On topographic controls of soil hydraulic parameter scaling at hillslope scales

Most upscaling efforts for soil hydraulic parameters developed thus far have opted to ignore the effect of topography in their derivation of effective parameter values. This approach, which considers a flat terrain with no lateral flows, is reasonable as long as the coarser support dimensions are of the order of a few hundred meters. In such a scenario, the upscaled characteristics of the parameters are governed predominantly by the texture and structure of the soil in the domain. However, when upscaling fine-scale hydraulic parameter data to much larger extents (hillslope scales and beyond), topography plays a bigger role and can no longer be ignored. Efforts to model hydrologic processes and phenomena, with particular emphasis on those occurring in the unsaturated zone, are conducted at various scales. We present here a study to isolate the influence of topographic variations on the effective, upscaled soil hydraulic parameters under different hillslope configurations. The power-averaging operator algorithm was used to aggregate fine-scale soil hydraulic parameters to coarser resolutions. Hydrologic scenarios were simulated using HYDRUS-3D for four different topographic configurations under different conditions to test the validity of the upscaled soil hydraulic parameters. The outputs from the simulations (fluxes and soil moisture states) were compared across multiple scales for validating the effectiveness of the upscaled soil hydraulic parameters. It was found that the power-averaging algorithm produced reasonably good estimates of effective soil hydraulic parameters at coarse scales. Further, a probable threshold dimension beyond which the topography dominates the soil hydraulic property variation was analyzed. On the basis of only the topography, the scaling algorithm was able to capture much of the variation in soil hydraulic parameters required to generate equivalent flows and soil moisture states in a coarsened domain.