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SIMULATED CHANGES IN RUNOFF AND SEDIMENT IN DEVELOPING AREAS NEAR BENSON, ARIZONA
Citation:
LEVICK, L., D. J. SEMMENS, D. GOODRICH, W. G. KEPNER, J. BRUSH, R. LEIDY, AND E. GOLDMANN. SIMULATED CHANGES IN RUNOFF AND SEDIMENT IN DEVELOPING AREAS NEAR BENSON, ARIZONA. U.S. Environmental Protection Agency, Washington, DC, 2006.
Impact/Purpose:
The primary objectives of this research are to:
- Provide information on the variability in water supply that can be expected under varying climatic conditions. Early efforts will be focused on assembling regional databases for at least two counties (Mecklenberg County and York County) within SEQL region that can be used for water supply generation and model development.
- Develop tools that will help improve our ability to evaluate, study, and model linkages between different types of environmental systems: hydrologic, geomorphic, ecological, and climatic.
- Explore the use of annual and seasonal measurements of large lake surface temperatures as a new ecological indicator of the overall thermal content of those lakes, and construct an estimator of seasonal large lake heat budgets.
Description:
Residential and commercial development is occurring with unprecedented speed throughout the American Southwest. It is projected that from 1995 to 2025, the population in the six Southwestern states of California, Nevada, Arizona, New Mexico, Utah and Colorado will increase by more than 50%, while the remainder of the country is projected to grow only 10 to 15%. More recently the Arizona Department of Commerce has projected the state's population will increase three-fold in the next several decades. This scale and rapid pace of development presents special challenges to the review and permitting process as required under Section 404 of the Clean Water Act (CWA) and the National Environmental Policy Act (NEPA). Many of the areas undergoing rapid development are in arid and semiarid regions whose watersheds and associated streams exhibit ephemeral or intermittent flow. The standard process for CWA permitting associated with new development rarely considers the special attributes and circumstances encountered in these environments. In addition, rapid urbanization can present a challenge in assessing the cumulative impacts of development on watersheds and landscapes when permitting is conducted piecemeal over multiple parcels in the same region.
Geospatial tools are needed to enable the rapid assessment of proposed developments to identify circumstances that may require more in-depth analysis. The Automated Geospatial Watershed ssessment (AGWA) tool is a GIS-based interface for the well established Soil and Water Assessment Tool (SWAT) and Kinematic Runoff and Erosion Model (KINEROS2) watershed models, and is capable of such assessments (see http://www.tucson.ars.ag.gov/agwa/ and/or http://www.epa.gov/esd/land-sci/agwa/). In this study AGWA was applied to an 8,200-acre study area proposed for development located near Benson, Arizona. Pre- and proposed postdevelopment land-cover conditions were simulated using KINEROS2 through the AGWA interface and subjected to several design storms. Changes in runoff and sediment yield due to the proposed changes in land cover were computed for five watersheds encompassing the study area and extending to the main-stem of the San Pedro River. The largest average changes in runoff volume (~+413%) and sediment yield (~ +231%) across the five watersheds occurred for the two-year, one-hour design storm. For the 10-year, one-hour design storm these changes in runoff and sediment yield were reduced to roughly +23%.
This analysis represents a preliminary, qualitative assessment of anticipated hydrologic change resulting from proposed development in the Benson area. Results definitively indicate that the proposed land-use changes will result in significant alteration of the hydrologic regime both within and downstream of the impacted watersheds where they empty into the San Pedro River. Quantitative predictions of these impacts would require substantial additional information that is not presently available for the ungauged study-area watersheds. Insufficient information on the design and placement of flood mitigation measures (detention basins, riparian buffers, water harvesting, recharge wells, open space infiltration galleries), for example, did not allow this study to assess the impacts of such structures on runoff changes. In addition, a larger-scale analysis of development within the San Pedro River Basin would be necessary to ascertain cumulative impacts to the river and identify areas of critical concern.