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United States Department of Agriculture

Agricultural Research Service


Location: Southwest Watershed Research

2010 Annual Report

1a. Objectives (from AD-416)
The existing project objectives (as given below) reflect the redirection towards quantification of climate change effects: 1. Develop methods and techniques for quantifying natural and anthropogenic induced ephemeral-channel runoff and subsequent recharge in cooperation with U.S. Geological Survey Tucson Science Center under current and projected climate scenarios. 2. Develop methods and techniques to quantify and predict water budgets of riparian ecosystems under current and projected climate scenarios through direct measurements of evaporation and plant transpiration and predict water savings by removal of invasive mesquite vegetation. 3. Develop methods and techniques to explicitly quantify the spatial and temporal distribution of vegetation, land use, and infiltration reduction using remotely sensed methods to improve prediction of basin scale semi-arid 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 under objective #3, 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.

3. Progress Report
A number of important advances were made in quantifying the changes in climate, plant, water, and carbon cycle interactions as a result of invasive species encroachment. In the first case, Lehmann Lovegrass invaded portions of the ARS Walnut Gulch Experimental Watershed due to a long-term drought. Carbon uptake and the ratio of post-storm evaporation to evapotranspiration was significantly altered due to the difference in plant structural form and its ability to more rapidly use seasonal rainfall for growth. In the case of observed worldwide woody plant encroachment into arid and semiarid grasslands, the current paradigm that more ecosystem carbon sequestration would occur was found to be incorrect for two grasslands being invaded by mesquite in southeast Arizona. This ecosystem was a found to be a source of carbon to the atmosphere in which carbon release increased with drought severity. All of these findings will provide critical information on how ecosystems will respond and possibly adapt to climate change and other stresses. Advances were also made in the ability of satellites to provide information about crop type, phenology and water status using a large number of radar images. The MU is taking a leadership role in formulation of the AGAVES (Assessment of Goods And Valuation of Ecosystem Services) interagency program and hosted an AGAVES meeting in January. Work continued on the rangeland portion of the congressionally mandated NRCS Conservation Effects Assessment Project (CEAP) including two briefings to NRCS senior management and input to the NRCS Resource Conservation Assessment (RCA) report. Validation of the nitrogen and phosphorous components of the coupled ARS models (KINEROS2 and OPUS; now K2-O2) was completed and a significant number of presentations, posters, proceedings paper and computer demonstration for K2-O2 and the Automated Geospatial Watershed Assessment (AGWA) tool were presented at the Federal Interagency Hydrologic Modeling Conference. The MU has continued its membership with the Congressionally recognized Upper San Pedro Partnership (USPP) ( to provide it with research and technical assistance. The MU monitored and calculated improved estimates of groundwater recharge from infiltration into flood detention ponds for the USPP. On the international front an OECD travel fellowship was completed to Spain and another was awarded for Spain for FY11. Cooperative work with Kazakhstan was presented at a NASA meeting and will continue as a result of a two year extension of funding for this project. Two new interagency agreements were initiated this fiscal year with the National Park Service and the EPA. Both involve training and use of AGWA for their watershed and management and permitting needs.

4. Accomplishments
1. Invasive grasses altered the water cycle in semiarid rangeland. This study investigated a transition from a grassland dominated by native species to a non-native Lehmann lovegrass monoculture at a semiarid grassland site in southeastern Arizona. Results from ARS scientists in Tuscon, AZ, showed that the post-storm daily water loss due to soil evaporation (E) [relative to the daily evapotranspiration from plants and soil (ET)] was greater in post-transition years than in pre-transition years. Results indicated that, compared to a native assemblage, Lehmann lovegrass dominance may be associated with an increase in the ratio of ED/ETD after precipitation in the summer growing season. Over the season, this resulted in a doubling of total seasonal ratio of E/ET after the Lehmann lovegrass invasion. These results will contribute to a comprehensive understanding of the impacts of species invasion in semiarid grasslands.

2. Monetary Valuation of Riparian Ecosystem Services: Conservation of freshwater systems is critical in the semi-arid Southwest where these systems are in high demand for competing human and environments uses. To address this conflict, natural scientists must evaluate how human associated changes to hydrologic regimes alter ecological systems. In this study the ARS scientists in Tuscon, AZ, and a multi-institutional team have developed a hydro-bio-economic framework for the San Pedro River Region in Arizona, and the Middle Rio Grande of New Mexico. Studies are being conducted for each site to examine how well values established at one site can be transferred to the other site. This system will translate management decisions into changes in ecosystem attributes which can be monetized providing a scientifically defensible management methodology housed within a easy to use Decision Support System. This will provide resource managers and decision makers with a suite of monetized ecosystem services values that heretofore have not been available for use in assessing trade-offs against costs for management and planning.

3. Drought Effects on Water and Carbon Cycling in a Desert Grassland Invaded by Mesquite. The increase in the amount of woody plants (shrubs and trees)enroaching into grassland ecosystems is one of the most wide-spread changes to ecosystems in the Southwestern U.S. This will affect water and nutrient cycling of these ecosystems. ARS scientists in Tuscon, AZ, measured water and carbon dioxide exchange between a woody-plant-encroached grassland and the atmosphere over a four-year period and determined how the amount of precipitation influenced these exchanges. We found that seasonal drought during this period strongly impacted these exchanges. In contrast to the current model that woody plant encroachment might result in more ecosystem carbon sequestration, we found that this ecosystem was a source of carbon to the atmosphere which increased with drought severity. These results highlight a complex relationship between vegetation change and climatic variation in precipitation that likely influences the carbon sequestration potential of these water-limited landscapes.

4. Invasive grass and grassland function. The invasive success of the South African grass, Lehmann lovegrass has been associated with a dramatic decrease in the diversity of native plant and animal populations, and is of considerable concern to land-managers across the arid and semi-arid Southwest. In co-operation with University of Arizona colleagues, ARS scientists in Tuscon, AZ, showed plots dominated by Lehmann lovegrass had higher soil water contents across the summer monsoon growing season, which resulted in higher seasonal evapotranspiration and greater carbon uptake activity compared to native grasses. These differences followed more rapid canopy development and allocation to leaf area display, indicating the invasive success of this exotic grass is due to rapid use of seasonal rains, thereby altering the tempo of basic ecosystem functioning in semiarid rangelands. Understanding these functional characteristics is important in efforts to control and manage the ecosystem-service consequences of this grass in important rangeland systems

5. Woody Shrubs in Western U.S. alter the spatial distribution of moisture in the soil. Because water is vital to life on this planet, knowing the amount of moisture in the soil is important for predicting the response of plants in natural ecosystems to global warming, increases in atmospheric carbon dioxide, and changing land-use patterns. Dry lands worldwide are currently experiencing an increase in the density and cover of woody plants, which in turn influence the availability of soil moisture, with potentially important effects on local and regional water cycles. ARS scientists in Tuscon, AZ, monitored soil moisture at a semiarid savanna near Tucson, AZ to determine the effect that the trees had on the amount of soil moisture in the soil. We found that the tree canopy reduced the amount of precipitation input into the soil, but also that the shade of the canopy had the tendency to reduce evaporative losses so that the soil dried less quickly than soil out in the open. These findings are consistent with, and may help to explain, the results of other investigations that have examined the role of woody plants in enhancing nutrient cycling and altering carbon cycling in arid ecosystems.

6. The accuracy of evaporation measurements. Measurements of evaporation over land are critical for determining local, regional and global hydrological budgets, model testing, and understanding ecosystem processes. This study conducted by ARS scientists in Tuscon, AZ, evaluated the accuracy of evaporation measurements made using a very common, state-of-the-art meteorological technique by comparing them with evaporation estimates derived from a seasonal or annual water balance over nearly twelve years and at three sites in Arizona, USA. Results indicate that the widely used technique resulted in estimates that were highly accurate, giving confidence to the accuracy of this type of data collected at sites worldwide.

7. Satellites provide information about crop type, seasonal changes in plant structure and water status. A time-series of 57 radar images of irrigated agriculture in La Mancha, SE Spain acquired by ARS scientists in Tuscon, AZ, in 2009 was used to assess the sensitivity of radar backscatter to crop and soil conditions for large fields of corn, barley, wheat, onion, alfalfa and oats. Preliminary results showed that radar backscatter detected by the satellite sensor was sensitive to 1) crop type, 2) crop seasonal growth and harvest date, 3) crop furrow size and orientation, and 4) soil moisture due to precipitation or irrigation. The multiple radar configurations and multi-date imagery were essential to discriminating this interrelated information. This information, derived from satellite images, provides distributed input for crop growth models for day-to-day crop management.

8. Riparian ecosystem carbon cycling. Identifying the dynamics of carbon dioxide cycling in ecosystems in semiarid regions will lead to the ability to better assess the ecological responses to future changes in climate. To make reliable predictions to change, scientists need to construct models that adequately represent and characterize how carbon dioxide exchange is influenced by weather, plant type and activity, and water availability. ARS scientists in Tuscon, AZ, developed a relatively simple computer model that is based on established theory and then applied this model at two sites in southern Arizona where measurements of carbon dioxide exchange have been made. We demonstrate that this model adequately reproduces the measurements, and we found that ecosystem carbon cycling was equal to or even more affected by the ecosystem composition than climate variation. This indicates a need to consider an ecosystem’s unique plant composition when determining the carbon sequestration potential of riparian landscapes.

9. Regional drought effects. The Southwest U.S. has experienced record drought intensity from 1999 to 2005. Prolonged droughts such as these are important ecological disturbances in deserts, resulting in widespread plant mortality that alters plant community structure and ecosystem functioning that can persist over very long time scales. Using continuously recorded monthly precipitation totals across 35 years and 67 weather stations, ARS scientists in Tuscon, AZ, found that higher whole-plant mortality and more extensive canopy die back in Sonoran Desert shrub populations was due to repeated failures of cool-season precipitation; these did not occur to as great an extent in the Mojave Desert, where rainfall accumulations are typically much lower than across the Sonoran. The biological diversity of Southwest U.S. deserts is of considerable aesthetic and commercial economic importance, and this research is important in that it shows which plant species are susceptible to severe drought-induced mortality, and establishes the mechanisms and long-term consequences of these community shifts across the region.

10. Improved forest carbon modeling. Changing global climate conditions have been predicted to increase the frequency and severity of disturbances such as hurricanes, ice-storms and insect outbreaks capable of dramatically altering forest canopy structure. Despite the importance of these disturbances, few forest carbon models directly incorporate canopy microclimate and structure into their operating parameters. In co-operation with a USDA Forest Service scientist, ARS scientists in Tuscon, AZ, compared the carbon balance of an oak/pine forest estimated with a canopy-constrained carbon assimilation model (the 4C-A model) to whole ecosystem net carbon exchange (NEE) measured directly with instrumentation prior to and following severe defoliation by gypsy moth. 4C-A carbon balance estimates were 10% of directly measured NEE, and successfully captured the switch in the forest functioning as a carbon sink to being a carbon source following defoliation. Thus, the 4C-A model provides a valuable tool to assess short- and long-term consequences of canopy disturbance to forest carbon balance dynamics, allowing for better prediction of novel, climate-induced disturbance regimes to the carbon sequestration ability of forested systems.

11. Carbon and Water Use for Tropical Deciduous Forest. The North American Monsoon (NAM) dominates summer climate and is responsible for providing the majority of rainfall over a large portion of western North America. Knowing how ecosystems respond to these seasonal rains is critical to understanding how surface vegetation may affect monsoon intensity. To better understand the effects and relationship between precipitation, carbon sequestration and evaporation, ARS scientists in Tuscon, AZ, made measurements of these exchanges over a little studied tropical dry forest in northwest Mexico. Three markedly defined periods were found during the six-month study period: 1) A pre-rainy season period, where carbon and water exchange was close to zero; 2) A monsoon period characterized by relatively large evaporation losses and large carbon uptake; and, 3) A post-rainy season where the ecosystem returned to dormancy. Because of the strength of the monsoon in this type of ecosystem these forests can take up a large amount of carbon dioxide from the atmosphere in a relatively short period of time.

5. Significant Activities that Support Special Target Populations
In 2009/10, undergraduate student Maxwell Justice (Native American descent) worked with SWRC as a UA NASA Space Grant Internship to aid in assessing the use of computer models for simulating crop growth. The results from Max’s research were a very good contribution to our ongoing research and have influenced the ongoing direction of our work. Max wrote an abstract describing his research results and presented the results (with a powerpoint presentation) at the Annual Space Grant Statewide Symposium. 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
Jenerette, G.D., Scott, R.L., Barron-Gafford, G.A., Huxman, T.E. 2009. Gross primary production variability associated with meteorology, physiology, leaf area, and water supply in contrasting woodland and grassland semiarid riparian ecosystems. Journal of Geophysical Research. 114: G04010. doi:10.1029/2009JG001074.

Xiao, J., Zhuang, Q., Law, B.E., Chen, J., Baldocchi, D.D., Cook, D.R., Oren, R., Richardson, A., Wharton, S., Ma, S., Martin, T., Verma, S., Suyker, A., Scott, R.L., Monson, R., Litvak, M., Hollinger, D., Sun, G., Bolstad, P., Burns, S., Curtis, P., Drake, B., Falk, M., Fischer, M., Foster, D., Gu, L., Hadley, J., Katul, G., Matamala, R., Mcnulty, S., Meyers, T., Munger, J.W., Noormets, A., Oechel, W., Paw U, K., Schmid, H.P., Starr, G., Torn, M., Wofsy, S. 2010. A Continuous Measure of Gross Primary Production for the Conterminous U.S. Derived from MODIS and AmeriFlux Data. Remote Sensing of Environment. 114:576-591.

Potts, D.L., Scott, R.L., Bayram, S., Carbonara, J. 2010. Woody plants modulate the temporal dynamics of soil moisture in a semi-arid mesquite savanna. Ecohydrology. 3:20-27.

Jenerette, G.D., Scott, R.L., Huete, A.R. 2010. Functional differences between summer and winter season rain assessed with MODIS derived phenology in a semi-arid region. Journal of Vegetation Science. 21:16-30.

Schafer, K.V., Clark, K.L., Skowronski, N., Hamerlynck, E.P. 2010. Impact of insect defoliation on forest carbon balance as assessed with a canopy assimilation model. Global Change Biology. 16:546-560.

Scott, R.L. 2010. Using watershed water balance to evaluate the accuracy of eddy covariance evaporation measurements for three semiarid ecosystems. Agricultural and Forest Meteorology. 150:219-225.

Lizarraga-Celaya, C., Watt, C., Rodriguez, J., Garatiza-Payan, J., Scott, R.L., Saiz-Hernandez, J. 2010. Spatio-temporal variations in surface characteristics over the North American Monsoon region. Journal of Arid Environments. 74:540-548.

Perez-Ruiz, E., Garatuza-Payan, J., Watt, C., Rodriguez, J., Yepez, E., Scott, R.L. 2010. Carbon dioxide and water vapour exchange in a tropical dry forest as influenced by the North American Monsoon System (NAMS). Journal of Arid Environments. 74: 556-563.

Brookshire, D.S., Goodrich, D.C., Dixon, M.D., Brand, A., Benedict, K., Lansey, K., Thacher, J., Broadbent, J., Stewart, S., Mcintosh, M., Doosun, K. 2010. Ecosystem Services and Reallocation Choices: A Framework for Preserving Semi-Arid Regions in the Southwest. Journal of Contemporary Water Research and Education. 144:60-74.

Shuttleworth, W.J., Serrat-Capdevila, A., Roderick, M.L., Scott, R.L. 2009. ON THE THEORY RELATING CHANGES IN AREA-AVERAGE AND PAN EVAPORATION. Quarterly Journal Royal Meteorological Society. 135: 1230-1247. doi: 10.1002/qj.434

Scott, R.L., Jenerette, G., Potts, D., Huxman, T. 2009. Effects of seasonal drought on net carbon dioxide exchange. JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 114: 1-13. G04004.

Schaffner, M., Unkrich, C.L., Goodrich, D.C. 2010. Application of The KINEROS2 Site Specific Model to South-Central NY and Northeast PA: Forecasting Gaged and Ungaged Fast Responding Watersheds. NWS Eastern Region Technical Attachment No. 2010-01. ( . Popular Publication.

Hamerlynck, E.P., Scott, R.L., Moran, M.S., Keefer, T.O., Huxman, T.E. 2010. Growing season ecosystem and leaf-level gas exchange of an exotic and native semiarid bunchgrass. Oecologia. 163:561–570.

Moran, M.S., Scott, R.L., Hamerlynck, E.P., Green, K.N., Emmerich, W.E., Holifield Collins, C.D. 2009. Soil Evaporation Response to Lehmann Lovegrass (Eragrostis lehmanniana) Invasion in a Semiarid Watershed. Agricultural and Forest Meteorology. 149:2133-2142.

Mcauliffe, J.R., Hamerlynck, E.P. 2010. Perennial plant mortality in the Sonoran and Mojave Deserts in response to severe, multi-year drought. Journal of Arid Environments. 74:885-896.

Entekhabi, D., Njoku, E.G., O'Neill, P.E., Kellogg, K.H., Crow, W.T., Edelstein, W.N., Entin, J.K., Goodman, S.D., Jackson, T.J., Johnson, J., Kimball, J., Peipmeier, J.R., Koster, R.D., McDonald, K.C., Moghaddam, M., Moran, M.S., Reichle, R., Shi, J.C., Spencer, M.W., Thurman, S.W. 2009. The Soil Moisture Active and Passive (SMAP) Mission. Proceedings of the IEEE. 98(5):704-716.

Last Modified: 2/23/2016
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