Long-term, process-based, continuous simulations for a small, nested rangeland watershed near Tombstone, AZ (USA): Extending model validity to include soil redistribution.

Soil or sediment redistribution prediction along hillslopes and within small watersheds is considered to be a great challenge for the application of watershed erosion models in predicting the impact of soil and water conservation measures as well as for the redistribution of pollution such as radioactive fallout. In this study, long-term soil loss and deposition were estimated for two nested semi-arid watersheds within the Walnut Gulch Experimental Watershed in Southeastern Arizona using the process-based Geo-spatial interface of WEPP (GeoWEPP). While soil parameters were previously parametrized and validated through watershed outlet runoff and sediment yields, the channel parameters were adjusted and validated based on reference values of soil redistribution generated from fallout radionuclide 137Cs samples within the watersheds. Two methods were applied for the soil redistribution analysis by comparing observed and simulated soil loss/deposition rates (a) at single pixels and reference values at the specific location of each 137Cs sample site; and (b) for average values of a 5 m radius around each 137Cs sample site to compensate for measurement and model uncertainties. Surprisingly, soil redistribution predictions improved as topographic data resolution increased from 5 m to 3 m and were best at 1 m without changing key model parameters that were originally derived at the watershed scale.

Projected climate change impacts in rainfall erosivity over Brazil.

The impacts of climate change on soil erosion may bring serious economic, social and environmental problems. However, few studies have investigated these impacts on continental scales. Here we assessed the influence of climate change on rainfall erosivity across Brazil. We used observed rainfall data and downscaled climate model output based on Hadley Center Global Environment Model version 2 (HadGEM2-ES) and Model for Interdisciplinary Research On Climate version 5 (MIROC5), forced by Representative Concentration Pathway 4.5 and 8.5, to estimate and map rainfall erosivity and its projected changes across Brazil. We estimated mean values of 10,437 mm ha-1 h-1 year-1 for observed data (1980-2013) and 10,089 MJ mm ha-1 h-1 year-1 and 10,585 MJ mm ha-1 h-1 year-1 for HadGEM2-ES and MIROC5, respectively (1961-2005). Our analysis suggests that the most affected regions, with projected rainfall erosivity increases ranging up to 109% in the period 2007-2040, are northeastern and southern Brazil. Future decreases of as much as -71% in the 2071-2099 period were estimated for the southeastern, central and northwestern parts of the country. Our results provide an overview of rainfall erosivity in Brazil that may be useful for planning soil and water conservation, and for promoting water and food security.

Sediment budgets and source determinations using fallout Cesium-137 in a semiarid rangeland watershed, Arizona, USA.

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 identify potential sediment sources using (137)Cs 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 (137)Cs 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. Soil redistribution measured using (137)Cs inventories for a small shrub-dominated subwatershed and a small grass-dominated subwatershed found eroding areas in these subwatersheds were losing -5.6 and -3.2tha(-1)yr(-1), respectively; however, a sediment budget for each of these subwatersheds, including depositional areas, found net soil loss to be -4.3tha(-1)yr(-1) from the shrub-dominated subwatershed and -0.1tha(-1)yr(-1) from the grass-dominated subwatershed. Generally, the suspended sediment collected at the flumes of the six other subwatersheds was enriched in silt and clay. Using a mixing model 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. The sediment budget studies also suggest that sediment yields measured at the outlet of a watershed may be a poor indicator of actual soil redistribution rates within these semiarid watersheds. Management of these semiarid rangelands must consider techniques that will protect grass-dominated areas from shrub invasion to improve rangeland conditions.