Stormwater takes on a new dimension during times of drought and amid concern for climate change, especially in the desert Southwest of the United States where our rivers are mostly dry beds of sand and gravel. Stormwater Phase II Municipal Separate Storm Sewer System (MS4) communities seeking appropriate best management practices (BMPs) to address sediment load — the principal nonpoint source pollutant in the arid Southwest — may find base flow conditions exhibiting no water and a high potential for in-channel scour and sediment loading. In this environment, water supply and water quality concerns have equal importance for decision makers, and hydrologic alteration is an important source of impairment.

Unlike small MS4 communities in other regions of the country, Southwestern BMPs will focus on stormwater retention and water harvesting to slow or reduce flow to our dry river beds, and to enhance groundwater recharge and augment water supply. During the first year of Stormwater Phase II, new municipal ordinances requiring the construction of retention features and enforcing water-harvesting practices within urban landscaping will be implemented, following the lead of the Stormwater Phase I efforts by both Phoenix and Tucson, Ariz.

Arid, Semiarid Climate Stormwater Characteristics: Phoenix

All four of the deserts of North America can be found 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 conditions in Arizona are distinctly different from other parts of the country. In addition, as the fastest growing region in the United States, the desert Southwest exhibits growth and development concerns that are global in implication: one-third of the earth’s land mass is classified as arid or semiarid.

As the fastest growing region in the United States, the desert Southwest exhibits growth and development concerns that are global in implication: one-third of the earth’s land mass is classified as arid or semiarid.

The city of Phoenix, a Phase I MS4 community, is surrounded by numerous Phase II Small MS4s over the vast urban corridor along Interstate Routes 10 and 17. The city is located within the Salt River Watershed, a tributary of the Gila River. The U.S. Geological Survey (USGS), working in cooperation with the Flood Control District of Maricopa County, has published an extensive characterization of stormwater in the Phoenix urban area.¹

From October 1991 to October 1998, the USGS found that the duration of actual stream flow following a storm is short, typically 1 hour to 2 hours. Storm event characteristics vary significantly between the summer monsoon convection storms and winter regional cold front precipitation. Mean monthly precipitation varies from 0.12 inch (May) to 1.00 inch (December), with mean maximum temperatures ranging from 65.9 degrees Fahrenheit (January) to 105.3 degrees Fahrenheit (July). Summer monsoon storms are intense — they exhibit dramatic convectional thunderheads, they are usually local or less than a square mile in size, and they often generate spontaneous localized flooding and erosion. Total individual storm rainfall can vary 1 inch or more from one side of metropolitan Phoenix to the other. Stormwater discharge was found to increase suspended solids (sediment) concentrations; however, the mean concentration of dissolved solids in urban stormwater was lower than in the upgradient, non-urban stream flow reach along the Salt River.

Arizona NEMO

Southwestern MS4 communities are finding resources through the University of Arizona’s Cooperative Extension Nonpoint Education for Municipal Officials (NEMO) Program, which is partnered with the Arizona Department of Environmental Quality. Educational outreach is an important aspect of the NEMO program, with stakeholder-group workshops, the Arizona NEMO Web site and a toolbox of Arid Region BMPs under development. NEMO recognizes that management of nonpoint source pollutants is inherently spatial, and supports the use of geographical information systems (GIS) to simulate and predict impact of land-use change. Arizona NEMO integrates watershed management and planning to emphasize the linkages between water supply and quality with research-based, non-advocacy professional education to engage stakeholders and foster better water- and land-use decisions.

At the national level, the NEMO program has been very successful in helping to mitigate nonpoint source pollution. The goal of NEMO is to educate land-use decision makers and encourage voluntary actions that will mitigate nonpoint source pollution and protect our natural resources. Arizona NEMO was 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 focuses educational outreach efforts on the policy makers, planners and land-use decision makers impacted by nonpoint source water quality issues — specifically the 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, as well as supporting the educational effort.

Arid, Semiarid GIS AGWA Application: San Pedro Watershed

The San Pedro River, a tributary of the Gila River, flows north into southeastern Arizona from Sonora, Mexico, and is an exceptional example of desert biodiversity. Rural ranching land use is shifting to irrigated agriculture and urban development. Across the watershed are desert shrub-steppe, riparian ecosystems, grasslands, agriculture, oak and mesquite woodlands and, at higher elevations, pine forest. The basin supports among the highest number of mammal species in the world, and the riparian corridor provides nesting and migration habitat for more than 400 bird species.

GIS-based hydrologic watershed modeling software has been developed by the U.S. Department of Agriculture-Agricultural Research Service (USDA-ARS) Southwest Watershed Research Center and the University of Arizona in collaboration with the U.S. Environmental Protection Agency’s (EPA) National Exposure Research Laboratory. AGWA software performs hydrologic model parameterization and results visualization for existing watershed scale hydrologic simulation models. AGWA allows users 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 and land cover change within the San Pedro River Watershed.^2,3,4

Storm event characteristics vary significantly between the summer monsoon convection storms and winter regional cold front precipitation.

Satellite imagery of the San Pedro Watershed taken in June of 1973, 1986, 1992 and 1997 exhibited significant land use change, 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 percent increase in urban area and 38 percent decrease in desert scrub and grassland. Due to the increased municipal water consumption and irrigated agriculture, the watershed basin is shifting toward an increasing reliance on groundwater, as are most of the urban areas of Arizona. Runoff and sediment yield have been increasing in the urban subwatershed, and flashier flood response has been observed.

Hydrologically, the area is typical of the desert Southwest, exhibiting relative extremes in components of hydrologic cycle — low annual precipitation, dry heat and highly localized intense rainfall with high potential for runoff and erosion (as was reported by the USGS for the Phoenix metropolitan area). Stream channels are predominantly ephemeral, and those portions of the San Pedro River that are perennial in flow are at risk of losing baseflow due to declining groundwater table elevations.

Runoff and sediment yield from the urban Sierra Vista subwatershed was simulated using design rainfall events, and the hydrographs resulting from these events showed dramatic increases in runoff volume, runoff rate and sediment yield. Decreased infiltration capacities and roughness associated with urbanization increase the potential for localized large-scale runoff and erosion events. Simulation results for the percent increase in sediment load did not equal the simulated increase in runoff, an observation that contradicts findings in other urban settings where the causal relationship is more direct.

The Sierra Vista simulations suggest that the increased erosion observed within an arid watershed undergoing a transition to urbanization may be due to more than urban-generated sediments. Relatively clear urban storm runoff resulting from the increase in impervious surfaces is sediment starved, and may be responsible for excessive down-cutting in ephemeral streams and washes. If the contributed runoff enters the channel without a sufficient sediment load, ephemeral streams will degrade and erode the stream channel aggressively, contributing to the transport and re-distribution of streambed sediment. In the natural setting, intermittent storm flow acts as a conveyer belt, reloading with sediment with each storm event and transporting sediment on down gradient. With the increased flow volume associated with urbanization in the Sierra Vista area, the volume of mobilized sediment is significant. The GIS/AGWA modeling simulations for the 5-year, 30-minute design storm calculated runoff increasing by 177 percent, and sediment load increasing by 851 percent in response to urbanization over the 24 years between 1973 and 1997.

TMDLs in the Arid, Semiarid Southwest

EPA establishes criteria to evaluate pollutant contribution to streams from land-use impacts and other nonpoint sources under Section 303(d) of the Clean Water Act. Total maximum daily loads (TMDLs) are designated as a tool for watershed-based management decision making and identification of the adequacy of existing water quality management practices. In Arizona, sediment is the principal nonpoint source pollutant with 125.4 stream miles (<1 percent of the assessed stream miles and 36 percent of the impaired stream miles) classified as impaired due to excessive sediment, which is over three times greater than impairment caused by the next leading constituent.

The goal of NEMO is to educate land-use decision makers and encourage voluntary actions that will mitigate nonpoint source pollution and protect our natural resources.

BMPs to address sediment discharge for the arid, semiarid small MS4 operators are consistent with the standard Stormwater II technical guidance measures for maintenance and control of erosion. Examples include such measures as grading and excavation ordinances, post-construction runoff reduction and development of municipal authority to inspect construction sites and enforce control measures. MS4 operators also are implementing dry weather inspection on a semiannual basis in response to the two cycles of stormwater flow, with inspections prior to the summer monsoon and during the fall prior to the winter frontal storms.

Where the Arizona MS4 best management practices differ from other regions is the emphasis on retention of storm flow and water harvesting. The City of Tucson, a Phase I MS4, is currently developing a Stormwater Ordinance, but has already promulgated the Land Use Code (LUC), which states that (urban) landscaping is intended to accomplish energy, water and other natural resource conservation, and “reduce soil erosion by slowing stormwater runoff” and “assisting in ground water recharge” LUC Sec. 3.7.1.1.A.. Suggested water harvesting techniques include the construction of “microbasins,” drainage swales, french drains to enhance subsurface drainage and gabions within streambeds and washes. Many of these structures can be retrofitted on existing landscapes, such as parking lots, so as to retain stormwater. As the Phase II MS4s begin implementation of their Stormwater Management Programs (BMPs), engineered storm water retention structures and water harvesting on urban landscapes will take the lead in BMPs.

Summary

The arid climate and unique stormwater hydrology of the Southwest, coupled with increased urbanization, will only exacerbate the trend toward increasing erosion and stream sediment load. The limited perennial stream reaches, and their vulnerability to baseflow loss due to increased reliance on groundwater, requires careful management and better land use decisions so as to assure the sustainability of water resources, community character and long-term economic health of Arizona. With the ongoing implementation of Phase II SMPs and the support of the Arizona NEMO program, Arizona Small MS4s will meet their goals with the support of GIS models to simulate and predict impact of land-use change and appropriate management of nonpoint source pollutants.

Arizona NEMO is a Charter Member of the National NEMO Network. Affiliated University of Arizona Cooperative Extension programs include: Arizona Project WET, Master Watershed Steward Program and Arizona FireWise.

References

1. Fossum, Kenneth D., C. M. O’Day, B. J. Wilson and J. E. Monical: 2001. Statistical Summary of Selected Physical, Chemical, and Toxicity Characteristics and Estimates of Annual Constituent Loads in Urban Stormwater, Maricopa County, Arizona. U.S. Geological Survey Water Resources Investigations Report 01-4088, prepared in cooperation with the Flood Control District of Maricopa County. 33 pp.

2. Hernandez, Mariano, W. G. Kepner, D.J. Semmens, D. W. Ebert, D.C. Goodrich and S. N Miller: 2003. Integrating a Landscape/Hydrologic Analysis for Watershed Assessment, in proceedings of First Interagency Confine on Research in the Watersheds, Oct. 27-30, 2003. United States Department of Agriculture, Agricultural Research Service.

3. Levick, L.R., R.L. Scott, S.N. Miller, D.J. Semmens, D.P. Guertin and D.C. Goodrich, 2003. Application of a land cover modification tool and the Automated Geospatial Watershed Assessment tool (AGWA) at Fort Huachuca, AZ. Society for Conservation GIS (SCGIS) Annual Conference, July 3-5, 2003, Pacific Grove, CA.

4. Miller, Scott N., W. G. Kepner, M.H. Mehaffrey, M. Hernandez, R. C. Miller, D.C. Goodrich, K.K. Devonald, D.T. Heggem and W. P. Miller: 2002. “Integrating Landscape Assessment and Hydrologic Modeling for Land Cover Change Analysis,” in the Journal of the American Water Resources Association, Vol. 38, No. 4, August. P. 915- 929.

e-Sources

• University of Arizona’s School of Natural Resources — www.ag.arizona.edu/srnr


• Automated Geospatial Watershed Assessment (AGWA) — www.tucson.ars.ag.gov/agwa.

This article originally appeared in the May/June 2004 issue of Water & Wastewater Products, Volume 4, Number 3.