Characterizing Hydraulic Properties in An Arid Watershed to Aid in Flood Control and Uncertainty Analysis

Hydraulic properties of desert soils significantly affect rainfall/runoff processes within watersheds and are of critical importance for hydrological modeling in arid and semiarid environments. The recent urbanization of ungaged watersheds of the desert southwest has left flood control districts with the difficult task of predicting and managing unknown quantities of flood waters. The purpose of this study was to rapidly characterize the distribution and uncertainty of soil hydraulic properties found in a remote basin in southern Nevada. The basin contains a variety of surfaces ranging from steep bedrock highlands to gently sloping interfluvial areas. In this work, we characterized runoff potential and saturated hydraulic conductivity (K_s), and developed a site-specific pedotransfer function (PTF). Geomorphic mapping, soil sampling and analysis, rainfall simulation, and tension infiltrometer (TI) tests were all used as field characterization techniques on five distinct geomorphic surfaces determined within the study watershed.

The use of detailed geomorphic mapping significantly reduced the variance in K_s resulting in statistically distinct hydrologic groups which could be scaled to the watershed. Measurements from TI tests showed higher K_s values on the younger soils and eroded older surfaces, and lower values on the older and well-paved surfaces. In addition, the site-specific PTF was showed that, by using only soil texture and bulk density, K_s could be estimated at R² = 0.89. The measured curve numbers (CN) were inversely correlated to K_s resulting in a R² = 0.93. Therefore, at this field site, using soil texture and bulk density values collected throughout the watershed and the site-specific relationships obtained by field work, CNs and other parameters for this watershed could be easily and quickly estimated. This study suggests that field verification, which in the past was deemed cost-prohibitive in large remote watersheds, can be both cost-effective and efficient using a similar approach.