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Title: TRENDS IN PRECIPITATION RUNOFF AND IN CHANNEL VEGETATION IN THE USDA-ARS WALNUT GULCH EXPERIMENTAL WATERSHED 1735

Author
item Nichols, Mary
item Nearing, Mark
item Shipek, David

Submitted to: Proceedings of the World Water and Resources Congress
Publication Type: Proceedings
Publication Acceptance Date: 3/10/2005
Publication Date: 5/14/2005
Citation: Nichols, M.H., Nearing, M.A., Shipek, D.C. 2005. Trends in precipitation runoff and in channel vegetation in the usda-ars walnut gulch experimental watershed. Proc. ASCE, World Water and Environmental Resource Congress, May 15-19, Anchorage, AK.

Interpretive Summary: Channels in the semiarid southwestern US are shaped by infrequent flows that occur primarily during the summer monsoon season. These flows erode, transport, and deposit sediment. The amount of sediment that is stored in the channel or delivered to connecting rivers varies with runoff. Changes in the sediment in the channels can affect water supply by altering water storage and water quality by altering sediment delivery to downstream locations. The number of large flood flows decreased on the Walnut Gulch Experimental Watershed from 1956-1996, and during the same time period winter precipitation increased. Research was conducted using air photos and field measurements to identify and quantify changes in the Walnut Gulch channel that have occurred in response to the trends in precipitation and runoff. Since 1935 there has been an increase in vegetation in the channel. The vegetation increase may be a response to a combination of increased available moisture during non-summer months and fewer large magnitude flow events during the summer months.

Technical Abstract: Precipitation and runoff play a critical role in adjustments to channel morphology and in channel vegetation establishment in semiarid regions. In southeastern Arizona, more than 60% of the precipitation occurs during July, August, and September, and almost all of the runoff is produced during these months. Data collected from the network of raingages and runoff measuring flumes on the USDA-ARS Walnut Gulch Experimental Watershed were analyzed to quantify trends in rainfall and runoff. Since the mid 1950s there has been an increase in precipitation during non-summer months. There was a decrease in annual runoff and a decrease in the magnitude of the maximum annual runoff event during the same time period. Air photos and field measurements were combined to quantify vegetation changes in the main Walnut Gulch channel. In channel vegetation was found to have increased since the mid 1930s. The vegetation increase may be a response to a combination of increased available moisture during non-summer months and fewer large magnitude flow events during the summer months. Although the trends identified may represent short term fluctuations in rainfall and runoff, an improved understanding of the relationships among precipitation, runoff, and channel vegetation establishment is important for understanding the potential impacts of larger-scale climate changes on water quality and quantity.