Combining Pedotransfer Functions and Detailed Geomorphic Mapping to Characterize Runoff Potential on an Arid Alluvial Fan Complex

A common practice for predicting surface runoff from hydrographic basins is to assign curve numbers (CN), which relate to hydrologic soil groups, land use, and vegetative cover, and that can be used in rainfall-runoff models to generate hydrographs. The purposes of this study were to characterize the soils of a remote basin in southern Nevada, specifically runoff potential and soil hydraulic conductivity (Ks), and to examine the use of a site- specific pedotransfer function (PTF) that would relate soil texture and bulk density to CNs. Geomorphic mapping, soil sampling and analysis (n=79), rainfall simulation (N=19), and tension infiltrometer (n=47) tests were all used as characterization techniques on the five distinct geomorphic surfaces identified on this 19.5 km2 site. The field- measured CNs for the older Qf5 and Qf6 surfaces (87 and 83, respectively) were close to default CN values for similar surfaces classified as hydrologic soil group C. The higher CN values obtained for the Qf4 and QTt surfaces (92.5 and 94, respectively) reflect the well-developed desert pavements covering these surfaces. The field values of Ks from the infiltrometer were higher on surfaces younger in age or otherwise eroded or incised, and lower on desert pavement surfaces and old terrace deposits. Higher CN values correspond to lower Ks. The use of detailed geomorphic mapping significantly reduced the variance in Ks across the watershed resulting in statistically distinct hydrologic groups, which could be scaled to the watershed. When average Ks was regressed onto field-measured CNs, a linear relationship was found with R2=0.928. The results show that measurement of Ks may be used to directly estimate CN in this watershed. The site-specific PTF method was also tested using the entire database of 79 soil texture and bulk density data, in combination with field measured Ks and a multiple linear regression approach. The results showed that this simple PTF approach, using only soil texture and bulk density, provided an excellent estimate of field Ks (R2=0.890). This finding means that, at this field site, an easy way to characterize the hydrographic basin would be to analyze soil samples collected from across the site, and use the site-specific relationship obtained from the field work to estimate CN, or any other hydraulic properties. Though other relationships may exist for different watersheds, the method appears to be robust for this site, making rapid assessments of flood potential possible.