GULLY INITIATION AND EXPANSION IN LESOTHO: A CASE OF THE BUASONO AREA

Abstract

ABSTRACT Gullies are common in Lesotho especially in the lowlands. They occur mostly in soils that have an abrupt textural change between surface layers (A horizons) and subsurface layers (B horizons). A few studies that deal with factors influencing gully formation and extension in Lesotho have been published. This study was conducted in the Buasono catchment in Lesotho. The objectives of the study were (1) to determine the factors that affect gully initiation and development in an area; and (2) to determine the rate of gully expansion and extension over a nineteen-year period between 1978 and 1997. An active gully was selected within the Buasono catchment. The gully was regularly surveyed, and studied from 1978 to 1997. After nineteen years the area of the gully has increased by 49%; the pipe on the southern side of the gully has increased by 61% while that on the northern side by 50%. Initial observations show that the intensity and the amount of rainfall play a major role in the rate of extension of a gully. The rate of extension increases with the increasing amount and intensity of rainfall. Soils within this area were studied in terms of their morphology, physical and chemical properties. Field observations show that soil morphology plays a major role in the initiation, development and extension of a gully. The failure of the gully head and the slumping of the overhang, the undercutting of the sidewalls by flowing water, as well as piping were found to be the major processes by which gully expansion occurs. Duplex soils were found to be more erodible than non-duplex soils, in terms of both morphology and other inherent characteristics. Duplex soils in general have significantly (p<0.05) lower amounts of exchangeable calcium and magnesium than the non-duplex soils, the former have lower organic matter content than other soils in the same study area. However, they have significantly (p<0.05) higher exchangeable sodium and exchangeable sodium percent (ESP) than the non-duplex soils found in the same catchment. The results show that the Duplex soils are uniform among themselves, but are different from the non-duplex soils. Particular care should therefore be taken in the utilization of catchments dominated by duplex soils, in order to minimize the threat of gullying.