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United States Department of Agriculture

Agricultural Research Service

Research Project: QUANTIFYING LANDSCAPE FACTORS INFLUENCING SOIL PRODUCTIVITY AND THE ENVIRONMENT Title: Sediment Budgets and Source Determination Using Fallout Cesium-137 in Semiarid Rangelands

Authors
item Ritchie, Jerry
item Nearing, Mark
item Rhoton, Fred

Submitted to: International Atomic Energy Agency
Publication Type: Proceedings
Publication Acceptance Date: November 3, 2007
Publication Date: N/A

Interpretive Summary: Analysis of soil redistribution and sediment sources in semiarid and arid rangelands can provide information for understanding watershed sediment budgets and rangeland conditions so that management practices can be implemented to improve rangeland conditions and reduce sediment loads in streams. Sediment budgets and sediment source determination using fallout 137Cs can provide useful information for understanding soil redistribution patterns and sediment sources areas to estimate which portions of a semiarid rangeland watershed are producing the suspended sediment loads in a stream. Two methods (sediment budgets and sediment source analyses) indicate that shrub dominated systems provide more suspended sediments to the stream systems and that sediment yields measured at the outlet of a watershed are a poor indicator of actual soil redistribution within a watershed. Management of these semiarid rangelands must consider conservation techniques that will protect grass dominated areas from shrub invasion. Expanding sampling areas to include more geomorphological surfaces (i.e., streambanks, gully faces, etc.) would allow inferences to be made about the relative contribution of streambank versus gully versus upland soil erosion contributions. There is also a need to determine which soils or slope conditions are contributing so that site specific remediation can be planned. Eventually, the ability to identify primary sediment sources in watersheds will contribute to a more efficient design of the best management practices to reduce pollutant loads, both suspended sediments and chemicals, from watersheds.

Technical Abstract: Analysis of soil redistribution and sediment sources in semiarid and arid rangelands can provide information for understanding watershed sediment budgets and rangeland conditions and for implementing management practices to improve rangeland conditions and reduce sediment loads in streams. The purpose of this research was to develop sediment budgets and to identify potential sediment sources using Cesium-137 in a series of small semiarid rangeland watershed on the USDA ARS Walnut Gulch Experimental Watershed near Tombstone, Arizona, USA. Soils were sampled in a grid pattern on two small watersheds and along transects associated with soils and geomorphology on six larger watersheds. Soil samples were analyzed for Cs-137 and selected physical and chemical properties (i.e., bulk density, rocks, particle size, organic carbon). Suspended sediment samples collected at flume sites on the Walnut Gulch Experimental Watershed were also analyzed. Sediment budgets measured using 137Cs inventories for a small shrub and a small grass watersheds found eroding areas were losing 5.6 and 3.2 t/ha/yr, respectively; however, a sediment budget for each of the small watersheds, including depositional areas, found net soil loss to be 4.3 and nearly zero t/ha/yr for the shrub and grass watersheds, respectively. Generally, the suspended sediments collected at the flumes of the larger watersheds were enriched in silt, clay, and K-40. Sediment source analyses of the data indicated that the shrub dominated subwatersheds were contributing most of the suspended sediments that was measured at the outlet flume of the Walnut Gulch Experimental Watershed. The two methodologies (sediment budgets and sediment source analyses) indicate that shrub dominated systems provide more sediments to the stream systems. These studies indicate that sediment yields measured at the outlet of a watershed may be a poor indicator of actual soil redistribution within a watershed. Management of these semiarid rangelands must consider conservation techniques that will protect grass dominated areas from shrub invasion

Last Modified: 8/27/2014
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