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- article by Kristine Uhlman, AZ NEMO Program
Arizona NEMO is the first attempt to adopt the national NEMO approach to conditions in the semiarid, western United States. For Arizona, the program is structured within a watershed-defined template and we are focusing our educational outreach efforts on the policy makers, planners, and land use decision makers impacted by nonpoint source water quality issues—including the Stormwater Phase II MS4 communities. GIS-based tools, such as the Automated Geospatial Watershed Assessment (AGWA) tool are being used to illustrate the effects of land use change on runoff and erosion, and to model sediment load—the principal nonpoint source pollutant in Arizona. All four of the deserts of North American occur in Arizona, and with a geographical extent equivalent to the combined size of the states of Maine, New Hampshire, Vermont, Massachusetts, Connecticut, Rhode Island and New York, stormwater could not be more distinct than from other parts of the country. GIS-based hydrologic watershed modeling software has been developed by the USDA-ARS Southwest Watershed Research Center and the University of Arizona in collaboration with the EPA’s National Research Laboratory. The Automated Geospatial Watershed Assessment software (AGWA) performs hydrologic model parameterization and results visualization for existing watershed scale hydrologic simulation models of stormwater runoff. AGWA allows the user to spatially visualize changes in hydrologic response through the use of remotely sensed land cover scenes. GIS-AGWA modeling has assisted in characterizing stormwater flow response to urbanization within the San Pedro River Watershed (Miller, et. al., 2002; Hernandez, et. al., 2003), and to model. Satellite imagery of the San Pedro Watershed taken in June of 1973, 1986, 1992 and 1997, exhibited significant land use change and urbanization, especially within the subwatershed running through the developing community of Sierra Vista. The Sierra Vista subwatershed exhibited profound changes in land use, with nearly a 415% increase in urban area and 38% decrease in desert scrub and grassland. Runoff and sediment yield have been increasing in the urban subwatershed, and flashier flood response has been observed. It is expected that the decreased infiltration capacities and roughness associated with urbanization would increase the potential for localized large-scale runoff and erosion events, but this was not found to be true.
Images above: Land cover change on the Upper San Pedro, after Miller, et al., 2002. (The San Pedro Watershed straddles the Arizona, Mexico boundary, shown as a horizontal line in each of the figures above.) The Sierra Vista simulations suggest that the increased erosion observed to occur within an arid watershed undergoing a transition to urbanization may be due to more than urban-generated sediments. Sediment starved urban storm runoff due to the increase in impervious surfaces may be responsible for excessive down cutting in ephemeral streams and washes. Without sufficient sediment load, ephemeral streams will down-cut and erode the stream bed aggressively, contributing to the transport and re-distribution of streambed sediment. In the natural south west desert setting, intermittent storm flow acts as a conveyer belt, reloading with sediment with each storm event and transporting sediment on down-gradient. The GIS/AGWA modeling simulations of the Sierra Vista urban subwatershed for the 5-year, 30-minute design storm calculated runoff increasing by 177%, and sediment load increasing by 851% in response to urbanization over the 24 years between 1973 and 1997. Hydrologic modeling and watershed assessments of the Upper Gila, Bill Williams, and Verde Watersheds are nearing completion, and examples from this work will be posted on the Arizona NEMO website in the future. |
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