Physical Controls of Soil Hydraulic Parameter Scaling

Understanding how soil hydraulic parameter values are affected at different scales by the spatial variability of influencing factors such as soil structure and texture, vegetation, and topography, as also the atmospheric forcings such as precipitation, is an inherent requirement of efficient scaling schemes. Inter-connections often exist between information across scales. However, the mathematical and/or physical nature of these connections is generally unknown. In order to provide better input data to hydrological, ecological and meteorological models, it is very much essential to understand the physical processes underlying the spatial variations of these soil hydraulic properties. We present here a study where we investigated the influence of topographic variations on the effective, upscaled soil hydraulic parameters under different hill-slope configurations. A mathematical form to describe the effect of the physical controls at this scale is presented. The power averaging operator algorithm is adapted to aggregate fine scale soil hydraulic parameters to coarser resolutions. Soil moisture state and water flow scenarios are simulated using the HYDRUS-3D software for different topographic configurations to test the equivalence of the upscaled soil hydraulic parameters. The algorithm produces reasonably good estimates of effective soil hydraulic parameters at coarse scales. The simulated soil moisture and hydraulic parameters are compared across scales, time, soil type distributions, vegetative covers, and with respect to topographic indices. Based only on the topography, the scaling algorithm was able to capture much of the variations in soil hydraulic parameters required to generate equivalent flows and soil moisture states in a coarsened domain. Further, the theoretical basis is validated with data from two different sites - one at the Panola Mountain Research Watershed in Georgia, and the other in the Little Washita watershed in Oklahoma. This approach takes a first step towards addressing the need to include physical controls into upscaling algorithms for soil hydraulic parameters.