Location: Southwest Watershed Research Center2017 Annual Report
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.
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.
This is the final report for this project which expired in February of 2017. Please see the report for the new project, 2022-13610-012-00D, “Understanding Water-Driven Ecohydrologic and Erosion Processes in the Semiarid Southwest to Improve Watershed Management”, for additonal information. This project is carried out under National Program 211 - Water Availability and Watershed Management. The first objective of this project is associated with National Action Plan Components 2 - Erosion, Sedimentation and Water Quality Protection; the second objective with Component 3 - Improving Conservation Effectiveness. This report summarizes progress for the five year term of the project. During the past five years, substantial results were realized on objective 1 to develop tools for quantifying climate and conservation management effects on rangeland sustainability as affected by runoff and erosion. Through field experiments and mathematical modeling, new databases, knowledge, and information were developed that improved our understanding of semiarid rangeland erosion and sediment transfers resulting in applied technologies to conserve soil and water. Rainfall and runoff data collected since 1953 are available to scientists, decision makers, and the public through the USDA ARS online data access project (DAP), which serves as the repository for data collected in support of the current project. During the past 5 years, additional sediment, meteorologic, ecologic, and remotely sensed data have been made available online. One of the longest-term, spatially distributed sets of event scale runoff measurements collected at small watershed outlet stock tanks was assimilated in the DAP. These data were used to determine that the largest source of uncertainty associated with runoff monitored at watershed outlet ponds is measurement of overflow through spillways, a finding that improved field methods for reducing uncertainty and improving the quality of field measurements. An isotopic technique assessing Caesium-137 as an indicator of average erosion and deposition over approximately the last 50-60 years was used to quantify decadal scale semiarid erosion rates and short-term erosion variability. The new technique was used to measuring stock pond reservoir sedimentation rates using cesium and lead signatures in accumulated sediment with results comparable to traditional measurement techniques. A very-high temporal resolution ground-based time-lapse photography was developed and used to capture gully erosion process and riparian area grazing use. Mass wasting and headcut plunge pool erosion were the most frequent erosion mechanisms driving headcut advance, however the previously undocumented process of subsurface erosion preceded that largest geomorphic change. The time-lapse system has wide applicability and was used to determine that riparian areas are grazed differently by cattle and elk; elk exhibited the unique behavior of standing in and traveling within stream channels while grazing, and grazed while lying down. The system is being used to document wild horse impacts on U.S. Forest Service lands. Vegetation differences explain morphological differences in the shape of semiarid lands. Specifically, as woody species replace grasses the land erodes and becomes more incised, water drains much more quickly during rain events, and the landscape tends to hold less water and becomes generally less productive for grazing and wildlife. Rock check dams can be used to restore degraded lands by retaining channel sediment and reducing in-channel gradients, however the impact of check dams on watershed outlet sediment yield is relatively short-lived. Rainfall erosivity is the capability of rainfall to cause soil loss from hillslopes by water. An historical review of rainfall erosivity determined that the Universal Soil Loss Equation erosivity empirically explains much of the variance in the soil loss from natural rainfall erosion plots. The dominant erosion process changes from raindrop detachment to flow detachment as an ecological site transitions from the reference state to degraded states. A technique for mapping woody cover using free, publicly available satellite imagery was developed. An integrated multi-platform approach for assessing brush management conservation efforts in semiarid rangelands is available, providing a cost-effective method for monitoring both conservation effects and cost/benefits ratios of brush management on the 188 million hectares of central and western rangelands and grazed forests that were treated at a cost of $19 million between 1977 and 2003 by the USDA Natural Resources Conservation Service (NRCS). In fulfillment of Objective 2, the Rangeland Hydrology and Erosion Model (RHEM) was developed in collaboration with ARS units in Boise, Idaho, and Reno, Nevada. A new approach was developed for parameterizing the splash and sheet flow erodibility parameter to enhance RHEM applications on disturbed rangelands. The enhancements to RHEM expand its applicability as a practical land management tool for conservation planning and quantifying environmental benefits of alternative conservation practices. An improved rainfall disaggregation algorithm was developed to better represent observed maximum rainfall intensities and thus improve erosion predictions. The model was extended to incorporate risk assessment. The RHEM Risk Assessment Tool was developed to evaluate and illustrate the risk of excessive runoff and soil erosion on rangeland sites relative to desired or optimal rangeland conditions. This tool facilitates the process of developing Ecological Site Descriptions, which are formal documents that are currently being developed in a large and active program across the United States by the Natural Resources Conservation Service, Forest Service, and Bureau of Land Management, to describe the hydrologic and vegetation functions of land resources, particularly for grazing lands. The RHEM model has been integrated into the GIS-based Automated Geospatial Watershed Assessment (AGWA) tool which is used by watershed, water resource, land use, and resource managers and scientists investigating the hydrologic impacts of land-cover/land-use change in small watershed to basin-scale studies. A watershed scale model incorporating process-based research and the RHEM was developed and validated to assess water-driven rangeland soil erosion rates. The overall impact of the combined research accomplishments is the availability of new information and tools for hillslope and waershed scale erosion assessments and decision making across the Western U.S.
1. Measuring small watershed sediment delivery using isotopic signatures. ARS scientists in Tucson, Arizona, quantified erosion dynamics of the past 90 years on three small semi-arid watersheds in Arizona with histories of grazing and vegetation change. Sediment cores were collected from sediment accumulated in stock ponds, and isotopes were measured using a gamma spectrometer. Collected sediment was dated, and past variations in sedimentation rates were identified and correlated with recorded history of anthropogenic disturbance. This study represents the first time that reservoir sediment accumulation rates determined from fallout isotopes have been verified in comparison with traditional measurements of aggradation based on topographic surveys and demonstrates that there is now an opportunity to quantify historic erosion and sediment transfer dynamics across the Southwest in areas that have not been well studied or instrumented.
2. Validation of the Rangeland Hydrology and Erosion Model (RHEM) Risk Assessment Tool. ARS scientists in Tucson, Arizona, formally documented the RHEM tool to show its capability for evaluating the risk of excessive runoff and soil erosion on rangeland sites relative to desired or optimal rangeland conditions. The capability of RHEM V2.3 for simulating flow and soil erosion was tested on a small watershed in Arizona and on 124 sites in Arizona and New Mexico. This model provides a tool that will have significant impact on helping to understand, assess, and manage western rangelands in the United States. In addition, this tool will greatly facilitate the development of Ecological Site Descriptions which are formal documents that are currently being developed in a large and active program across the U.S. by the Natural Resources Conservation Service, Forest Service, and Bureau of Land Management to describe the hydrologic and vegetation functions of land resources, particularly for grazing lands.
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Al-Hamdan, O.Z., Pierson, F.B., Nearing, M.A., Williams, C.J., Hernandez, M., Boll, J., Nouwakpo, S.K., Weltz, M.A., Spaeth, K.E. 2017. Developing a parameterization approach of soil erodibility for the Rangeland Hydrology and Erosion Model (RHEM). Transactions of the ASABE. 60(1):85-94.
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