Abstract
In a small river basin of the Mossi Plateau, Burkina Faso, the distribution of degraded bare ground identified on aerial photographs was examined in relation to pedo-geomorphic conditions, especially for soil texture and the estimated rate of annual soil loss by the universal soil loss equation (USLE). On a glacis located in the upper reaches of a basin belonging to a tributary of the White Volta River, sandy soils are distributed. Here, the occupation rate of degraded bare ground was 2.1% in 1955 and 2.7% in 1988, while on a riverain plain where silt-rich soils are distributed, it was 1.7% in 1955 and 21.9% in 1988. The land of the latter landform unit seems to be less intensively used than the former. Thus, it is suggested that the riverain plain is more susceptible to degradation than the upper glacis. The degraded bare ground, which has been levelled and covered with a thin surface crust, has a finer topsoil or a finer subsoil outcropping due to soil erosion. The bare soil profiles are dry and very hard (31-34 mm using a Yamanaka-type soil hardness meter) even in the rainy season. This condition must contribute substantially to degradation as revegetation or plant recovery does not occur. The erodibility factor (K) of the topsoil of the study area ranges between 0.38-0.55 in US customary units, and is very high on the riverain plain. The estimated annual soil loss from the riverain plain is, however, quite low (3 t ha^<-1>) due to the gentle slope (0.25%). This indicates that the high occupation rate of degraded bare ground on the riverain plain cannot be explained by the annual soil loss rate. The very low occupation rate at the glacis of the upper reaches may be explained by a low soil loss rate and the fragile consistency of sandy soil, of which the surface crust is easily destroyed. On the other hand, the high occupation rate on the riverain plain can be explained by the presence of a silt-rich topsoil which becomes very hard when dry. This condition may often result in abandonment of cultivated land.
