Relationships between gully characteristics and environmental factors in the Zhulube Meso-Catchment: Implications for Water Resources Management

Abstract

The objectives of this study were to determine the accuracy of using satellite imagery and Orthophotos in gully identification and to test for significant relationships (p<0.05) between gully characteristics and environmental factors. The results showed that gully characteristics (depth, width and area) are significantly (p<0.05) explained by soil characteristics, environmental factors, slope gradient, sediment loadings and the erosive power of streams. Statistical analysis focused on the correlation and regression of soil chemical properties, vegetation type and gully characteristics and identification of susceptible areas. GIS and remote sensing techniques showed that 36% of major gullies were discernible using the Landsat TM imagery, 56% from the Spot panchromatic and 77% from the Orthophoto. There was an evident significant (p<0.05) relationship between gully depth and bulk density at r2= 0.873 were the soil clay content was another soil property that showed a significant (p<0.05) relationship with gully development with its related minerals (Manganese, magnesium, Sodium and Calcium), indicating a decline in erosion with an increase in proportions. A significant (p<0.05) relationship between gully depths and slope gradient showed a resultant increase of r2= 0.62. There was a significant (p<0.05) relationship between gully development and the erosive power of stream while sediment loadings of the streams indicated a non-significant effect on the gully depth with an r2= 0.02 were p<0.05. It can be concluded that remote sensing and GIS techniques are applicable in gully identification, their accuracy levels varying greatly depending on the spatial, spectral and temporal resolution of the imagery used. The inherent susceptibility of soils to detachment and transport by various erosive agents was a function of soil properties including among others, physical and chemical soil properties. The effects of each soil property were different between sites thereby influencing the degree of vulnerability of any given soil to destructive erosion forces. In addition, the interactive effects of the topography, vegetation cover and rainfall factors greatly influenced erosive agents. Soil erodibility assessment using simulated stream erosive forces and sediment loadings revealed that sediment yield or the erosive power of the streams in the study area increased with increasing slope gradient depending on the clay content of the soils.