Multi-dimensional approach for evaluating land degradation in the Savanna belt of the White Volta Basin

The White Volta Basin (WVB) is located within the Savanna Ecological Zone shared by Ghana, Burkina Faso and Togo. It is of national importance to the development of these countries through agriculture, urban water and hydro-electric supply, transportation, tourism and others. Despite its benefits and potentials, it is currently under threat due to land degradation (LD) driven by both anthropogenic and natural forces. A multidimensional approach was employed to assess the degradation at two spatial scales (i.e. Basin and sub-basin scales) and propose mitigation measures. Spatial data from remote sensing historical land cover (1990-2007); existing GIS database of soil erosion obtained from RUSLE was integrated to determine LD hotspots in the area at a basin scale. Sub-basin scale assessment was also done to complement the basin scale analysis to ensure information complementarities. Observable indicator system developed from literature and expert consultation was used in conjunction with FAO field protocol for mapping land degradation in the sub-basin. Additionally, data on socio-ecological determinants of land degradation and mitigation measures were also gathered through interviews and group discussions. The results indicated that land degradation is persistently occurring in the basin and can be effectively mapped using these indicators. Soil loss through erosion and negative land use/cover conversion (NLUCC) were common indicators identified and used to map land degradation at both basin and sub-basin‘s scales. About 82% of the basin is degraded due to negative land use/cover conversion or soil erosion. Of this, 33% of the basin‘s area is experiencing severe degradation. Degradation hotspots were found around areas where urbanization was on the increase. A cross-scale analysis of the different indicators at the two scales showed that, there exist matches and divergences between some indicators. The best indicator matches was between the net soil loss at the basin scale with that of soil erosion state at the sub-basin scale (92.6%) and erosion extent (92 %). There exists a great divergence between negative land use/cover conversions at the basin scale and erosion severity at the sub-basin scale with a divergent value of 87 %. Poor agricultural soil and rangeland management, deforestation as well as climate change were perceived to be the direct drivers of LD in the basin. Increased in human population, change in demand and consumption for food and fuel wood, poverty and inadequate labour were the main indirect drivers responsible for the degradation in the basin. Identified possible mitigation measures to combat the degradation in the basin include: controlled bush burning or no burning, minimum tillage and crop rotation. Others were stone bunds, organic manures and mineral fertilization. The study revealed that land degradation is occurring in the basin and can be mapped using both proxy and observable indicators. Thus, this will reduce the cost and time involved to assess land degradation at the sub-basin or field levels using traditional methods. It is therefore recommended that the appraisal of land degradation should embrace these three pillars (multi-scale, multi-indicators and actor-based) as proposed in this study for effective and accurate results as well as development of appropriate mitigation interventions.

Source: KNUST