Location: Southwest Watershed Research2008 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
FY2008 was a banner year for the MU as the culmination of a multi-year effort was realized with the publication of a special section of 20 papers in Water Resources Research, one of the premier journals in hydrology, 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 USDA-ARS Walnut Gulch Experimental Watershed (WGEW). Substantial progress has also been made within the MU in planning and conducting research for the rangeland portion of the congressionally mandated NRCS Conservation Effects Assessment Project (CEAP) in cooperation with multiple ARS labs, NRCS locations and NASA Ames. The MU was heavily involved in organizing the annual international conference of the Soil and Water Conservation Society held in Tucson in late July. The effects of a long-term drought on plant mortality and subsequent replacement by an invasive species within the WGEW on the watershed runoff and erosion response has been observed. This provides an unparalleled opportunity to assess the hydro-ecological impacts associated with this extreme climatic event as the MU can contrast this to the long, and well documented, watershed records, observations, and research knowledge base accumulated over the prior 50 years. Fortuitously, the MU successfully recruited a rangeland plant physiologist/ecologist, Dr. Erik Hamerlynck, who started this FY. Dr. Hamerlynck and several other scientists of the MU are actively conducting research on this front and a number of important results are anticipated in the coming year(s). The MU joined an informal user group of public land managers across Arizona that was established to evaluate both a time series of remotely sensed estimates of canopy cover across the state and tools for web access to the cover estimates for improved management. In cooperation with the EPA and the University of Arizona the MU released version 2.0 of the Automated Geospatial Watershed Assessment (AGWA) tool (www.tucson.ars.ag.gov/agwa) and the web-based WEPPCAT tool (http://typhoon.tucson.ars.ag.gov/weppcat/index.php). Both are tools that make ARS technology more accessible and usable for watershed management. The MU has continued is close association and membership with the Congressionally recognized Upper San Pedro Partnership (http://www.usppartnership.com/) to provide research and technical assistance for the Partnership to meet it objective for sufficient water, now and in the future, for residents of the Sierra-Vista subwatershed of the San Pedro and the San Pedro National Riparian Conservation area. Research results from the MU supplied new and improved estimates of riparian water use for basin water balance calculations. In addition, new methods employing remote sensing techniques were developed which will markedly simplify future estimates for riparian water use. Remote sensing methods were also developed to provide large area estimates of soil moisture, CO2 fluxes and leaf area indices that will greatly enhance our ability to characterize watershed states and fluxes. NP 211, Problem 5.
1. Fifty Years of Hydrologic Research and Data Collection: USDA ARS Walnut Gulch Experimental Watershed (WGEW). SWRC Scientists completed a multi-year, unit-wide Data Access Project (DAP) with the goal to promote analyses and interpretations of historic and current WGEW high-resolution hydrological data by improving data access. This special section of the Journal of Water Resources Research and the associated Web site (http://www.tucson.ars.ag.gov/dap/) describe 50 years of data collection and the most recent research results at the USDA ARS Walnut Gulch Experimental Watershed (WGEW) in southeast Arizona. Ten of the twenty special section journal publications address the hydrology and climatology of the WGEW and well as providing experimental watershed history, metadata, data access and recent analysis results. The goal of this compilation was to encourage cooperative, interdisciplinary studies of semiarid ecohydrology at WGEW based on continuing long-term, high-resolution, measurements of hydrology, ecology, and climate. This summary, like similar summaries from the ARS Reynolds Creek and Little River Experimental Watersheds, demonstrate the exceptional value of this transformative research infrastructure to the advancement of the hydrological sciences. NP 211 Problem Area #5 Watershed Management, Water Availability, and Ecosystem Restoration.
2. How water movement in tree roots affects ecosystem water balance and productivity. Plant roots capture, store and transport soil water, and in doing so alter the ecosystem water, energy, and nutrient balance. One potentially important process facilitated by roots, and neglected in research, is the redistribution of water from moist to dry soil layers; this process has important ecological and hydrological consequences and should be accounted for in many plant/soil models. The study, by ARS scientists in the Southwest Watershed Research Unit in Tucson, AZ, investigated the movement of water in the roots of mesquite trees in a semiarid rangeland. It was found that when the surface soil layers were wet, mesquite trees moved water from the surface and downward in their taproot even during the winter when the canopy was leaf-less. When the surface was dry, mesquites brought deep moisture up to the surface in their taproot, and this moisture was either used to support transpiration or, at night, it was sent out laterally into the near surface soil. These results directly show that mesquites can move significant amounts of water both upward and downward in the soil and represents a profound new finding in the ecohydrology sciences regarding plant water movement mechanisms. Redistribution has important ecological and hydrological consequences, and thus, this process should be accounted for in many plant/soil models. NP 211 Problem Area #5 Watershed Management, Water Availability, and Ecosystem Restoration.
3. Satellite-based approach to mapping net CO2 flux over large areas. There is a need for techniques to accurately quantify Carbon Dioxide (CO2) sources and sinks over large areas because of the importance of atmospheric CO2 levels on the climate. In this study, ARS scientists in the Southwest Watershed Research Unit in Tucson, AZ developed an approach for producing estimates of daily net CO2 flux using a combination of satellite imagery, the Water Deficit Index, and Bowen ratio energy balance measurements. This approach can be used to map daily net CO2 flux at the landscape scale for a better understanding of the role semiarid grasslands play in the carbon cycle. NP 211 Problem Area #5 Watershed Management, Water Availability, and Ecosystem Restoration.
4. A Prototype Rangeland Decision Support Tool for Public Land Management. Public land managers in the West need remotely sensed information to focus limited time and resources available for field data collection. A prototype website with imagery for all of Arizona from early 2000 to 2008 was developed in a collaboration between the Southwest Watershed Research Center in Tucson, Arizona, Applied Geosolutions in New Hampshire, and Michigan State University. The tool allows rangeland managers to plan field work, assess changes in cover over time, and compare cover estimates for areas with similar potential vegetation communities. With refinements in cover estimates and integration of climate information, the prototype could become the first interagency operational tool for remote sensing of rangelands in Arizona. This accomplishment contributes to Objective 3 of the NP 211 Coordinated Plan of Work (Appendix A) for the Application of KINER-OPUS and SWAT using improved estimates of vegetation cover.
5. Airborne LIDAR successfully classifies riparian vegetation species and estimates cottonwood canopy characteristics and leaf area. Better methods for estimating water use in semiarid riparian systems are needed. Airborne and ground based LIDAR (LIght Detecting And Ranging) technology was used, by ARS scientists in the Southwest Watershed Research Unit in Tucson, AZ, to estimate biophysical properties of young, mature, and old cottonwood trees in the San Pedro River Basin near Benson, Arizona, USA. Lidar derived estimates of canopy height, crown diameter, stem diameter at breast height, canopy cover, and mean intensity of return laser pulses from the canopy surface are well correlated with ground measurements. In addition, lidar-derived images were successfully used to differentiate age classes of cottonwood trees for riparian areas quickly and accurately. The lidar-derived canopy information has the potential to improve riparian corridor water use estimates for the Upper San Pedro Basin and other semiarid riparian systems. NP 211 Problem Area #5 Watershed Management, Water Availability, and Ecosystem Restoration. Conducted in cooperation with Univ. of Arizona and Univ. of Florida scientists.
6. Partitioning Evapotranspiration in Semiarid Ecosystems Using Time Series of Soil Surface Temperature. Overall plant/soil water use is dependent on the partitioning of evapotranspiration (ET) into its components of evaporation (E) and transpiration (T) but is difficult to estimate except over very small scales. In this study, a new approach was used to partition measurements of ET into daily evaporation (E) and daily transpiration (T) in a semiarid watershed based on the low-cost addition of an infrared thermometer and soil moisture sensors to existing eddy covariance and Bowen ratio systems. The method was tested, by ARS scientists in the Southwest Watershed Research Unit in Tucson, AZ, using Bowen ratio estimates of ET and continuous measurements of surface temperature with an infrared thermometer at a grassland and shrubland site within the Walnut Gulch Experimental Watershed in southeast Arizona USA in years 2004-2006. For these sites during the study period in these years, the season-long T/ET was higher for the grass-dominated site than for the shrub-dominated site, and did not vary systematically with variation in amounts and timing of rainfall. Information about the ratio of T/ET is related to critical global change concerns, including shrub encroachment and non-native species invasion. NP 211 Problem Area #5 Watershed Management, Water Availability, and Ecosystem Restoration.
7. Appropriate scale of soil moisture retrieval from high-resolution radar imagery. Large area estimates of soil moisture are important in a wide range of applications from agriculture to military trafficability yet are difficult to obtain. An operational approach was developed, by ARS scientists in the Southwest Watershed Research Unit in Tucson, AZ, to determine the optimal spatial resolution for soil moisture retrieval from high resolution radar imagery. This assessment approach was demonstrated at watersheds located in Arizona (AZ), Oklahoma (OK) and Georgia (GA), where optimum ground resolution was determined to be 162 m, 310 m, and 1131 m respectively. Results showed that optimum ground resolution will depend on the spatial distribution of land surface features that affect radar backscatter. This work offers insight into the accuracy of soil moisture retrieval without relying on ground verification of soil moisture for validation and only requires a satellite image and average roughness parameters of the site. NP 211 Problem Area #5 Watershed Management, Water Availability, and Ecosystem Restoration.
8. Remotely sensed estimates of soil moisture to infer soil texture and hydraulic properties. There is a need for being able to gather accurate data about water infiltration, runoff, and evapotranspiration from a remote sensor. This study, by ARS scientists in the Southwest Watershed Research Unit in Tucson, AZ, used estimates of near-surface soil moisture derived from microwave remote sensing to calibrate hydraulic properties in a land surface model and infer information on the soil conditions of the region. Results demonstrated the potential to gain physically meaningful soil information using simple parameter estimation with few but appropriately timed remote sensing retrievals. The results was improved predictions of water infiltration, runoff and evapotranspiration by accurately representing soil texture and hydraulic properties in land surface models. NP 211 Problem Area #5 Watershed Management, Water Availability, and Ecosystem Restoration.
9. Riparian water use in a semiarid watershed. Accurate information about riparian water use is needed to improve water budgets within these basins so that stakeholders will have accurate information to make water resource management decisions. Riparian water use was determined, by ARS scientists in the Southwest Watershed Research Unit in Tucson, AZ, by developing a model that coupled remote sensing data from satellites with local evaporation measurements. The model was then applied over the entire riparian region within the San Pedro Sierra Vista watershed determine evaporation and groundwater use for the years 2001 through 2005. This paper provides the first yearly estimates of groundwater use by riparian vegetation in the San Pedro Valley, which in turn will improve the groundwater budgets used by scientists and management agencies to better manage the region’s water resources. It also provides an effective approach to estimate riparian water use in other basins using widely available remotely sensed data. NP 211 Problem Area #5 Watershed Management, Water Availability, and Ecosystem Restoration.
10. Determining the components of ecosystem water and carbon flux in a riparian woodland. There is a compelling need to understand how precipitation influences sources and sinks of carbon dioxide in seasonally dry ecosystems because these ecosystems would likely be more sensitive to changes in precipitation than other global changes. Natural and anthropogenic changes in the climate system anticipated this century will have large impacts on ecosystem structure and function. In this study, ARS scientists in the Southwest Watershed Research Unit in Tucson, AZ investigated how the distribution of precipitation over a growing season influenced the dynamics of carbon and water cycling in semiarid riparian mesquite woodland. We found that where the trees access groundwater, photosynthesis and ecosystem respiration were often unrelated because of their different sensitivities to rainfall. Also, the majority of the precipitation that fell on this ecosystem is returned to the atmosphere via plant uptake and transpiration. These results suggest that future changes precipitation will likely have the greatest impact on the carbon cycling of riparian woodlands with increases in precipitation likely leading to reduced carbon storage and decreases leading to storage gains. NP 211 Problem Area #5 Watershed Management, Water Availability, and Ecosystem Restoration.
11. Largest uncertainties in semiarid flash flood forecasting associated with radar-rainfall estimates. There is need for a real-time forecast and warning system for flash floods based on a rainfall-runoff model. This study, by ARS scientists in the Southwest Watershed Research Unit in Tucson, AZ, used an ARS semiarid, physics-based, and spatially distributed watershed model driven by high-resolution radar rainfall input to evaluate such a system. The predictive utility of the model and dominant sources of uncertainty were investigated for several runoff events within the USDA-ARS Walnut Gulch Experimental Watershed located in the southwestern US. Results indicated that the highest predictive uncertainty was heavily dominated by biases in the radar rainfall depth estimates. This indicates that if possible, biases in radar-rainfall estimates should be removed in near-real time with raingage observations if available. NP 211 Problem Area #5 Watershed Management, Water Availability, and Ecosystem Restoration. This research was carried out in cooperation with Univ. of Arizona and National Weather Service scientists.
12. An improved electronic raingage with digital recording, telemetry and optional soil moisture measurement capability. As sensing technological improvements increase accuracy and precision of data, a firm understanding of the differences between newer and older measurement and data collection systems is necessary so that differences in the time series statistics of observed values can be correctly interpreted. A side-by-side comparison, by ARS scientists in the Southwest Watershed Research Unit in Tucson, AZ, of two types of recording raingages was conducted demonstrating the improvements of the new technology and that differences between the two raingages have limited impact on rainfall-runoff models. More accurate and precise precipitation and soil moisture measurements support improved quantitative estimates of water balance components and rainfall-runoff relationships. NP 211 Action Plan Component Problem Area 5, Watershed Management, Water Availability, and Ecosystem Restoration.
13. Evaluation of multiple types of soil moisture sensors for monitoring southwestern rangelands. Various types of soil moisture sensors have been developed for automated and unattended use for scientific and land management applications; however, their effectiveness and limitations have never been studied. These sensors are being installed and used for a wide range of soil moisture applications such as drought forage prediction or validation of large scale remote sensing instruments. The results of this analysis, by ARS scientists in the Southwest Watershed Research Unit in Tucson, AZ, underscore the need to recognize the limitations of soil moisture sensors and the factors that can affect their accuracy in predicting soil moisture in situ. NP 211 Action Plan Component Problem Area 5, Watershed Management, Water Availability, and Ecosystem Restoration.
14. Soil-dependent mortality of desert shrubs. Soil and landform features that control plant water availability need to be better understood in order to help reduce plant mortality during drought. Plant mortality of white bursage (Ambrosia dumosa) and creosotebush (Larrea tridentata) was quantified, by ARS scientists in the Southwest Watershed Research Unit in Tucson, AZ, in varying soils, and watershed position (hillslope vs. channels). Mortality was more widespread for bursage than creosotebush, and dead plants tended to be smaller than surviving plants, especially in channel and hillslope locations. Die back was greater in young, weakly-developed soils that fostered extensive plant growth in previous years, while plants growing in older, well-developed soils showed markedly lower mortality, especially in bursage. We attributed these differences in plant response in part to variation in distributions of large rocks within soil profiles. These soil-specific changes in community and canopy structure could alter seasonal fluxes of water and carbon in these water-limited systems. NP 211 Problem Area #5 Watershed Management, Water Availability, and Ecosystem Restoration.
15. Precipitation interpolation methods and multi-decadal persistence in rainfall variability. Spatial and temporal rainfall variability over watersheds directly impacts the runoff response over virtually all watersheds. In complimentary studies we evaluated selected interpolation methods for monsoonal rainfall events over a dense gauge network at the USDA-ARS Walnut Gulch Experimental Watershed in southeast Arizona. With these interpolated rainfall fields ARS scientists in the Southwest Watershed Research Unit in Tucson, AZ then evaluated the daily, seasonal, and annual precipitation volumes and intensities for multi-decadal trends over a range of watershed sizes and related them to trends in runoff. For interpolation methods we found that the multi-quadric-bi-harmonic method outperforms the inverse-distance-weighting (IDW) method. For spatial rainfall variability trends we found that the variability of cumulative precipitation decreases rapidly with time, and, on average, became uniform after 20 years. There were no significant temporal trends in basin wide precipitation but a long-term decrease in runoff was found from 1966 to 1998. This study provides a framework to assess changes in the precipitation regime over decades which may contribute to changes in vegetation, water supply, and, over longer timescales, landscape evolution. NP 211 Problem Area #5 Watershed Management, Water Availability, and Ecosystem Restoration.
16. Impact of errors in land cover on runoff and erosion prediction. In water scarce, arid and semiarid regions it is important improve our predictive capability to predict runoff and erosion from rainfall to better manage water and avoid hazards caused by floods. Land cover and land use is an important factor in how rainfall is transformed to runoff, yet land cover, which is often derived from satellite images, is often incorrectly classified into a different land use. In this study ARS scientists in the Southwest Watershed Research Unit in Tucson, AZ assessed the impact of land cover classification error on a commonly used USDA-ARS rainfall-runoff-erosion watershed in the southwestern US. Simulation results indicated that as watershed size increases, misclassification of land cover has a greater impact on simulated runoff and erosion but decreases as rainfall event size increases. NP 211 Problem Area #5 Watershed Management, Water Availability, and Ecosystem Restoration. This research was conducted in cooperation with Univ. of Arizona and Wyoming scientists.
5. Significant Activities that Support Special Target Populations
5. Significant Activities that Support Special Target Programs 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. 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.
Goodrich, D.C., Unkrich, C.L., Keefer, T.O., Nichols, M.H., Stone, J.J., Levick, L., Scott, R.L. 2008. Event to multidecadal persistence in rainfall and runoff in southeast Arizona. Water Resour. Res., 44, W05S14, doi:10.1029/2007WR006222.