Spatial Soil Erosion Hazard Assessment and Modelling in Mbire District, Zimbabwe: Implications for Catchment Management

Abstract

Soil erosion is a natural geomorphological process resulting from water and land interactions but accelerated to become an environmental hazard by human activities such as clearing of forests for cultivation, poor farming practices and encroachment into marginal lands. Mbire District is no exception, with soil erosion manifesting in the form of gullies causing onsite adverse impacts on agricultural land, settlements and engineering structures. The main objective of this study was to assess soil erosion hazard and gully erosion occurrence in Mbire District, Zimbabwe and asses the implications for catchment management. In this study an improved method of the Soil Loss Estimation Model for Southern Africa (SLEMSA) for soil erosion hazard assessment was used in a GIS to identify the spatial extent of potential erosion hazard for the District. The association between distribution of gullies and flood hazard, landcover, soil and terrain variables was assessed at Mushumbi Pools which forms part of Wards 4, 9, 12 and 17 of the district. Ten factors were linked to gully occurrence namely; landcover, soil type, slope, elevation, distance from river, distance from road, Sediment Transport Index (STI), Stream Power Index, (SPI) Wetness Index (WI) and flood hazard. Using factors showing significant correlation with gully occurrence, a gully erosion hazard map was developed by means of Weight of Evidence Modelling. Results from the SLEMSA model show that Mbire District has a low and moderate erosion hazard covering 72% of the district, with pockets of high hazard occurring in areas with a combination of steep slopes, low erodibility soils, high settlement density and cultivation. Out of the 17 wards in Mbire District, 10% or more of the area of each of the Wards: 1, 2, 3, 5 and 6 was in the very high erosion hazard class. Results also show that out of the ten factors affecting gully erosion studied, seven were significantly correlated (p < 0.05) to gully occurrence namely; landcover, soil type, elevation, flood hazard, distance from river, STI and SPI. Three factors namely slope, WI and distance from road were not significantly correlated (p > 0.05) to gully occurrence. The gully erosion hazard map showed that 90% of the very high hazard class is within a distance of 250 meters from rivers. Model validation indicated that 75% of the validation set of gullies were in the high hazard and very high hazard class. The model had a prediction accuracy of 74.7 %. In this study it is concluded that the SLEMSA model can be applied to assess erosion hazard in the District but cannot predict occurrence of gully erosion and Weight of Evidence Modelling has great potential in gully erosion hazard mapping. It is recommended that further studies could be done to establish the actual quantities of soil lost and its deposition into hydrological structures. Catchment management targeted at soil conservation may have to focus on areas with high settlement density and cultivation, especially if cultivation is expanding into fragile lands. For planning purposes, it is recommended that highly erodible soils should be allocated to landuse activities such as wildlife management which do not reduce vegetation cover. It is also recommended that policy makers have to address increases in settlement densities along rivers.