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United States Department of Agriculture

Agricultural Research Service


Location: Southwest Watershed Research

2007 Annual Report

1a. Objectives (from AD-416)
Quantify primary semiarid water and energy balance components with emphasis on rainfall, storm water recharge, and evapotranspiration. 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. Quantify ecosystem influence and feedbacks on water fluxes and states over a selected range of arid and semiarid primary vegetation types.

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. Formerly 5342-13610-007-00D (12/06).

3. Progress Report
During FY07 a transition was made from the prior research plan to the current plan starting Jan. 31, 2007. The prior project was entitled “Hydrologic Processes, Scale, Water Resources, and Global Change for Semiarid Watershed Management” (Project Number: 5342-13610-007-00D; Project Period: 05/01/2002 to 01/30/2007). In addition to beginning research on the new project and addressing the milestones noted above, a major coordinated effort by the entire MU was carried out during FY07. This effort was the development of a set of papers describing and analyzing over 50 years of data (precipitation, runoff, sediment, weather, soil moisture, vegetation, carbon and water flux, remote sensing, and geographic data) collected at the U.S.D.A. Walnut Gulch Experimental Watershed (WGEW). These papers have been submitted for an upcoming special issue of Water Resources Research and are in various stages of review. The MU has also been heavily involved in gearing up for the Conservation Effects Assessment Project (CEAP) move to the western United States. The Tucson MU and the ARS Northwest Watershed Research Center in Boise are leading development of the Rangeland Hydrology and Erosion Model (RHEM) both for CEAP and NRCS use in the west. Several planning meetings have been held and model development for the event based model is well underway. The following subordinate ARS projects are being conducted under the auspices of this project to assist in achieving the milestone and research objectives of this project and its predecessor. 5342-13610-007-14S - Remote Sensing and GIS for Improved Characterization of Landscapes for Hydrologic Modeling and Estimation of Soil Moisture 5342-13610-010-01R - Spatially Integrated Environmental Modeling 5342-13610-010-02R – Prediction of Seasonal to Inter-Annual Hydro-Climatology including the Effects of Vegetation Dynamics and Topography over Large River Basins 5342-13610-010-03R - Forecasting Water Quality and Quantity Hazards Using Spatially Distributed Watershed Models and Biophysical Data 5342-13610-010-05S - San Pedro Riparian National Conservation Area (SPRNCA) Riparian Water Needs Study 5342-13610-010-06S – Geospatial Watershed Modeling for Improved Water Management

4. Accomplishments
Understanding the consequences of woody plant encroachment in Western rangelands. Encroachment by woody plants into former grasslands has been a widely reported phenomenon across many semiarid landscapes around the world, yet we do not understand how this pervasive on-going change in vegetation will affect water and nutrient cycling on rangelands. Scientists at the SWRC, Tucson, AZ investigated how the water and carbon dioxide are cycled in a Chihuahuan Desert shrubland in southeastern Arizona. Results suggest that the ecosystem lost the most carbon at the start of the summer rainy season when the shrubs were still dormant, but once the shrubs became active they were able to efficiently acquire both water and carbon dioxide throughout the growing season. The dataset collected in this study is one of the longest of its kind and will be an important benchmark for future studies on woody plant encroachment. NP 201/211, Problem Area 5, Problem Statement: More accurate quantitative components of basin water budgets that consider ecosystem feedbacks affecting watershed states and fluxes and enhanced instrumentation (in-situ soil moisture, eddy-covariance, etc.) applications coordinated with ecosystem and biogeochemical observations. The effect of rainfall events on riparian ecosystems in semiarid environments. The influence of different hydrological processes on the structure and functioning of riparian vegetation in semiarid basins is not well understood and this information is critical to the management of these important and ecologically diverse ecosystems, which are threatened by human groundwater use in the western U.S. The results of many studies along the San Pedro River conducted in southeastern Arizona were synthesized to show that rooting depth and access to groundwater are key factors that control the vegetations water use and carbon dioxide exchange. Depth to groundwater, which varies substantially across the riparian landscape, is a key factor controlling the sensitivity of cottonwood transpiration, leaf photosynthetic metabolism, and plant water sources to pulsed inputs of growing season moisture. Because mesquite accesses groundwater in these habitats, ET and gross ecosystem production are decoupled from precipitation, but ecosystem respiration remains highly coupled to rainfall due to the dominant contribution of litter and bulk soil organic matter decomposition. Responses of net ecosystem exchange of carbon dioxide to rainfall variability in riparian floodplain is therefore not simple, but depends on vegetation structure and the connection of dominant plants to the water table. The complex vegetation patterns, hydrologic setting and disturbance dynamics in the riparian landscape offer unparalleled opportunities to investigate fundamental processes linking water and carbon exchange. NP 201/211, Problem Area 5, Problem Statement: More accurate quantitative components of basin water budgets that consider ecosystem feedbacks affecting watershed states and fluxes and enhanced instrumentation (in-situ soil moisture, eddy-covariance, etc.) applications coordinated with ecosystem and biogeochemical observations. A space-based approach for mapping regional surface roughness. When runoff occurs on a watershed, the roughness of the surface of the watershed plays an important role in how rapidly water moves on the watershed, which in turn is very important determining the magnitude of flooding and erosion. Surface roughness, as it affects water flow and erosion, is typically estimated is a relatively subjective manner and is very difficult to do consistently over large areas. Often empirical methods are used or gross extrapolations are made from laboratory studies. In this study, ARS and Univ. of Arizona scientists developed a new approach to derive roughness data from satellite-based radar images. The result is a quantitative, and consistently derived, roughness map applicable to large drainage areas. This estimates can be directly input into hydrologic models to improve estimates of erosion and runoff. NP 201/211, Problem Area 5, Problem Statement: Improved watershed simulation, plant growth, and weather generation model components and data assimilation tools for water budget, water quality assessment, and flood and drought risk and impact assessment. Estimating soil texture and hydraulic properties from remote sensing data and modeling. Soil texture and hydraulic properties are critical components of energy and water balance studies, but are not easily determined over heterogeneous regions. This study conducted by NASA, ARS scientists in Tucson, AZ and the U.S. Army used estimates of soil moisture derived from satellite imagery to infer soil texture and hydraulic properties from a land surface model. This operational approach is useful for mapping soil information using available technology for a better understanding of water availability in semiarid watersheds. Research is useful for farmers and managers of semiarid watersheds. NP 201/211, Problem Area 5, Problem Statement: Improved watershed simulation, plant growth, and weather generation model components and data assimilation tools for water budget, water quality assessment, and flood and drought risk and impact assessment. (Also addresses Obj. 1 in Coordinated Plan of Work (CPW) entitled “Remote Sensing for Parameters and States of Watershed Models for Improved Watershed Management."

4. Accomplishments

5. Significant Activities that Support Special Target Populations
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 four years, twelve students (Hispanic, African-American, women) have come through the program. This year two women completed the program under this project. 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
Bryant, R., Moran, M.S., Thoma, D., Holifield Collins, C.D., Skirvin, S.M., Rahman, M., Slocum, K., Starks, P.J., Bosch, D.D., Gonzalez-Dugo, M.P. 2007. Measuring surface roughness height to parameterize radar backscatter models for retrieval of surface soil moisture. IEEE Geosci. and Rem. Sens. Letters 4(1): 137-141.

Nagler, P.L., Glenn, E., Kim, H., Emmerich, W.E., Scott, R.L., Huxman, T., Huete, A. 2007. Relationship between evapotranspiration and precipitation pulses in a semiarid rangeland estimated by moisture flux towers and modis vegetation indices. Journal of Arid Environments. 70:443-462.

Nichols, M.H. 2007. The walnut gulch experimental watershed - 50 years of watershed monitoring and research. In: Monitoring and Evaluation of Soil Conservation and Watershed Development Projects. de Graaff, J, Cameron, J., Sombatpanit, S., Pieri, C., Woodhill, J.(eds.). World Association of Soil and Water Conservation, Bangkok. 496 p.

Potts, D.L., Huxman, T.E., Scott, R.L., Williams, D.G., Goodrich, D.C. 2006. The sensitivity of ecosystem carbon exchange to seasonal precipitation and woody plant encroachment. Oecologia 150(3): 453-463.

Santanello, Jr., J.A., Peters-Lidard, C.D., Garcia, M.E., Mocko, D.M., Tischler, M.A., Moran, M.S., Thoma, D.P. 2007. Using Remotely-Sensed Estimates of Soil Moisture to Infer Soil Texture and Hydraulic Properties across a Semi-arid Watershed. Remote Sensing of Environment. 110:79-97. doi:10.1016/j.rse.2007.02.007.

Tischler, M., Garcia, M., Peters-Lidard, C., Moran, M.S., Miller, S., Thoma, D., Kumar, S., Geiger, J. 2007. A GIS framework for surface-layer soil moisture estimation combining satellite radar measurements and land surface modeling with soil physical property estimation. J. of Environmental Modeling and Software 22:891-898.

Last Modified: 2/23/2016
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