Research Overview

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Research Programs

SWRC scientists conduct research in collaboration with local landowners and ranchers, local government and organizations, university partners, other state, federal and international scientists and agencies. SWRC research ranges from understanding, describing and modeling hydrologic processes to applications of technology at the field level.

Field research in support of these program areas historically has been conducted primarily at the two main SWRC instrumented research areas Walnut Gulch Experimental Watershed (WGEW) and the Santa Rita Experimental Range (SRER). Paralleling the shifting and expanding nature of research conducted by SWRC scientists, field research has been geographically extended regionally to rangelands, forests and riparian areas in southeastern Arizona, nationally to other states and internationally to Mexico, China and Kazakhstan.

Research at the Southwest Watershed Research Center is currently conducted within the USDA-Agricultural Research Service’s National Program 201:Water Resource Management. The two major emphasis areas at SWRC are:

  1. Hydrology and
  2. Erosion and Sedimentation.


Arid and semiarid regions constitute over a third of the world’s landmass yet are under increasing population pressure. In the semiarid southwestern U.S. population is projected to increase over 50% by 2030 in comparison to 5-15% in other U.S. regions. This will dramatically increase society’s need to manage its water, soil, and nutrient resources to support people, agriculture and the environment. Hydrology research at SWRC is designed to improve our ability to manage watershed resources under the stress of increasing population and climatic variation. Methods of investigation include field and laboratory experimentation, as well as the development and use of state-ofthe- science watershed models. The goal of this project is to improve the ability to manage watersheds for reliable water supply, water quality, and ecosystem health by improving our ability to quantify semiarid water budget components; developing new model components and develop decision support systems that utilize remotely sensed data more fully; and, which consider the influence of ecosystems and their feedbacks. The specific objectives are:

  1. Quantify, and provide tools to estimate the impacts of urban development on the urban-rural interface as it affects surface runoff and groundwater recharge. This objective focuses on quantifying hydrologic impacts of rapid land-use change occurring in the semiarid southwest due to significant population growth and human relocation from other parts of the country. The urban-rural interface is changing the character of rangeland watersheds drastically. Ironically, as compared to development in more humid watersheds, this may lead to greater groundwater recharge.
  2. Develop improved watershed model components and decision support systems that more fully utilize and assimilate remotely sensed data for parameterization, calibration, and model state adjustment. Objective 2 is directed toward developing remote sensing and modeling tools to estimate evapotranspiration and CO2 fluxes and to extrapolate our findings from objectives 1 and 3 to larger and more diverse regions.
  3. Quantify primary semiarid water and energy balance components with emphasis on determining how surface processes and states influence water and carbon fluxes over a selected range of primary semi-arid vegetation types. A major component of objective 3 is to quantify the hydrologic and carbon cycle impacts and feedbacks of less rapid, non-development related, vegetation change that is occurring in many semiarid regions throughout the world with the encroachment of woody species into grasslands.

The products of the proposed research include better technologies and strategies to manage water and carbon resources and the information necessary to inform national policy on watershed management for both the present and future. The expected benefit of the program, working in conjunction with other scientific, political, and action agencies, is the long-term ability of our watersheds to sustain their residents, agriculture and the environment.

Erosion and Sedimentation

Erosion research at the SWRC addresses the lack of knowledge and decision tools to quantify the climatic and management effects on the sustainability of rangelands as affected by runoff and erosion. Federal action agencies have requested that a hydrologic and erosion model be developed to contribute to the ecological site description and National Resource Inventory data bases, to assess the efficacy of conservation practices for the Conservation Security Program, and to provide estimates of runoff and erosion for rangeland monitoring and ranch planning. To achieve these goals, three overall objectives have been identified.

  1. The first objective is to provide data bases, knowledge, and information on rangeland erosion at a range of spatial scales for the development, validation, and implementation of erosion decision tools. Within this overall objective, we are quantifying erosion, determining sources and sinks of sediment, and working to understand the biotic and abiotic influences on sediment yield at scales ranging from plots to watershed. The resulting data and knowledge will be used to validate hydrologic and erosion relationships and for parameter estimation equations for the rangeland erosion model. We also use remote sensing techniques to provide parameter estimation for large scale applications of the erosion model and rangeland health assessments.
  2. The second objective is to develop decision tools, including a rangeland specific hydrology and erosion model, for the planning and evaluation of sustainable rangeland management. Objective 2 consists of the development of a Rangeland Hydrology and Erosion Model , as well as an Economic Decision Support System (EDSS). The Erosion Model will be developed for a wide range of erosion related applications from the national scale – evaluating NRI data, the state scale - parameterizing NRCS ecological site descriptions, and the local scale - ranch planning. The EDSS will be used to calculate the cost benefit ratios of upland conservation management.
  3. Finally, the third objective is to develop design criteria for local ranchers and other land managers for conservation structures, to document the effectiveness of conservation structures on the land, and to provide the erosion model with data on conservation practices so that they are appropriately considered in assessing their impacts.

The anticipated products of this work will be 1. A method for interpreting semiarid sediment yield measured at the watershed outlet with respect to internal watershed erosion, transport, and deposition processes. 2. Sediment source identification, sediment tracking, and sediment budgets for model validation. 3. Criteria for the design of small rock-dam structures for purposes of sediment retention in semiarid rangeland landscapes. 4. New approaches for assimilating remote sensed data in the parameterization of rangeland hydrology and erosion models. 5. A hydrology and erosion model specifically developed for rangeland applications for use by action agencies and landuse managers. 6. Decision support tools for NRCS and public land managers on rangelands.

Experimental Watersheds

Research conducted at the Southwest Watershed Research Center is supported by outdoor laboratories at the Walnut Gulch Experimental Watershed near Tombstone, Arizona (since 1953) and at the Santa Rita Experimental Range south of Tucson (since 1975). The Walnut Gulch Experimental Watershed is the most densely gaged and monitored semiarid rangeland watershed in the world and is critical to improving scientific understanding of semiarid ecosystems.

Walnut Gulch Experimental Watershed

Studies on the Walnut Gulch Experimental Watershed at Tombstone, Arizona are part of the comprehensive research initiated in 1951 by the Research Division of the Soil Conservation Service. After considerable screening of prospective areas in Arizona, New Mexico, and Colorado, active research was begun in 1953 on the Walnut Gulch Experimental Watershed.

In 1954, the research and personnel were transferred to the Agricultural Research Service; and in 1961, the Southwest Watershed Research Center was established with headquarters in Tucson, Arizona.

Research is being conducted in cooperation with the Natural Resource Conservation Service, the local Soil Conservation Districts, and the ranchers who own the land of the watersheds.

Walnut Gulch enters the San Pedro river at Fairbank, Arizona. The study area comprises the upper 150 sq. km of the drainage basin. There is evidence that much of the area was grassland less than 100 years ago; but now shrubs dominate about 2/3 of the watershed. Creosote, tarbush, mortonia, and whitethorn are the most common shrubs. The remaining 1/3 is still grassland, dominated mainly by black grama, curly mesquite grass, and tobosa grass.

Santa Rita Experimental Watershed

The SWRC currently maintains eight subwatersheds within the Santa Rita Experimental Range (SRER), the oldest continuously maintained experimental range in the western United States (est. 1903). Established in 1975, these eight small watersheds are set up as paired watersheds.

Each watershed is instrumented with one recording raingage and one flume. The watersheds range in size from 1.05 ha to 4.01 ha. Four of the watersheds (#1 - 4) are in the shrub-dominated lower portion of the experimental range; the remaining four watersheds (#5 - 8) are located in the grass dominated upper portion of the experimental range.

The Santa Rita Experimental Range is currently administered by the University of Arizona, the SRER serves as a primary research location for members of the SWRC working in cooperation with the University.