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Title: Simulated canges in runoff and sediment in developing areas near Benson, Arizona

Author
item LEVICK, L.
item SEMMENS, D.
item Goodrich, David - Dave
item KEPNER, W.
item BRUSH, J.
item KEUDY, R.
item GOLDMANN, E.

Submitted to: Environmental Protection Agency
Publication Type: Other
Publication Acceptance Date: 12/10/2006
Publication Date: 12/30/2006
Citation: Levick, L.R., Semmens, D., Goodrich, D.C., Kepner, W.G., Brush, J., Keudy, R.A., Goldmann, E. 2006. Simulated canges in runoff and sediment in developing areas near Benson, Arizona. EPA, Region 9 Report-(EPA/600/R-06/158). 1-21.

Interpretive Summary: The unprecedented speed and scale of development throughout the American Southwest presents special challenges to the review and permitting process as required under the Clean Water Act (CWA) and the National Environmental Policy Act (NEPA). Many 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 for new development rarely considers the special attributes and circumstances encountered in these environments. In addition, rapid urbanization can present a challenge in assessing cumulative impacts when permitting is conducted piecemeal over multiple parcels in the same region. The study area, located near Benson, Arizona, in the San Pedro Valley, will convert approximately 8,200 acres of previously undisturbed land that drains directly into the San Pedro River to a planned development. The San Pedro River, nationally known as being one of the last free-flowing rivers in the Southwest, is a critical migration corridor for hundreds of bird species and serves as important habitat for many other regionally-declining species of plants, fish, and wildlife. Upstream from the proposed development is the San Pedro Riparian National Conservation Area (SPRNCA), created in 1988 to protect nearly 40 miles of river and riparian area, and its biological, educational, recreational and cultural resources. Although not federally protected as an RNCA, the San Pedro River downstream (north) of the study area also contains many of the same highly valued attributes and is critical to maintaining the ecological integrity of upstream areas. Changes to the hydrology of that environment, such as increases or decreases in flow or sediment volumes, can have serious impacts on the aquatic ecosystem and the health of those areas. This study examines the effects of development on the hydrology of this portion of the San Pedro River watershed. The proposed development resulted in substantial relative changes in runoff and sediment yield that are attributable to modifications in land cover, as predicted by the AGWA/KINEROS2 rainfall-runoff-erosion model. Consistent with established principles, the relative changes are largest for the smallest design storms and decrease with increasing design-storm size. In all cases, net runoff and sediment yield increased at the watershed outlets due to urbanization and more impermeable surfaces. Expected adverse environmental consequences from such increases may include degraded water quality from sediment and pollutant transport, erosion and alteration of the stream channel, habitat destruction, decreased biological diversity, and increased flooding.

Technical Abstract: The large scale and rapid pace of residential and commercial development throughout the American Southwest 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). The watersheds and associated streams in this arid and semiarid region 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, assessing the cumulative impacts of development on watersheds and landscapes when permitting is conducted piecemeal over multiple parcels in the same region is challenging. The Automated Geospatial Watershed Assessment (AGWA) tool and the Kinematic Runoff and Erosion Model (KINEROS2) watershed model were applied to an 8,200-acre study area proposed for development near Benson, Arizona. Pre- and proposed post-development land-cover conditions were simulated using 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%. Results definitively indicate that the proposed land-use changes due to development 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.