Validation of GLASOD Map for Sediment Sources and Erosion Processes Identification in the Nyumba Ya Mungu Reservoir Catchment

Abstract

The main objective of this paper is to report on the preliminary validation results of the Global Assessment of Soil Degradation (GLASOD) as a tool for mapping sediment sources in Tanzania. This study was carried out in a well studied catchment, the Nyumba Ya Mungu (NYM) reservoir catchment located in the upstream of Pangani River Sub-basin. Previous studies in the same catchment used quantitative approach that entailed comprehensive sediment sampling programme and numerical modelling to identify sediment sources and erosion processes. Although previous researchers’ findings were satisfactory, the methods used were demanding in terms of resources (time, funding, and personnel) and impractical to a large ungauged catchment. The quest to validate GLASOD map is evident as it was qualitatively developed through collating expert judgments of many soil scientists to produce a world map of human-induced soil degradation at a scale 1:10,000,000. In the current study sediment sources mapped from qualitative method (GLASOD) plus supplement field visit observations and quantitative approaches are compared and discussed in detail. Preliminary results suggest that the paired information on sediment sources, field based data versus GLASOD, for upper catchments or upland locations are more strongly correlated than lower reaches. The results of this study have further emphasized the fact that GLASOD map is satisfactory to depict large regional differences in soil degradation but it is not capable of explaining local degradation. Besides, GLASOD map does not capture erosion processes dynamics compared to comprehensive sediment sampling programme. Notwithstanding, GLASOD map might be a useful tool for sediment sources and erosion processes identification scoping studies in the study area. Based on this study, it is therefore recommended to complement the GLASOD map with field based data for detailed study initiatives.