Submitted to: Water Resources Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/15/2010
Publication Date: 9/9/2010
Publication URL: http:// 46, W09512, doi:10.1029/2009WR009001.
Citation: Polyakov, V., Nearing, M.A., Nichols, M.H., Scott, R.L., Stone, J.J., Mcclaran, M. 2010. Long-term runoff and sediment yields from small semi-arid watersheds in southern Arizona. Water Resources Research. 46: W09512, doi:10.1029/2009WR009001. Interpretive Summary: Soil erosion on rangelands is one of the most common problems that affects arid ecosystems of the Southwestern US. In order to preserve and manage these ecosystems one needs to understand relationships between rainfall, runoff and erosion process. Scientists at USDA-ARS Southwestern Watershed Research Center in Tucson, Arizona studied eight small, 2.5 to 9 acre, semi-arid rangeland watersheds located in Santa Rita Experimental Range on the northern slopes of the Santa Rita mountains 30 miles south of Tucson. The watersheds were equipped with instruments that recorded precipitation, surface runoff, and amount soil eroded from the area and data was collected over the period of 34 years. Average annual precipitation ranged between 354 mm and 458 mm with 53% of the total rainfall occurring from July through September. On average 9.2% of annual rainfall water run off the watersheds into channels. The amount of eroded sediment varied greatly among watersheds, ranging between 0.3 and 2.8 ton per acre per year. This may result in the loss of soil productivity and development of gullies. The authors developed mathematical relationships that help to predict amount of runoff and erosion that will occur based on the amount of precipitation. It was found that high rainfall intensity rather than the total amount of rainfall was primarily responsible for producing surface runoff. Long term monitoring was found to be essential for accurate characterization of erosion process. On one of the watersheds a single large rainfall generated nearly a quarter of all sediments eroded from the area in 34 years. Further research is ongoing to investigate the role of changes in vegetation and the affect of drought on hydrological processes.
Technical Abstract: This study presents analysis of 34 years of precipitation, runoff and sediment data collected from 8 small (1.1 to 4.0 ha) semi-arid rangeland watersheds in southern Arizona, USA. Average annual precipitation ranged between 354 mm and 458 mm with 53% of the total rainfall occurring from July through September. Runoff depth was 3.5% to 13.9% of annual precipitation depth for individual watersheds and 9.2% on average. Measured sediment yields were highly variable, ranging between 0.85 t ha-1 y-1 and 6.69 t ha-1 y-1 with an average of 2.4 t ha-1 y-1. Ten percent of rainfall events with the largest sediment yields produced over 50% of the total sediment yield during the 34 year period. Linear regression models were developed to relate precipitation and runoff characteristics to watershed sediment yield. Maximum 30 min precipitation intensity was the primary factor affecting runoff, and runoff was the best predictor for sediment yield, explaining up to 90% of its variability. Runoff events with missing sediment data were estimated to account for 30% of the total sediment yield.