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Research Project: Soil Erosion, Sediment Yield, and Decision Support Systems for Improved Land Management on Semiarid Rangeland Watersheds

Location: Southwest Watershed Research Center

2013 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:
Soil samples for 137Cs analysis were collected at 12 field locations at the Empire Ranch for the Loamy Upland ecological site, several of which have been previously measured by rainfall simulation studies. Vegetation characteristics at all sites were used to determine the state of the site within the context of the state and transition model for the ecological site. Analyses of the less than 2-mm soil fraction for 137Cs was made by gamma-ray spectrometry. Rainfall simulator experiments were conducted on four sites of the Loamy Upland ecological site at the Walnut Gulch Experimental Watershed and at the Empire Ranch. Data from the overland flow part of the experiment were used with overland flow data from ARS Boise, Idaho, to determine the structure of the concentrated flow erodibility parameter estimation equation for the Dynamic Rangeland Hydrology and Erosion Model (DRHEM). Progress was made to scan and upload historic photos to the website. Photo points have been relocated in the San Simon Watershed. Tools for creating tiepoints and co-registering temporally sequential photos have been developed and implemented through the website. In addition, tools for annotating, storing, and managing user contributed content have been implemented. Time-lapse hardware has been built and deployed at 3 field sites in Arizona. A geodatabase of sediment control and retention structures (including stock ponds) has been developed in cooperation with the University of Arizona Department of Arid Lands and the Graham County Cooperative Extension Office. In addition, the Arizona Department of Water Resources stock tank permit database has been evaluated with respect to aerial imagery to verify stock tank locations in both the San Simon and Las Cienegas Watersheds. The unit's stock tank runoff data through 2012 has been integrated and made available through the unit's database website. The steady-state solution of the RHEM equations were solved within the dynamic framework of KINEROS2, with additional capability to dynamically update soil erodibility parameters for both concentrated and sheet flow within storm events. The resulting model, DRHEM, was integrated into a web site application. Results of research in collaboration with ARS in Boise, Idaho, were used to develop parameter estimation equations for concentrated flow on disturbed rangeland sites. DRHEM was fully integrated into the KINEROS2 model for use in rangeland Conservation Effects Assessment Project (CEAP) evaluations done in collaboration with the Natural Resources Conservation Service (NRCS). A spatial database of grazed and ungrazed management units across Arizona was refined by developing a method to resolve inconsistent boundaries and attribute tables. The database has gaps where public data are not available. Further improvement of this layer will require coordination between the different land management agencies, for example, agreement on common boundaries between adjoining allotments held by the Forest Service and Bureau of Land Management. A spatial database of areas with NRCS conservation practices applied between 2003 and 2006 was assessed.

4. Accomplishments
1. A hillslope erosion model for disturbed rangelands. Analysis of data from fire-disturbed rangeland sites has illustrated the importance of intra-storm dynamics on soil erosion because of major changes in the soil surface during erosion events. The commonly used steady-state solutions of erosion equations, such as those in the Water Erosion Prediction Project (WEPP) and the Rangeland Hydrology and Erosion Model (RHEM), are unable to represent these intra-storm dynamics and changes. Therefore, ARS researchers at the Southwest Watershed Research Center in Tucson, Arizona, solved the RHEM equations within the dynamic framework of the watershed model KINEROS2, with capability to dynamically update soil erodibility parameters within storm events. This work represents a major step forward for ARS hillslope-scale erosion models in general, and for rangeland hydrologic and erosion assessment in particular, as it is the first major ARS model that is focuses primarily on soil erosion that incorporates intra-storm dynamics and thus changing characteristics of the site during a rainfall event. In certain conditions, such as those found after fire on natural rangeland sites, this will be an important factor in accurately assessing erosion rates.

2. Sediment budget developed for a small southwestern watershed. Semiarid areas are among the highest sediment producing regions in the world, but sediment budgets that quantify the long-term movement of sediment at the small watershed scale are rare. ARS researchers in Tucson, Arizona, developed a sediment budget for a 43.7 ha and a nested 3.7 ha semiarid, shrub dominated watershed. The sediment budget is based on hydrologic, geomorphic, erosion, and sediment data collected from 1963 through 2006 on watershed 223 within the USDA, ARS Walnut Gulch Experimental Watershed in the southeastern Arizona. Although the channel network is well developed and incising in the steeper reaches of the watershed, hillslopes are the dominant source of sediment, contributing 85% of the overall total sediment yield. Knowledge of the source area of sediment (hillslope or channel) will assist land use managers in both the targeting of new and evaluation of existing conservation practices.

Review Publications
Nichols, M.H., Nearing, M.A., Polyakov, V.O., Stone, J.J. 2012. A sediment budget for a small semiarid watershed in southeastern Arizona, USA. Geomorphology. 180-181: 137-145.
Cerdà, A., Brazier, R., Nearing, M.A., De Vente, J. 2013. Scales and erosion. Catena. 102:1-2.
Al-Hamdan, O.Z., Pierson Jr, F.B., Nearing, M.A., Williams, C.J., Stone, J.J., Kormos, P.R., Boll, J., Weltz, M.A. 2012. Concentrated flow erodibility for physically-based erosion models: temporal variability in disturbed and undisturbed rangelands. Water Resources Research. DOI: 10.1029/2011WR011464.
Nichols, M.H., Mcreynolds, K., Reed, C. 2012. Short-term soil moisture response to low-tech erosion control structures in a semi arid rangeland. Catena. 98:104-109.
Reike-Zapp, D., Nichols, M.H. 2011. Headcut retreat in a semiarid watershed in the southwestern United States since 1935. Catena. 87(1):1-10.
Al-Hamdan, O.Z., Pierson Jr, F.B., Nearing, M.A., Williams, C.J., Stone, J.J., Kormos, P.R., Boll, J., Weltz, M.A. 2013. Risk assessment of erosion from concentrated flow on rangelands using overland flow distribution and shear stress partitioning. Transactions of the ASABE. 56(2):539-548.
Delgado, J.A., Nearing, M.A., Rice, C. 2013. Conservation practices for climate change. Advances in Agronomy. 121:47-115.
Ascough II, J.C., Flanagan, D.C., Nearing, M.A., Engel, B.A. 2013. Sensitivity and first-order/Monte Carlo uncertainty analysis of the WEPP hillslope erosion model. Transactions of the ASABE. 56(2)/437-452.
Guzmán, G., Quinton, J., Nearing, M.A., Mabit, L., Gómez, J. 2013. Sediment tracers in water erosion studies: Current approaches and challenges . Journal of Soils and Sediments. 13:816–833.
Polyakov, V.O., Nearing, M.A., Hawdon, A., Wilkinson, S., Nichols, M.H. 2013. Comparison of two stream gauging systems for measuring runoff and sediment yield for a semi-arid watershed. Earth Surface Processes and Landforms. 38: 383–390.
Sanches Oliveira, P., Wendland, E., Nearing, M.A. 2013. Rainfall erosivity in Brazil: A Review. Catena. 100:139–147.