Submitted to: Federal Interagency Sedimentation Conference Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 10/31/2005
Publication Date: 5/1/2006
Citation: Nearing, M.A., Kimota, A., Nichols, M.H., Ritchie, J.C. 2006. Spatial patterns of soil erosion and deposition in two small, semi-arid watersheds. Proc. 8th Federal Interagency Sedimentation Conference, April 2-6, 2006, Reno, Nevada. Interpretive Summary: This study was undertaken to use radioactive Cesium in soils to measure the distribution and rates of soil erosion in two small semi-arid watersheds located in southeastern Arizona. The radioactive Cesium that we measured was deposited in soils across the entire world as a result of atmospheric atomic bomb testing that was conducted by various nations in the period largely around the early 1960s. For a long time scientists have been measuring the amount of sediment that leaves a watershed. This is important to know, however, the amount of sediment that a watershed generates to river and stream systems does not tell the whole story about soil erosion. What scientists are generally not able to measure on a routine basis are the hillslope erosion rates and the spatial distributions of erosion in watersheds. Our results indicate that erosion rates in these watersheds were actually much greater on average than we expected for rangelands. Also, we found that the measurement of sediment leaving a watersheds told us very little about the amounts of erosion that were taking place on hillslopes within the watersheds. In this case the amount of sediment measured in the traditional manner at the watershed outlets were extremely different from watershed to watershed, while erosion rates on the hillslopes inside the two watersheds were not that much different at all. This was due to the fact that most of the sediment generated in one watershed was deposited before it left the outlet, while nearly all of the sediment in the other watershed left the watershed outlet. This study has significant implications for improving our ability to manage the soil and water resources of this nation by improving our knowledge of erosion rates in rangelands of southern Arizona and providing spatial data needed to test and improve the tools we use for conservation planning.
Technical Abstract: This work investigates spatial patterns of hillslope erosion in a semiarid ecosystem considering influences of vegetation, slope, rocks, and landscape morphology. 137Cs inventories were measured on one shrub and one grassed watershed in southeastern Arizona. Mean erosion rates in eroding areas were 5.6 and 3.2 t ha-1 yr-1, and net erosion rates for the entire watershed, including depositional areas, were 4.3 and nearly zero t ha-1 yr-1 for the shrub and grass watersheds, respectively. Differences in hillslope erosion rates between the two watersheds were apparently due to vegetation and erosion rates within the watersheds were not correlated to slope gradient or curvature, but were correlated to rocks in the upper soil profile. The study showed that measurement of sediment yield from a watershed can be a poor indicator of erosion taking place within the watershed.