A Topography-Based Scaling Algorithm for Soil Hydraulic Parameters at Hill-slope Scales

Understanding how soil hydraulic parameter values are affected at different scales by the spatial variability of influencing factors such as soil structure & texture, vegetation, and topography, as also the atmospheric forcings such as precipitation, is an inherent requirement of efficient scaling schemes. While available soil hydraulic parameter aggregation or upscaling schemes ignore the effect of topography, their application becomes limited at hillslope scales and beyond, where topography plays a dominant role in soil deposition and formation. Hence an upscaling algorithm accounting for topographic controls of the soil hydraulic parameter variations across space was considered necessary. In this study soil hydraulic parameters were upscaled from a 30-m resolution to a 1-km resolution using a new aggregation scheme where the scale parameter was based on the topography. The new upscaling algorithm was tested at the hill-slope scale (1-km) across two locations: 1) the Little Washita watershed in Oklahoma, and 2) the Walnut Creek watershed in Iowa. The watersheds were divided into pixels of 1-km resolution and the effective soil hydraulic parameters obtained for each pixel. Each pixel/domain was then simulated using the physical-based HYDRUS-3D modeling platform. In order to account for the surface (run-off/on) and sub-surface fluxes between pixels, an algorithm to route infiltration-excess run-off onto downstream pixels at daily time steps and to update the soil moisture states of the downstream pixels was applied. Simulated soil moisture states were compared across scales, and the coarse scale values compared against the airborne soil moisture data products obtained during the hydrology experiment field campaign periods (SGP97 and SMEX02) for selected pixels with different topographic complexities, soil distributions, and land cover. Results from these comparisons show good correlations between simulated and observed soil moisture states across time, topographic variations, location, elevation, and land cover. Stream discharge comparisons made at two gauging stations in the Little Washita watershed also provide reasonably good results as to the suitability of the upscaling algorithm used. Based only on the topography of the domain, the new upscaling algorithm was able to provide coarse resolution values for soil hydraulic parameters which effectively captured the variations in soil moisture across the watershed domains.