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United States Department of Agriculture

Agricultural Research Service


Location: Southwest Watershed Research

2009 Annual Report

1a. Objectives (from AD-416)
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. 2. Develop improved watershed model components and decision support systems that more fully utilize and assimilate economic and remotely sensed data for parameterization, calibration, and model state adjustment. 3. Quantify primary semiarid water and energy balance components with emphasis on determining how surface processes and states influence water fluxes over a range of primary semi-arid ecosystem types. Improve the ability to manage watersheds for reliable water supply, water quality, and ecosystem health by improving the ability to quantify semiarid water budget components.

1b. Approach (from AD-416)
Methods of investigation include field and laboratory experimentation, as well as the development and use of state-of-the-science watershed models and the use of remote sensing for watershed characterization. Satellite derived rainfall will be evaluated using raingages for large area rainfall estimation, the enhancement of recharge due to urbanization will be examined in adjacent, well instrumented, natural and residentially developed catchments. High-resolution remotely sensing and rainfall simulator experiments will be used to evaluate the capability to remote estimate infiltration rates on compacted and constructed surface common to development at the urban-rural interface. Remote spectral surface responses will be combined with energy balance models and radiative transfer theory to estimate surface water, carbon and energy fluxes based on observations from a network of five eddy-covariance and two Bowen ratio towers. A number of modeling components for the Automated Geospatial Watershed Assessment (AGWA) will be developed or enhanced to enable a more realistic representation of watershed processes and best management practices. AGWA will be migrated to both the internet and ARCGIS platforms to enhance usability and access. In addition we will quantify the physical mechanisms and component fluxes that are responsible for the observed ecosystem-scale water and CO2 fluxes. Scientists will carry out this research at sites located across both a riparian and an upland woody plant encroachment gradient. Continue existing research and add activities to develop methods and techniques to quantify and predict water budgets under current and projected climate scenarios through direct measurements of evaporation and plant transpiration, and predict water savings by removal of invasive mesquite vegetation. FY09 Program Increase $228,600.

3. Progress Report
A number of important advances were made in plant water use and its interaction with carbon cycling which have important implications for plant response to global climate change. As noted last year, portions of our Walnut Gulch Experimental Watershed were overtaken by invasive Lehmann Lovegrass due to a long-term drought. In the transitional year between native and lovegrass-dominated plant communities, extensive ephemeral forb cover resulted in higher runoff velocities and the first measureable erosion in over 20 years. Subsequent spread by lovegrass has more than doubled soil water evaporation over the growing season, likely due to differences in lovegrass structure, which has dramatically lower basal area compared to the native grasses it replaced. Data from our ecosystem water and carbon flux monitoring stations was used to better understand the response from a variety of semiarid ecosystems to monsoon rainfall. Advances were also made in a multi-institutional research effort to assess the market value of ecosystem services. Work continued on the rangeland portion of the NRCS Conservation Effects Assessment Project (CEAP). Substantial progress was made within the MU in planning and conducting field experiments and research for this effort. Initial coupling of two well known ARS models (KINEROS2 and OPUS) was completed and testing is currently underway. The resulting couple model will be able to address a wider range of water quality, agriculture and land management practices, and nutrient cycling, including carbon. The coupled model will then be incorporated into the Automated Geospatial Watershed Assessment (AGWA) tool to facilitate its ease of use and application. The climate assessment tool developed under the companion Erosion project within the MU was also incorporated into AGWA enabling assessments of watershed impacts from generalized global climatic changes (AGWA was awarded a 2008 ARS Technology Award). Other significant technology transfer undertaken involved development of a major review of the importance of ephemeral and intermittent streams at the request of EPA in response to recent Supreme Court decisions as to whether these streams constitute “waters of the United States” under the Clean Water Act. The MU has continued its membership with the Congressionally recognized Upper San Pedro Partnership (USPP) ( to provide it with research and technical assistance. Research results from the MU supplied improved estimates of runoff increases resulting from urbanization. The USPP and all of its member agencies were awarded the 2008 U.S. Dept. of Interior’s Cooperative Conservation Award. On the international front an OECD travel fellowship was completed in New Zealand and another was awarded for Spain for FY10. Cooperative work with Kazakhstan continued. The MU was also quite successful in obtaining extramural grants either directly to the MU or to university collaborators in which MU scientists are Co-Investigators. Two awards were received from NASA, one from the NOAA COMET program; and one from the DOD Strategic Environmental Research and Development Program (SERDP).

4. Accomplishments
1. Rainfall effects on the uptake and release of carbon dioxide from ecosystems. Carbon dioxide (CO2) concentrations in the atmosphere have risen dramatically since the start of the Industrial Revolution. There is major concern about how this rise in CO2 has and will affect global climate. All ecosystems take in and release CO2 and so there has also been a large research effort to understand how these CO2 increases will affect and modify the cycling of carbon dioxide within ecosystems. ARS scientists in Tucson, AZ developed a method to separate measurements of total ecosystem carbon dioxide exchange into carbon dioxide release (respiration) and uptake (photosynthesis) and applied this method to data from a network of CO2 flux measurement sites in southern Arizona. Results indicate that the influence of precipitation on respiration varies depending on landscape position and ecosystem type. Therefore, future changes in precipitation patterns associated with global warming will likely result in widely variable ecosystem responses in CO2 cycling in this region.

2. Dynamics of riparian plant water in arid and semiarid basins. Riparian systems in arid and semiarid areas harbor and maintain a far greater percentage of the biodiversity than the area they occupy on the landscape but are under increasing stress due to competing water demands. Coordinated, interdisciplinary investigators by ARS researchers in Tucson, AZ, Univ. of Arizona, Arizona State Univ., Geological Survey, and Univ. of Wyoming scientists quantified groundwater and surface water conditions required to maintain several distinct levels of riparian habitat quality. In addition, ground-based and remotely sensed methods to estimate the amount of water used by dominant riparian systems in the Southwest and in particular the San Pedro River. This review summarizes the results of this broad area of research. These new methods and the resulting knowledge have enabled improved management of basin water resources to preserve and restore riparian river systems in the Southwest where there are competing water needs for agriculture, municipal, military, and industrial use.

3. Integrating science and policy for water management. Very few elected officials and decision makers read scientific journals which often results is decisions being made without the best science and the agonizingly slow transfer of new research into the decision making arena. Via a long-term membership and commitment to the Upper San Pedro Partnership, ARS researchers at the ARS-Southwest Watershed Research Center and other Federal agencies have, designed, conducted, published and translated original research to directly meet the needs of elected officials and policy makers of the Partnership. The Partnership is viewed as a model for integrating science with policy and decision making to address complex natural resource and economic challenges. It has been Congressionally recognized and has received the 2008 Department of Interior Cooperative Conservation Award. This interaction and partership sets an example for similiar watershed partnerships as it was designated by the UNESCO HELP (Hydrology, Environment, Life, and Policy) program as an operational example basin where science is being effectivey integrated into policy and decision making.

4. A novel technique for measuring the amount of moisture in the soil. Moisture in the soil moderates regional climate, much as the ocean does. Measuring soil moisture is crucial for weather and short-term climate forecasting, but soil moisture measurements useful for these applications are difficult to make. ARS scientists, in Tucson, AZ developed and tested a novel technique that uses the dependence of the amount of naturally-occurring, low-energy, cosmic rays above the ground on the water content of soil. The measurement with a standard neutron detector placed a few meters above the ground takes minutes to hours, permitting high-resolution, long-term monitoring of undisturbed soil moisture conditions. The large area of measurement makes the method suitable for weather forecast models and for calibration of satellite sensors, and the measurement depth makes the probe ideal for studies of atmosphere, plant, and soil interactions and has the potential to revolutionize the way soil moisture is measured.

5. Soil evaporation response to the invasion of lehmann lovegrass (Eragrostis lehmanniana) into a semiarid watershed. The invasion of the exotic grass, Lehmann lovegrass, into native desert grasslands is of great concern to ranchers and land managers throughout the Southwestern United States. Lehmann lovegrass displaces native grasses and reduces plant and animal diversity. There is far less known about the impact of Lehmann lovegrass invasion on ecosystem hydrology, despite the fact that it is a common invasive species in the desert southwest where water is scarce. ARS scientists at Tucson, AZ used multiyear measurements of a naturally occurring vegetation transition to quantify the change in surface water balance associated with Lehmann lovegrass invasion. Results showed that the water loss from soil evaporation over the growing season doubled with Lehmann lovegrass invasion, which in turn will determine the persistence and management of Lehmann lovegrass in desert grasslands.

6. How access to groundwater affects mesquite ecosystem growth. Mesquite have expanded their range in many dry parts of the U.S. With this change in ecosystem structure, associated changes in how ecosystems cycle carbon and water are expected but largely unknown. ARS scientists at Tucson, AZ examined the sensitivity of the carbon cycling in ecosystems invaded by mesquite to precipitation in southern Arizona for a range of shrub sizes and in a riparian and upland setting. For mesquite in the upland setting, mesquite activity was closely linked to the onset of the summer rainy season. In contrast, only the smallest of riparian mesquite were physiologically responsive to the summer rainfall while the larger riparian mesquite were physiologically insensitive to the rains. These results show that woody plant encroachment in riparian areas will have a bigger impact on ecosystem carbon cycling due to the deeper roots of shrubs which access the groundwater more readily.

7. How soil moisture affects ecosystem water loss in southwestern U.S. Evapotranspiration (ET) is a process that links the surface water, energy and carbon balances with the physiological activity of plants, especially for water-limited ecosystems. While the influence of the greening of monsoon-dominated ecosystems on the surface energy exchange has been recognized, little is known of its effects on the relationship between soil moisture and ET. ARS scientists at Tuscan, AZ utilized a set of multi-year observations in southwestern U.S. and northwestern Mexico to determine the relationship between soil moisture and ET and how it changes in response to vegetation activity. This analysis can be used to improve the understanding and prediction of land-atmosphere interactions in the water-limited ecosystems of this region.

8. Plant responses to extreme drought in the Southwest U.S. The Southwest U.S. has experienced record drought intensity from 1999 to 2005. Prolonged droughts are important ecological disturbances in deserts, resulting in widespread plant mortality that alters plant community structure and species composition. ARS scientists in Tuscan, AZ demonstrated that basic soil hydrological features strongly influenced patterns of whole-plant mortality and plant canopy die-back in deciduous and evergreen desert shrubs, and that long-lived evergreens with different leaf sizes and canopy displays used distinct mechanisms to physiologically “scrub” excess light energy and protect photosynthetic function under severe drought conditions. These studies can be used to better predict variation in the ecological effects prolonged drought can have across these water-limited systems.

5. Significant Activities that Support Special Target Populations
SWRC scientists are actively participating in a mentoring program through collaboration with the University of Arizona/NASA Space Grant Program. The program matches outstanding undergraduate students from underrepresented groups with mentors from the SWRC to work together on projects developed by mentors. The ultimate goal of the program is to "grow our own" scientists from underrepresented groups. Over the last five years, twelve students (Hispanic, African-American, women) have come through the program. ARS researcher completed her Masters of Science in the School of Natural Resources at the University of Arizona. SWRC scientist served as a co-advisor for her research. SWRC scientists and another researcher mentored two female undergraduates in environmental research during the summer of 2009. The undergraduates were recipients of a National Science Foundation’s Research Experience for Undergraduates fellowships through the University of Arizona. SWRC scientists are heavily involved in the NSF Sustainability of semiArid Hydrology and Riparian Areas (SAHRA) Science and Technology Center. This program has a very active program in outreach and education of Native American communities to increase hydrologic awareness career opportunities in science.

Review Publications
Jennerette, G.D., Scott, R.L., Huxman, T.E. 2008. Whole ecosystem metabolic pulses following precipitation events. Functional Ecology. 22:924-930. doi: 10.1111/j.1365-2435.2008.01450.x

Moran, M.S., Scott, R.L., Keefer, T.O., Emmerich, W.E., Hernandez, M., Nearing, G.S., Paige, G.B., Cosh, M.H., O'Neill, P.E. 2009. Partitioning evapotranspiration in semiarid grassland and shrubland ecosystems using time series of soil surface temperature. Agricultural and Forest Meteorology. 149:59-72.

Hamerlynck, E.P., Huxman, T.E. 2009. Ecophysiology of two Sonoran Desert evergreen shrubs during extreme drought. Journal of Arid Environments. 73: 582-585.

Xiao, J., Zhuang, Q., Baldocchi, D., Law, B., Richardson, A., Chen, J., Oren, R., Starr, G., Noormets, A., Ma, S., Verma, S., Wharton, S., Wofsy, S., Bolstad, P., Burns, S., Cook, D., Curtis, P., Drake, B., Falk, M., Fishcer, M., Foster, D., Gu, L., Hadley, J., Hollinger, D., Katul, G., Litvak, M., Martin, T., Matamala, R., Mcnulty, S., Meyers, T., Monson, R., Munger, J., Oechel, W., Paw U, K., Schmid, H., Scott, R.L., Sun, G., Suyker, A., Torn, M. 2008. Estimation of Net Ecosystem Carbon Exchange for the Conterminous United States by Combining MODIS and AmeriFlux Data. Agricultural and Forest Meteorology. 148:1827-1847.

Vivoni, E.R., Moreno, H.A., Mascaro, G., Rodriguez, J.C., Watts, C.J., Garatuza-Payan, J., Scott, R.L. 2008. Observed relation between evapotranspiration and soil moisture in the North American monsoon region. Geophysical Research Letters. 35, L22403. doi:10.1029/ 2008GL036001.

Zreda, M., Desilets, D., Ferre, T., Scott, R.L. 2008. Measuring soil moisture content non-invasively at intermediate spatial scale using cosmic-ray neutrons. Geophysical Research Letters, 35, L21402. doi:10.1029/2008GL035655.

Potts, D.L., Scott, R.L., Cable, J.M., Huxman, T.E., Williams, D.G. 2008. Sensitivity of mesquite shrubland CO2 exchange to precipitation in contrasting physiographic settings. Ecology. 89(10):2900–2910.

Williams, D., Scott, R.L. 2009. VEGETATION-HYDROLOGY INTERACTIONS: DYNAMICS OF RIPARIAN PLANT WATER USE. In: Ecology and Conservation of the San Pedro River. Ed. by J. C. Stromberg and B. J. Tellman. Tucson: University of Arizona Press. p. 37-56.

Stromberg, J., Dixon, M.D., Scott, R.L., Maddock, T., Baird, K., Tellman, B. 2009. Status of the Upper San Pedro River (United States) Riparian Ecosystem. In: Ecology and Conservation of the San Pedro River. Ed. by J. C. Stromberg and B. J. Tellman. Tucson: University of Arizona Press. p.371-387.

Hamerlynck, E.P., Mcauliffe, J.R. 2008. Soil-dependent canopy-die back and plant mortality in two Mojave Desert shrubs. Journal of Arid Environments. 72:1793– 1802. . Journal of Arid Environments.

Cosh, M.H., Jackson, T.J., Moran, M.S., Bindlish, R. 2008. Temporal persistence and stability of surface soil moisture in a semi-arid watershed. Remote Sensing of Environment. 112:304-313.

Scott, R.L., Huxman, T.E., Williams, D.G., Hultine, K.R., Goodrich, D.C. 2008. Quantifying Riparian Evapotranspiration. Southwest Hydrology. 7(1): 26-27.

Richter, H., Goodrich, D.C., Browning-Aiken, A., Varady, R. 2009. CHAPTER 9: INTEGRATING SCIENCE AND POLICY FOR WATER MANAGEMENT. In: Ecology and Conservation of the San Pedro River. Ed. by J. C. Stromberg and B. J. Tellman. Tucson: University of Arizona Press. p. 388-408.

Semmens, D.J., Goodrich, D.C., Unkrich, C.L., Smith, R.E., Woolhiser, D.A., Miller, S.N. 2008. KINEROS2 and the AGWA Modeling Framework. Chapter 5: In Hydrological Modelling in Arid and Semi-Arid Areas (H. Wheater, S. Sorooshian, and K. D. Sharma, Eds.). Cambridge University Press, London. Pp. 49-69.

Last Modified: 2/23/2016
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