Physical Controls On Soil Hydraulic Parameter Scaling.

Soil hydraulic parameter variability is affected at different scales by the spatial variability of influencing factors such as soil texture and structure, topography, vegetation, and climatic factors such as precipitation and temperature patterns. Understanding the nature of the linkage between the physical controls and the soil hydraulic parameters is critical in developing efficient scaling schemes for large domains. Since 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. We present the results from a study where soil hydraulic parameters were upscaled from a 30-m resolution to a 1-km resolution using a new aggregation scheme. The scale parameter in this study was based on the topography of the domain. Further, the effects of parameter correlations on the upscaled effective values were also studied. The new upscaling algorithm was tested at the hill-slope (1-km) scale across different test locations. 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. The 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.