Spatial Patterns of Soil Erosion and Deposition in Two Small, Semiarid Watersheds.

This work investigates spatial patterns of hillslope erosion and sediment yields 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.  Calculated 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, over the past four decades.  Differences in hillslope erosion rates between the two watersheds were apparently due to vegetation: while on the shrub site runoff pathways were unobstructed, on the grass site, runoff was obstructed by vegetation patches and litter.  Hillslope erosion rates within the watersheds were not correlated to slope gradient or curvature, but were correlated to rocks in the upper soil profile.  These results are interpretable in terms of slope-velocity equilibrium wherein overland flow velocities became independent of slope gradient because of differential rock cover, which evolved as a result of preferential erosion of fine material on the steeper slopes prior to 137Cs deposition. Watershed morphology and channel incision controlled sediment yield.  Most of the eroded soil was deposited in swales of the grassed watershed.  Most of the soil eroded in the shrub watershed was exported from the watershed outlet by way of a well-incised channel system.  The study shows that measurement of sediment yield from a watershed can be a poor indicator of erosion taking place within the watershed.