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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

2015 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 report summarizes progress through the 48th month of the five-year project plan. Progress is ongoing on Objective 1) to provide databases, knowledge, and information on rangeland erosion at a range of spatial scales for the development, validation, and implementation of erosion decision tools, and Objective 2) to develop decision tools including a rangeland specific hydrology and erosion model for improved planning and evaluation of rangeland management practices. A vacant scientist position resulting from a retirement in FY13 continued to impact progress. Progress was made on Objective 1 by developing Cesium137 databases to quantify decadal scale erosion rates. Soil erosion can play a significant role in how one plant community transitions to another, for example, as a site degrades from a healthy condition to a poor one. Two existing tools were used to measure soil erosion at different time scales to better understand the role that soil erosion might play in such transitions. Soil erosion was measured using an isotopic technique with 137Cs, which quantifies erosion as an average over approximately the last 50-60 years, and with a rainfall simulator, which characterizes erosion at essentially a single point in time. A comparison of these measurements showed the importance of site conditions, such as recent fires, on measured soil erosion. The longer term 137Cs measurements probably have more meaning in terms of characterizing the long-term differences in soil erosion rate as a function of plant community, but the rainfall simulator measurements are more useful to identify processes that have the potential to impact the ecological system and potentially cause a change in the state of the site. Additional progress on Objective 1 was made through development and application of high resolution time-lapse photography. Hardware, electronic, and photographic system advances were made documenting geomorphic change that occurs either infrequently or on time steps that are too long for standard measurement and observation techniques to interpret. A high resolution time-lapse camera system was deployed in the ARS Walnut Gulch Experimental Watershed (WGEW), and images were collected to document and quantify the dominant erosion process in response to an abrupt lowering of the channel bed. The previously unobserved process of piping was identified through the photography as an important erosion process on the WGEW. In the absence of time-lapse images the occurrence of subsurface erosion would not have been known. Progress was made on Objective 2 to incorporate updated algorithms to address the impacts of fire on rangeland erosion rates leading to the release of a new version (2.3) of the Rangeland Hydrology and Erosion Model (RHEM). The new version updated the parameter estimation equations to estimate the splash and sheet erodibility coefficient on natural rangeland conditions, and improved the rainfall disaggregation algorithm. The acquisition of a ground based lidar system in FY15 has allowed us to develop very high resolution topographic models that support applied field research and simulation model development and testing.

4. Accomplishments
1. Improved prediction of rangeland hydrologic and erosion processes. A new version of the Rangeland Hydrology and Erosion Model (RHEM) was developed and released by ARS scientists in Tucson, Arizona, in collaboration with ARS scientists in Boise, Idaho. The new release (version 2.3) uses expanded data sets covering an extended geographical range and new splash and sheet equations were developed to identify the breakpoint or threshold at which the relationship between splash and sheet erodibility and ground cover changes significantly. The rainfall disaggregation algorithm was improved by increasing the number of intervals in the step function describing the double exponential distribution from 10 to 20 to better represent the observed maximum rainfall intensities. These model improvements will result in better erosion predictions to improve rangeland conservation planning.

2. Assessing brush management conservation efforts in semiarid rangelands. Millions of dollars have been spent on brush management, or removal of unwanted woody vegetation, as a conservation practice to control the presence of woody species. Land managers need an inexpensive means of monitoring the effects of brush management conservation methods for decreasing degradation in rangeland systems. ARS scientists in Tucson, Arizona, developed a technique combining two types of free, publicly available satellite imagery to map woody cover. The resultant maps can be used to track brush removal, as well as monitor presence or lack of subsequent reemergence. This work provides land managers and action agencies with an operational means of determining where to allocate resources to implement brush management and to monitor the effectiveness of brush management conservation investments.

Review Publications
Flores Cervantes, J., Istanbulluoglu, E., Vivoni, E., Holifield Collins, C.D., Bras, R. 2014. A geomorphic perspective on terrain-modulated organization of vegetation productivity: Analysis in two semiarid grassland ecosystems in Southwestern United States. Ecohydrology. 7:242-257.
Holifield Collins, C.D., Stone, J.J., Cratic Iii, L. 2015. Runoff and sediment yield relationships with soil aggregate stability for a state-and-transition model in southeastern Arizona. Journal of Arid Environments. 117:96-103.
Polyakov, V.O., Nichols, M.H., Mcclaran, M., Nearing, M.A. 2014. Effect of check dams on runoff, sediment yield and retention on small semi-arid watersheds. Journal of Soil and Water Conservation. 69:414-421.
Sanchez-Cohen, I., Díaz-Padilla, G., Velasquez-Valle, M., Slack, D., Heilman, P., Pedroza-Sandoval, A. 2015. A decision support system for rainfed agricultural areas of Mexico. Computers and Electronics in Agriculture. 14:178-188.
Garbrecht, J.D., Nearing, M.A., Shields, D., Tomer, M.D., Sadler, E.J., Bonta, J.V., Baffaut, C. 2014. Impact of weather and climate scenarios on conservation assessment outcomes. Journal of Soil and Water Conservation. 69(5):374.