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United States Department of Agriculture

Agricultural Research Service

Research Project: Quantifying Relative Contributions of Salt Mobilization and Transport from Rangeland Ecological Sites Within the Upper Colorado River Basin

Location: Great Basin Rangelands Research

Project Number: 5370-13610-001-17
Project Type: Interagency Reimbursable Agreement

Start Date: Sep 01, 2013
End Date: Sep 30, 2015

Objective:
The Rangeland Hydrology and Erosion Model (RHEM) was developed exclusively on data collected from a large number of geographically distributed rangeland erosion experiments. RHEM was designed to require minimal input that is readily available for most rangeland ecological sites: surface soil texture; slope length, steepness and shape; dominant plant life-form (i.e., shrub, shortgrass, annual grass etc.); canopy cover; ground cover (rock, litter, basal area, and microbiotic crust); and precipitation. RHEM estimates runoff, soil erosion, and sediment delivery rates and volumes at the spatial scale of the hillslope and the temporal scale of a single rainfall event. RHEM can be utilized to evaluate alternative management scenarios (i.e., a change in vegetation state as represented by changes in canopy and ground cover or vegetation composition by life form) to reduce environmental and economic vulnerability and track environmental benefits of management actions within watersheds if the hydrologic response of the alternative vegetation states are known on rangelands. However, the current limitation in conducting national and regional assessment of the benefits of conservation on rangelands is lack of scientific data concerning the hydrologic response of changes in land cover and land use of rangelands at the ecological site scale to effectively parameterize RHEM. The project goal is to rank ecological sites to determine relative contribution to reduce salt mobilization and transport to the upper Colorado River. A second goal is to evaluate the RHEM model for assessing hydrology, erosion, and salt mobilization and loading responses associated with management of vegetation within the upper Colorado River. A third goal is to share data with USGS for improvement of the SPARROW model for estimating salt loading and transport into Upper Colorado River Basin.

Approach:
Rainfall simulation will be used to quantify the hydrologic, erosion, and salt mobilization and transport response on targeted ecological sites defined by the BLM in the upper Colorado River Basin. Four to six plots per ecological site will be used to quantify salt mobilization and loading as a function of existing soil and vegetation condition. Rangeland Health, National Resource Inventory and BLM Annual Inventory and Monitoring assessments will be collected at each experiment site to allow for correlation to measured infiltration, runoff, erosion and salt transport. To quantify the hydrologic response of the different ecological site, we will use a rainfall simulator (2 m wide x 6 m long) with rainfall rates at 5 and 50-year return period rainfall events (Appendix I). We will follow, in general, the rainfall simulation procedures that were developed during the USDA Water Erosion Prediction Project to quantify soil characteristics, infiltration, runoff, and erosion rates. Rainfall simulation plots will be characterized prior to rainfall simulation to document vegetation (canopy and ground cover, standing biomass, and plant height). Post rainfall simulation ground cover will be reassessed to quantify changes in distribution in ground cover as a function of runoff and soil loss. Soil surface attributes (topography, bulk density, aggregate stability, surface texture, and soil salinity) will be quantified before and after rainfall simulation. Depth of wetting front will be quantified after each rainfall simulation experiment to estimate transport of salts to vadose zone. Three digital cameras will be mounted on the rainfall simulator to provide for a synoptic view of the entire plot (1-mm pixel resolution) and will be used to identify the gap frequency between shrubs. A digital camera will be located on a trolley mounted on the outside frame of the simulator (2 meter AGL) and a picture will be taken along the entire perimeter of the plot with 90% overlap for the development of a plot digital elevation model. Images will be taken before and after rainfall simulation allowing for determination of soil movement (both loss and deposition) to be calculated. In addition, time-lapse photography from the overhead cameras during the rainfall simulation will allow us to trace fluorescent dye applied at the top of the plot as it is transported down slope. We will quantify the number and distribution of concentrated flow paths and how they change as a function of rainfall intensity using commercially available image analysis software, such as ERDAS and ESRI (ArcGIS 10). These data will also be used to develop model parameters for RHEM for predicting runoff, sediment yield, and salt loads. RHEM model performance will be evaluated using the Nash and Sutcliffe coefficient by using observed data versus modeled predicted runoff volume, peak flows and sediment yield.

Last Modified: 8/30/2014
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