instrumented headwater basin in the Upper Coastal Plain of South Carolina. Both models showed good agreement with observed perched water depth, interflow discharge and soil moisture observations from 110m long open interflow interception trench whose 5.6 hectares of contributing hillslope were fitted with nested piezometers, soil moisture sensors, and series of V-notch weir boxes.