Simulating Soil-Water Movement through Loess Veneered Landscapes.

Soil Survey Geographic Database (SSURGO) data are available for the entire United States and so are incorporated in many regional and national models of hydrology and environmental management.  However, SSURGO does not provide an understanding of spatial variability and only includes saturated hydraulic conductivity (K_sat) values estimated from particle size analysis (PSA).  This study documented model sensitivity to the substitution of SSURGO data with locally described soil properties and alternate methods of measuring K_sat in a grassland basin that is part of the Shawnee Hills Loess-Catena Soil-Systems Project.  Incorporation of these different soil datasets significantly changed the results of hydrologic modeling as a consequence of the amount of space available to store soil water and how this soil water is moved downslope.  Field-measured K_sat from locally described soil profiles was different than that estimated from PSA.  Subsequently, this caused differences in which soil layers were incorporated into the hydrologic simulations using TOPMODEL, ultimately changing the simulations of soil-water storage.  Simulations of free-flowing soil water, the amount of water traveling through pores too large to retain water against gravity, were compared to field observations of water in wells at five slope positions along a catena.  A comparison of simulated data to observed data showed that ability to model the range of conditions observed in the field varied as a function of these three soil datasets (SSURGO and local field descriptions using PSA-derived K_sat or field measured K_sat) and that comparison of absolute values of soil-water storage are not valid if different characterizations of soil properties are used.  The ability to simulate and compare soil-water storage is critical to demonstrating the benefits of conservation agriculture for the drier conditions forecasted for most of the continental U.S. by the end of the century.