Influence of grazing, vegetation life-form, and soil type on infiltration rates and interrill erosion on a Somalion rangeland

Abstract

Heavy communal grazing pressure and rapid phytomass decomposition reduce shrub interspace cover in Somalia from 100% at the end of the growing season to 5% at the end of the dormant season. Intense monsoonal rain, characteristic of Somalia and other areas of subsaharan Africa, combined with sparse vegetative cover at the beginning of the rainy season, may result in overland flow and excessive erosion, even where sand content of the soil exceeds 9%. Little watershed research has been conducted in this region other than to document that the problem is extreme. The objectives of this study were to assess the seasonal hydrologic responses as influenced by 2 soils (sand vs. clay), grazing intensity (exclusion vs. heavy communal grazing), and cover types (shrub understory vs. interspace) in Somalia. Infiltration rate and interrill erosion on the sand site were significantly greater than on the clay site regardless of cover type or season. The clay site was dominated by annual forbs which rapidly decomposed. The sand site had greater annual and perennial grass cover which decomposed slower than forbs, providing longer and perhaps better protection from raindrop impact energy. Three growing seasons of livestock exclusion did not significantly increase soil cover on shrub interspaces; consequently, infiltration rates and interrill erosion remained similar to the communally grazed sites. Interspace cover left by livestock was instead removed by termites and other microorganisms. Restricted ability of livestock to graze beneath thorny shrubs and increased phytomass from shrub leaf-fall resulted in a greater accumulation of cover and litter beneath shrubs, which aided infiltration on clay sites, regardless of season.