Effects of vegetation change on interrill runoff and erosion, Walnut Gulch, southern Arizona

During the past 100 years grassland has been replaced by shrubland in many parts of the American Southwest. The effects of this vegetation change on interrill runoff and erosion are investigated by performing field experiments on small and large runoff plots located on contemporary grassland and shrubland hillslopes in Walnut Gulch Experimental Watershed. The experiments indicate that the vegetation change causes runoff and erosion to increase in interrill areas by decreasing resistance to overland flow, decreasing runon infiltration, increasing the spatial heterogeneity of the plant canopy, and possibly increasing the susceptibility of the soil to frost action. Increased runoff and erosion result in stripping of the soil A horizon, the formation of desert pavement in intershrub areas, and the development of rills. Fines remain and accumulate only under shrubs, where they form islands of fertility. Shrubs are able to regenerate in these islands, but it is difficult for any plants to become established in intershrub areas. Consequently, there is an increase in the spatial heterogeneity of soil resources, and this spatial heterogeneity is self-perpetuating. Increasing spatial heterogeneity of soil resources is considered to be characteristic of the process of desertification.