Location: Southwest Watershed Research Center2014 Annual Report
1a. Objectives (from AD-416):
Objective 1. Provide databases, knowledge, and information on rangeland erosion at a range of spatial scales for the development, validation, and implementation of erosion decision tools. Objective 2. Develop decision tools including a rangeland specific hydrology and erosion model for improved planning and evaluation of rangeland management practices.
1b. Approach (from AD-416):
This project addresses the need for rangeland decision tools to assess the climatic and conservation management effects on rangeland sustainability as affected by runoff and erosion. The Natural Resources Conservation Service (NRCS) has requested that the Rangeland Hydrology and Erosion Model (RHEM) be integrated with a watershed scale model for the Conservation Effects Assessment Project (CEAP). Techniques to assess natural drivers and management practice effects on rangeland hydrologic and erosion processes at the hillslope or ecological site scale are primarily qualitative. Techniques are needed to quantify erosion rates and processes, particularly under disturbed conditions where data are lacking. At larger scales, the impact of conservation structures on sediment dynamics is poorly understood. Tools are needed that integrate the effect of management practices implemented at the hillslope scale with watershed scale processes to assess their environmental impact and cost effectiveness. The general approach of this project is to develop databases, knowledge, and information that will be used to evaluate conservation practices and quantify the physical and effectiveness of those practices on reducing runoff and erosion at the hillslope and watershed scale. The expected outcomes of the research are 1. Databases and improved measurement techniques to quantify a) decadal-scale hillslope erosion rates, b). overland flow erosion for disturbed conditions, c) sediment transfers as impacted by conservation structures and d). landscape change; 2. Integration of RHEM and KINEROS2 in the GIS based AGWA framework for rangeland conservation practice assessment; and 3. A framework to assess effectiveness of conservation practices.
3. Progress Report:
This research is performed under National Program 211, Water Availability and Watershed Management in support of ARS Goal 6, Protect and Enhance the Nation’s Natural Resource Base and Environment. This report summarizes progress through the 36th month of the five-year project plan. We made progress on both objectives. On Objective 1, progress included interpretation of cesium-137 (Cs-137), a radioactive isotope of cesium, measurements in the context of state and transition models, as well as of long-term small watershed runoff and sedimentation rates measured in stock tanks. Soil samples from five rainfall simulation sites on the Loamy Upland ecological site have been analyzed and soil erosion rates estimated. Erosion rates determined from Cs-137 and rainfall simulation are being related to vegetation characteristics in the context of the state and transition model for the ecological sites. Soil erosion rates by slope-wash and the various factors that control these rates in the Mohave Desert have been summarized. Cs-137 in soils collected from a series of flumed terraces in Bushland, Texas is being analyzed to study wind erosion. We also collected and analyzed alluvial samples from three ponds in Walnut Gulch to determine erosion and sedimentation dynamics on watershed scales as related to state and transition models. Additional progress on Objective 1 to develop tools to improve hydrologic assessment and watershed management is advancing through development and application of high resolution time-lapse photography. Progress was made to advance hardware, electronic, and photographic systems for documenting ecological and geomorphic change as well as land use impacts that occur either infrequently or on time steps that are too long for standard measurement and observation techniques to interpret. A time lapse camera system capable of operating for up to 3 months collecting high resolution digital images on a 30 second time step was deployed and successfully tested. Images are currently being collected to quantify rates of headcut advance, ephemeral runoff, and elk and cattle grazing dynamics in a riparian meadow. Progress on Objective 2 to improve and enhance analytical and predictive tools continues through ongoing development of the Rangeland Hydrology and Erosion Model (RHEM). Version 2.2 of the model was released with a new set of parameter estimation equations developed for the Smith-Parlange infiltration equation. In addition, a minor adjustment has been made to the splash and sheet erodibility parameter estimation equation to account for an improved calibration factor and better represent slope steepness. The erodibility coefficient has been slightly increased to better represent concentrated flow erosion in undisturbed rangelands. New work at ARS Tucson with RHEM offers an ability to synthesize information across a wide variety of rangeland ecosystems and could be used as an analytical and predictive tool. There has been a great deal of hydrologic research on rangelands in the past 50 years that is relevant and important for the development of Ecological Sites and State and Transition Models. As Ecological Site work shifts more toward a hierarchical, multivariate approach, the ability to integrate a wider variety of differentiating factors into a systematic way to delineate and describe sites becomes critical. The hydrology of individual sites can determine the behaviors (production, erosion, runoff, water quality, vegetation structure) that provide the basis for grouping soils into sites. Managers and policy makers also require a predictive understanding of hydrology at the site and landscape scales to make decisions about land use, management and conservation practices. We convened a small working group with experience in Ecological Sites and rangeland hydrology to develop an initial approach to the evaluation and integration of hydrologic information into the Ecological Site Description system. In addition, a two day workshop that included transfer of model technologies was held in conjunction with the research unit’s 60th Anniversary celebration of the USDA-ARS Walnut Gulch Experimental Watershed. The workshop was attended by stakeholders including land owners, Federal and State land management agencies, scientists, and the public.
1. Improved ability to simulate erosion on disturbed rangelands. As there is no generally accepted erosion prediction model for rangelands, a long-term goal has been the development of the Rangeland Hydrology and Erosion Model (RHEM). The greatest danger of erosion, but most difficult to simulate, is following major disturbance, such as a fire. ARS researchers in Tucson, Arizona, and Boise, Idaho, were able to develop empirical equations that predict splash and sheet flow detachment in terms of ground and vegetation cover. RHEM was able to match the predicted effect of disturbances and treatments across a wide range of vegetation and ground cover conditions except for steep slope (> 20%) sites immediately after fire; where the newly developed splash and sheet equations were used together with concentrated flow detachment equations to estimate soil loss. The improved splash and sheet flow erosion modeling approach in RHEM creates a practical management tool for quantifying erosion and assessing erosion risk following rangeland disturbance. The enhancements to RHEM expand its applicability as a practical land management tool for conservation planning and quantifying environmental benefits of alternative conservation practices.
2. Uncertainty in stock pond runoff analyzed. There is very little measured runoff data available from rangeland watersheds, limiting understanding of fundamental hydrological processes. The Walnut Gulch Experimental Watershed stock pond database, recently completed, is one of the longest-term, spatially distributed sets of event scale runoff measurement in the world. The database was evaluated to quantify measurement uncertainty associated with runoff monitored at watershed outlet ponds. The effects of instrumentation, field methods, and QA/QC and data processing procedures were evaluated. The uncertainty analysis provides critical information to improve field methods, develop simulation models, and assess the quality of simulation model predictions.
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