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[1] This study reports sediment yields from seven small (0.18-5.42 ha) watersheds in Southern Arizona measured from 1995 to 2005. Sediment concentrations and total event sediment yields were related to storm-runoff characteristics, and statistical relationships were developed to estimate sediment yields for events with missing data. Precipitation ranged from 263 to 298 mm yr À1 , runoff ranged from 8.2 to 26.4 mm yr À1, and sediment yields ranged from 0.07 to 5.7 t ha À1 yr À1 , with an areal average of 2.2 t ha À1 yr À1 . For six of the seven watersheds, between 6 and 10 events produced 50% of the total sediment yields over the 11-year period. On the seventh watershed, two storms produced 66% of the sediment because of differences in the geomorphology and vegetation characteristics of that area. Differences between sediment yields from all watersheds were attributable to instrumentation, watershed morphology, degree of channel incision, and vegetation.

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[1] Watershed research is critical for quantifying the unique characteristics of hydrologic processes worldwide and especially in semiarid regions. In 1953, the United States Department of Agriculture established the Walnut Gulch Experimental Watershed (WGEW) near Tombstone, Arizona, to conduct hydrologic and erosion research. This manuscript (1) provides a historical context summarizing the evolution of the Southwest Watershed Research Center research program, (2) describes significant contributions to instrumentation development and contributions to science, and (3) describes the current WGEW data collection program in the context of contemporary research questions. The development of specialized flumes for streamflow measurement and the establishment of the core monitoring networks are described. WGEW data have been used to quantify semiarid rainfall, runoff, infiltration, and transmission losses; to develop and validate simulation models; and to support broader, regional, basin-scale research. Currently, rainfall, runoff, sediment, meteorology, and flux data collection continue at the WGEW, but the monitoring network has been expanded, and data use has evolved to support several multiple government agencies, universities, and international research programs.

Water quality modeling generally requires estimates of the amount of eroded material entering water courses. This information is necessary because sediment often transports adsorbed chemicals. Numerous models have been developed to assist with assessment of this problem. These models often contain some modification of the Universal Soil Loss Equation (USLE). A recently initiated effort to improve USLE technology has produced a computer‐based model, RUSLE (Revised USLE), which employs new relationships to estimate values of the six factors in the equation. Three input databases are required: climatic data, crop data, and field operations data. Although numerous specific entries for these data are contained in the program, in many cases users must supplement or modify the supplied data. Results of a sensitivity analysis help users tailoring the databases to specific conditions.

The climate of Southern Arizona is dominated by summer precipitation, which accounts for over 60 percent of the annual total. Summer and non‐summer precipitation data from the USDA‐ARS Walnut Gulch Experimental Watershed are analyzed to identify trends in precipitation characteristics from 1956 to 1996. During this period, annual precipitation increased. The annual precipitation increase can be attributed to an increase in precipitation during non‐summer months, and is paralleled by an increase in the proportion of annual precipitation contributed during non‐summer months. This finding is consistent with previously reported increases in non‐summer precipitation in the southwestern United States. Detailed event data were analyzed to provide insight into the characteristics of precipitation events during this time period. Precipitation event data were characterized based on the number of events, event precipitation amount, 30‐minute event intensity, and event duration. The trend in non‐summer precipitation appears to be a result of increased event frequency since the number of events increased during nonsummer months, although the average amount per event, average event intensity, and average event duration did not. During the summer “monsoon” season, the frequency of recorded precipitation events increased but the average precipitation amount per event decreased. Knowledge of precipitation trends and the characteristics of events that make up a precipitation time series is a critical first step in understanding and managing water resources in semiarid ecosystems.

Runoff Curve Numbers (a measure of a watershed's runoff response to a rainstorm) were determined using threedifferent methodsfor 18 semiarid watersheds in southeasternArizona. Each ofthe methodsproduced similar results. A relationship was then developed between optimum Curve Number and drainage area of the watershed used. Curve Numbers decreased with increasing drainage area. This response is a reflection of spatial variability in rainfall and infiltrationlosses in the coarse-textured material ofthe channels associated with larger drainage basins.