Estimating Soil Moisture in Small Watersheds, Using a Water Balance Approach

Abstract

Soil moisture content was estimated daily by a energy balance and water budget (EBWB) analysis of two small subwatersheds in the Walnut Gulch Experimental Watershed near Tombstone, Arizona. One watershed was 8.1 ha and covered with grass, and the other was 48.6 ha and covered with shrubs. The four-week experiment took place in July and August, during the summer rainy season that dominates the precipitation pattern of southern Arizona. Mean daily soil moisture (SM, mm) was estimated by a water balance at each watershed, using precipitation (P), runoff (RO) and evapotranspiration (ET). Daily ET estimates were derived from an energy balance on each watershed, using measured net radiation (Qn), soil heat flux (Qg) and sensible heat flux (Qh) to solve for latent energy (QLE, and hence ET) as a residual term. Next, independent estimates of watershed SM were obtained from Time Domain Reflectometry (TDR). The results of simple correlation analyses between the methods showed that the simple correlation coefficients for Lucky Hills and Kendall are 0.764 and 0.791 respectively. The agreement of the two sets of soil moisture measurements confirms that simple water and energy balance measurements can yield appropriate estimates of soil moisture in small watersheds.