Location: Southwest Watershed Research Center2018 Annual Report
1a. Objectives (from AD-416):
1:As part of the LTAR network, and in concert with similar long-term, land-based research infrastructure in the region, use the Walnut Gulch LTAR site to improve the observational capabilities and data accessibility of the LTAR network and support research to sustain or enhance agricultural production and environmental quality in agroecosystems characteristic of the semiarid Southwest region. Research and data collection are planned and implemented based on the LTAR site application and in accordance with the responsibilities outlined in the LTAR Shared Research Strategy, a living document that serves as a roadmap for LTAR implementation. Participation in the LTAR network includes research and data management in support of the ARS GRACEnet and/or Livestock GRACEnet projects. 1.1:Improve & continue long-term measurements & analysis of water budgets on WGEW & Santa Rita Experimental Range (SRER) watersheds. 1.2:Expand variables measured on WGEW & SRER watersheds based on recommendations of the LTAR Meteorology, Hydrology, CO2, Non-CO2 Gas, Soil, Biology, & Wind Erosion Committees. 1.3:Develop a long-term monitoring program. 1.4:Implement an experiment on the SRER watersheds to quantify the effects of brush management on a set of ecosystem services. 1.5:Compute trends in sub-daily & daily precipitation intensity across LTAR sites. 1.6:Evaluate National Weather Service dual pole radar precipitation data & its ability to improve flash flood forecasting. 2:Quantify how seasonal, annual, and decadal-scale variations in climate, plant community composition, and management impact processes controlling the cycling of water, energy, and carbon in semiarid rangelands 2.1:Determine how changes in vegetation structure & climate affect ecosystem-atmosphere water vapor & CO2 exchange using long-term flux tower observations. 2.2:Use isotopes in pond deposition sediments to understand & quantify erosion & sediment yields in semiarid landscapes as a function of ecological sites. 2.3:Quantify the impact of erosion control structures on runoff & sediment transfers in semiarid landscapes. 2.4:Estimate annual production & minimum total foliar cover using Landsat & MODIS satellite. 2.5:Develop methods to assess climate impacts on rangeland vegetation composition & production across the West. 3:Develop a new conceptual framework and corresponding experimental methods to understand and model the dynamics of semiarid upland and channel erosion processes. 3.1:Conduct experiments to quantify the effects of surface condition. 3.2:Conduct experiments to develop a remote sensing method to estimate hydraulic roughness. 4:Improve hillslope (RHEM) and AGWA/KINEROS2 watershed models and develop methods to incorporate new remotely sensed, meteorologic, & land surface information. 4.1:Complete development & post-disturbance testing of the RHEM for application in Western rangelands. 4.2:Develop a mechanism to extend the findings from the Walnut Gulch LTAR site across Arizona & New Mexico & support collaborative vegetation management of public lands to improve watershed function. 4.3:Incorporate a variety of KINEROS2 (K2) / AGWA model enhancements.
1b. Approach (from AD-416):
Objective 1: 1. Use co-located rain gauges to quantify uncertainties in long-term precipitation datasets. 2. Use radar stage measurements to test remote methods to measure runoff stage 3. Deploy mobile x-band Doppler radar and compare with Dual Pole radar rainfall rain-gauge observations, and runoff observations on the WGEW. 4. Meet LTAR objectives by: a) using observational datasets to quantify the individual components of the watershed water balance in Walnut Gulch Experimental Watershed WGEW), b) using satellite and ground measurements of vegetation to document changes in watershed vegetation, c) determining trends and magnitude of precipitation intensities and precipitation extremes across the continental US, and d) implementing the LTAR common experiment to assess the effects of brush management on a set of ecosystem services. Objective 2: 1. Use long-term flux tower observations to determine how changes in vegetation structure and climate affect ecosystem-atmosphere water vapor and carbon dioxide exchange. 2. Use 210Pb pond stratigraphy to determine erosion rates and their historical dynamics on small watersheds over the past 50-100 years. 3. Quantify runoff and sediment yields on watersheds to quantify the impact of erosion control structures on runoff and sediment transfers. 4. Use satellite, climate, site productivity and management data to estimate annual production and minimum total foliar cover. 5. Use LiDAR, point cloud, and new satellite datasets to construct canopy height models to assess climate impacts on rangeland vegetation composition and production. Objective 3: 1. Use rainfall simulator experiments to quantify the effects of surface condition on infiltration, runoff, concentrated flow dynamics, sediment transport processes, and surface evolution. 2. Use radar backscatter roughness and hydraulic roughness at a laboratory, rainfall simulator, and small watershed scales using airborne and satellite active radar imagery to develop a remote sensing methods to estimate hydraulic roughness. Objective 4: 1. Complete development and post-disturbance testing of the Rangeland Hydrology and Erosion Model (RHEM) for application in Western rangelands. 2. Create a web interface to identify problem areas in watersheds, compare across watersheds, and assess trends in time prior to KINEROS2 modeling. 3. Incorporate RHEM, improved process model representations, and higher-resolution, model inputs, sub-surface and variable width routing, and interstorm processes into KINEROS2.
3. Progress Report:
Substantial progress was made on all four objectives and their sub-objectives, all of which fall under the National Program 211 Water Availability and Water Management, Component 2 Erosion, Sedimentation, and Water Quality Protection, and Component 4 Watershed Management to Improve Ecosystem Services in Agricultural Landscapes. Objective 1: Improvements in our observational capabilities at the Walnut Gulch Experimental Watershed Long-Term Agroecosystem Research (LTAR) site were made. They included an assessment of uncertainties associated with legacy precipitation data collected and evaluating new radar-based sensors to measure runoff stage levels (Sub-0bjectives 1.1 and 1.3). Continuous soil carbon dioxide (CO2) efflux measurements were implemented at our grassland site (Sub-objective 1.2). A methodology for the LTAR network to compute trends in subdaily and daily precipitation intensity was developed and initially applied to Walnut Gulch observations (Sub-objective 1.5). 2017 and 2018 Monsoon experimental campaigns in collaboration with National Aeronautics and Space Administration (NASA), National Oceanic and Atmospheric Administration (NOAA), United States Geological Survey (USGS), and the University of Arizona were also conducted to study precipitation processes and measurements using rain gauge, ground-based radar and satellite observations as well as multiple runoff measurement methods (Subobjectives 1.1, 1.6). Finally, significant progress was made in improving understanding of treatment effectiveness of brush management to re-establish grassland and associated hydrologic function (Subobjective 1.4). Field campaigns were conducted at the Santa Rita Experimental Range to evaluate a large-scale mesquite herbicide treatment, and rainfall simulation experiments across the West were made to understand the effects of pinyon and juniper removal on reestablishing sagebrush steppe vegetation and improving hydrologic function on woodland encroached rangelands in the Great Basin. Data from multiple vegetation measurement and rainfall simulation field campaigns were analyzed to assess the effects of herbicides, prescribed fire, tree cutting, and tree shredding tree-removal treatments. Objective 2: We completed an analysis of land-atmosphere CO2 and evapotranspiration (ET) data from 25 sites across the Southwest region to better constrain satellite and land surface model estimates of these exchanges in dryland regions (Sub-objective 2.1). Further, a synthesis of open shrubland ecosystem water and carbon exchanges was completed. We have analyzed pre-treatment brush management effects on ecosystem water and carbon exchange and drafted a manuscript. Long-term riparian woodland flux tower data have been pulled together and analysis has begun. Additionally, we continue to maintain and operate numerous eddy covariance flux towers in this region. These data have been vital to our analyses as well as to the analyses of many other researchers worldwide who are interested in inventorying, measuring and modeling water and carbon cycling. A sampling protocol for collecting isotope samples in depositional areas of ponds was established and tested on a pond located in Walnut Gulch Experimental Watershed (Sub-objective 2.2). This work investigated erosion dynamics of the past 90 years on three small semi-arid watersheds with histories of grazing and vegetation change. Activity of Caesium 137 (137Cs) and excess lead radioisotopes (210Pb) from 18 cores collected from sedimentation ponds were measured using a gamma spectrometer. The sediment was dated using a constant rate of supply (CRS) model. This study represents the first time that reservoir sediment accumulation rates determined from fallout isotopes have been verified by direct volumetric measurements of aggradation based on topographic surveys. Past variations in sedimentation rates were identified and correlated with recorded history of anthropogenic disturbance. These methods (137Cs and 210Pb) are suitable for use in arid environments and can complement each other to increase reliability of erosion rate estimates. Progress to understand the hydrologic, geomorphic and sediment yield impacts of check dams was made toward improving low-tech erosion control practices for restoring degraded rangelands (Sub-objective 2.3). A 10-year post construction evaluation has been completed to measure and analyze runoff volumes and peak runoff rates, watershed outlet sediment delivery, channel geometry, and internal watershed sediment accumulation. These results are being summarized and are contributing to 1) updated Natural Resources Conservation Service (NRCS) conservation practice standards and 2) an international review of checkdams and their application. Objective 3: We applied simulated rainfall to runoff plots with different gradients (Sub-objective 3.1). A series of simulations were made for each treatment with measurements of runoff rate, velocity, rock cover, sediment, and surface roughness. Velocities measured at the end of each experiment were a unique function of discharge rates, independent of slope gradient or rainfall intensity. Physical surface roughness was greater at steeper slopes. A field campaign was established to evaluate sediment yield from natural rainfall events over a two-year period following fire on steeply sloping sagebrush rangelands. Advancement was made in the understating of erosion from snowmelt and winter season runoff events for snow-dominated rangeland ecosystems. Objective 4: A variety of improvements were made to the Rangeland Hydrology and Erosion Model (RHEM), KINematic runoff and EROSion 2 (KINEROS2), and Automated Geospatial Watershed Assessment (AGWA) models as well as the Facilitator decision support tool developed and maintained by our management unit. A tutorial was developed and delivered to NRCS to aid their staff and others in application of the RHEM in predicting rangeland hydrologic and erosion responses to disturbances and land management actions (Sub-objectives 4.1, 4.2). Furthermore, a tutorial was developed and posted on the RHEM Model website entitled: Rangeland Hydrology and Erosion Model Tutorial Guide: Desert Southwest Grassland. Upon completion of the tutorial, the user will be able to assess how plant community transitions in a State-and-Transition Model affect hydrologic function of the site as a function of changes in plant lifeform, plant canopy and ground cover. AGWA and the Facilitator were loosely coupled so that KINEROS2/RHEM outputs could be ingested into the Facilitator. A number of tools in AGWA were developed to analyze Light Detection and Ranging (LIDAR) watershed geometry/topography for better representation of channel cross-sections and watershed detection storage ponds (Sub-objective 4.3).
1. U.S. semiarid agro-ecosystems impact global water and carbon cycles. Computer simulations suggest semiarid lands dominate the interannual variability of atmospheric carbon dioxide and the increasing trend of carbon uptake by the Earth’s land surface. Paradoxically, the models used in such studies are poorly validated by measurements, which have been lacking in number compared with forests. ARS researchers in Tucson, Arizona, in coordination with university-based scientists, addressed this knowledge gap with 150 site-years of measurements from 25 agro-ecosystems in southwestern North America including deserts, grasslands, shrublands, and forests. Large precipitation variability drove larger spatial and temporal variability of carbon uptake than reported in wetter regions. Models predicted only 20 to 30 percent of the measured interannual variability of plant growth and net carbon uptake, suggesting that semiarid region impacts on the global carbon cycle could be as much as 3 to 5 times larger than currently estimated. Improvements in the computer models used by scientists and managers of forest, rangeland, and water resources are needed to better understand the actual impacts of year-to-year weather variability on ecosystem water and carbon cycling.
2. Southwestern intermittent and ephemeral stream connectivity. Intermittent and ephemeral streams are important for the physical, chemical, and biological integrity of the nation's waters. ARS scientists in Tucson, Arizona, along with researchers at the Environmental Protection Agency (EPA), conducted a comprehensive examination of factors affecting the hydrologic, chemical, and ecological connectivity of ephemeral and intermittent streams on downstream waters in the arid and semiarid Southwestern U.S. Connectivity, as influenced and moderated through the physical landscape, climate, and human impacts to downstream waters, was presented first at the broader Southwestern scale, and secondly drawing on a more detailed example of the San Pedro Basin due to its history of extensive observations and research. Observations from the USDA-ARS Long-Term Agroecosystem Research (LTAR) Walnut Gulch Experimental Watershed further demonstrated connectivity to downstream waters. A wide array of evidence clearly illustrates hydrologic, chemical, and ecological connectivity of ephemeral and intermittent streams throughout stream networks. These flows supply water to downstream alluvial aquifers, nutrients and sediment to support riparian vegetation communities, and food, cover, nesting, and breeding habitat for wildlife.
3. Effective treatments to re-establish sagebrush steppe vegetation impacted by juniper woodland encroachment. Current understanding of long-term vegetation and hydrological responses to tree removal is inadequate to guide management. ARS researchers in Tucson, Arizona, Boise, Idaho, and Reno, Nevada, evaluated effects of prescribed fire, tree cutting, and tree shredding on vegetation and hydrologic and erosion processes at encroached sagebrush rangelands nine years after tree removal. Native grass cover increased substantially after burning, which improved infiltration and reduced runoff and erosion; however, burning also increased cover of invasive grasses and reduced important limited sagebrush cover. Mechanical tree removal treatments were less effective at recruiting native grasses, but limited invasive grass recruitment and sagebrush mortality. Infiltration, runoff, and erosion were largely unaffected by the mechanical treatments, but runoff and erosion rates are expected to decrease over time as native grass and sagebrush cover increase. These results provide scientists and land managers insight into the timeframe needed for vegetation and hydrologic recovery on degraded woodlands following tree removal and will help improve management of these landscapes.
4. Increases in atmospheric dryness leads to greater plant water stress. Plants experience drought stress due to lack of moisture in the soil and by the degree of dryness in the air. The impact of these two drivers of drought stress has historically been difficult to disentangle. Using data collected throughout the U.S. over a broad variety of different ecosystems, an ARS scientist in Tucson, Arizona, collaborated with university-based researchers to show that, in many ecosystems, atmospheric dryness often independently limits both plant photosynthesis and water use more than soil moisture, especially for forests. Furthermore, they show that climate change will likely result in nearly universal increases in atmospheric aridity but with more widely varying and inconsistent changes in soil moisture. Plant responses to future drought stress could diverge from our present conceptual understanding, and management approaches; e.g., increasing irrigation amounts during drought may become increasingly ineffective at mitigating plant stress.
5. Runoff and erosion experiments in the western U.S. rangelands using the Walnut Gulch Rainfall Simulator. Erosion and runoff data collection on rangeland ecosystems is labor intensive, difficult, expensive, and relatively rare, and the scarcity of data inhibits our ability to understand rangeland hydrologic and sediment function, limiting our capacity to manage rangelands. ARS scientists in Tucson, Arizona, and Reno, Nevada, summarized a data set of hydrological, erosion, vegetation, ground cover, and other supplementary information from 272 rainfall simulation experiments conducted on 23 semi-arid rangeland locations in Arizona and Nevada between 2002 and 2013. The simulations were conducted under a wide range of rainfall intensities on plots with a variety of slopes, ground cover and vegetation cover. The scope of this data set, combined with state-of-the-art rainfall simulation equipment, makes it particularly valuable to advance our understanding of basic erosion and transport processes specific to rangelands. The data set can be used to evaluate and compare management practices, and study ecological states, transitions and thresholds. It can also support erosion model development and validation.
6. Evaluation of satellite rainfall estimates against three dense gauge networks. Accurate global estimates of precipitation are essential for understanding and managing essential water resources. The USDA-ARS Long-Term Agroecosystem Research (LTAR) Walnut Gulch Experimental Watershed (WGEW) was one of three dense rain gauge sites in the world used in a recent assessment of National Aeronautical and Space Administration’s (NASA) active and passive Global Precipitation Measurement (GPM) products. The evaluation was conducted at the level of individual satellite pixels with multiple gauges per satellite pixel and precise precipitation accumulations near satellite overpass time to ensure a representative comparison. ARS researchers at Tucson, Arizona, along with NASA scientists, found that the active radar precipitation retrievals generally performed better than the passive retrievals. They found that rainfall retrieval diminishes greatly under coastal conditions and the semiarid conditions at WGEW. Results from this effort will be used to improve algorithms for GPM rainfall retrievals.
7. Satellite images provide valuable information about agroecosystems. The intensity of the green hue of a crop, forest, or other vegetation type can be used to estimate plant growth. However, the relationship between greenness and growth is weak in rangeland. ARS researchers in Tucson, Arizona, in collaboration with university-based scientists, compared traditional greenness data measured by satellites with emerging datasets of solar-induced fluorescence (SIF), which more directly indicates plant growth. They compared these data with carbon uptake measurements, indicators of plant growth, in 21 diverse ecosystems in the Southwest region of North America. They found SIF better captured seasonal and annual variations, although greenness better captured spatial variations. This work suggests that the combination of new and existing technologies will produce superior results for assessing vegetation productivity using satellites.
8. Unmaintained conservation structures cause erosion. Legacy water-conservation and erosion-control structures are a primary control on hydrologic and geomorphic regimes in managed rangelands, especially if the structures are not maintained. ARS scientists in Tucson, Arizona, conducted a comprehensive assessment of earthen berms, water spreaders, and water control gates on a former commercial cattle ranch that was converted to a wildlife refuge 33 years ago. The results showed that almost half of the earthen berms, and approximately 20 percent of the water spreaders had failed. Gullying and altered surface runoff patterns, induced by unmaintained structures, are a threat to large areas of productive rangeland. This new knowledge of the impacts of legacy conservation structures on gullying and altered runoff patterns provides ranchers and land management agencies critical data for managing semiarid rangelands to prevent flooding and control erosion.
9. Improvements in the rangeland hydrology and erosion modeling. A process-based rangeland erosion model that can function as a practical tool for quantifying runoff and erosion rates specific to western U.S. rangelands, that provides reasonable runoff and soil loss prediction capacity for rangeland assessment, management, and research is a critical need. The Rangeland Hydrology and Erosion Model (RHEM) is a tool developed by the United States Department of Agriculture (USDA) for assessing runoff and soil erosion rates on western rangelands of the U.S. ARS researchers in Tucson, Arizona, presented an improved version (V2.3) of the RHEM model. The capability of RHEM V2.3 for simulating flow and soil erosion was tested on a small watershed in Arizona and on 124 plots placed in Arizona and New Mexico. Evaluation of the model predictions showed that RHEM V2.3 produces results of satisfactory quality when simulating large flow and soil erosion events, but a greater degree of uncertainty is associated with predictions of small runoff and soil erosion events. This model provides USDA with a tool that will have significant impact on helping to understand, assess, and manage western rangelands in the U.S.
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