The water budget for a period of one year on a small instrumented semiarid subwatershed was evaluated. The evaluation was accomplished by identifying and quantifying the primary components of the water budget, rainfall, surface runoff, soil moisture content, and (collectively) other water losses. In addition to the evaluation of the water budget, regression models of surface runoff, soil moisture content, and suspended sediment were developed. Finally, chemical analysis of the runoff waters and an evaluation of a water balance simulation model were made. The study site, a 6.5-hectare subwatershed which lies on the southeastern portion of the Atterbury Watershed, is located about 16 km east of Tucson, Arizona. It has an elevation of about 3200 feet (975 m), with an average slope of about 3 percent. The mean annual precipitation is about 28 cm falling during two distinct seasons, summer and winter. The soil textural classification on the Atterbury Watershed ranges from sandy to clay loam. The evaluation of the water budget provided an index of how much of the total precipitation for the study year is attributed to each of the primary components of the water budget. Surface runoff, soil moisture content, and other water losses accounted for about 2, 55, and 43 percent of the total rainfall. Rainfall and related rainfall characteristics, such as intensity and duration, were found to be significant variables in regression models predicting surface runoff, soil moisture content, and suspended sediment. The chemical constituents of the waters were either within or lower than the limits set by the United States Public Health Service, World Health Organization and the United States Environmental Protection Agency. The usefulness of the BUM water balance model in simulating surface runoff from the study area was inconclusive.