ASSESSING CLIMATE, SOIL AND LANDSCAPE PROCESSES AFFECTING AGRICULTURAL ECOSYSTEMS
Title: Sediment Budgets and Source Determinations Using Fallout Cesium-137 in a Semiarid Rangeland Watershed in Arizona, USA
Submitted to: Journal of Environmental Radioactivity
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 15, 2009
Publication Date: June 25, 2009
Citation: Ritchie, J.C., Nearing, M.A., Rhoton, F.E. 2009. Sediment budgets and source determinations using fallout Cesium-137 in a semiarid rangeland watershed, Arizona, USA. Journal of Enivronmental Radioactivity. 100:637-643.
Interpretive Summary: Both sediment budgets and sediment source analyses indicate that shrub-dominated ecosystems are providing more suspended sediment at the Walnut Gulch Experimental Watershed than the grass-dominated ecosystems. Sediment budgets and sediment source analyses using fallout 137Cs provided useful data for understanding soil redistribution patterns and sediment sources areas to estimate which portions of the semiarid rangeland watershed are producing the suspended sediment loads in a stream. The sediment budget studies indicate that significant soil redeposition is occurring within the watershed before soil particles reach the watershed outlet, thus sediment yields measured at the outlet of a watershed may be a poor indicator of magnitude of actual soil redistribution within a watershed. Expanding our sampling areas to include more eroding surfaces (i.e., streambanks, gully faces, etc.) as well as sheet erosion sites would allow inferences to be made about the relative contribution of streambank versus gully versus sheet erosion contributions from the subwatersheds. The ability to identify primary sediment sources in watersheds contributes to a more efficient implementation of management practices to reduce suspended sediment and chemicals load from watersheds. Our studies suggest that management of these semiarid rangelands must consider techniques that will protect grass-dominated areas from shrub invasion.
Analysis of soil redistribution and sediment sources in semiarid and arid watersheds provides information for implementing management practices to improve rangeland conditions and reduce sediment loads to streams. The purpose of this research was to develop sediment budgets and to identify potential sediment sources using 137Cs and other soil properties in a series of small semiarid subwatersheds on the USDA ARS Walnut Gulch Experimental Watershed near Tombstone, Arizona, USA. Soils were sampled in a grid pattern on two small subwatersheds and along transects associated with soils and geomorphology on six larger subwatersheds. Soil samples were analyzed for 137Cs and selected physical and chemical properties (i.e., bulk density, rocks, particle size, soil organic carbon). Suspended sediment samples collected at measuring flume sites on the Walnut Gulch Experimental Watershed were also analyzed for these properties. Sediment budgets measured using 137Cs inventories for a small shrub-dominated watershed and a small grass-dominated watershed found eroding areas in these watersheds were losing 5.6 and 3.2 t ha-1 yr-1, respectively; however, a sediment budget for each of these watersheds, including depositional areas, found net soil loss to be 4.3 t ha-1 yr-1 from the shrub-dominated watershed and 0 t ha-1 yr-1 from the grass-dominated watershed. Generally, the suspended sediment collected at the flumes of the six other subwatersheds was enriched in silt, and clay. Using mixing models to determine sediment source indicated that shrub-dominated subwatersheds were contributing most of the suspended sediment 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 suspended sediment to the stream systems. These studies also suggest that sediment yields measured at the outlet of a watershed may be a poor indicator of actual soil redistribution rates within a watershed. Management of these semiarid rangelands must consider techniques that will protect grass-dominated areas from shrub invasion.