Vegetation density assessment in the Upper Molopo River Catchment, South Africa

In arid and semi-arid environments like the rest parts of South Africa, the state of vegetation density in catchments is an important indicator of the state of the environment. Climate variability coupled with different anthropogenic activities could affect vegetation cover at varying levels. The Upper Molopo River Catchment (UMRC) in the North West Province of South Africa is under this combined human use and climatic pressure. This study aimed at assessing long-term changes in vegetation density in the Upper Molopo River Catchment area. Two pairs of bi-seasonal Landsat images, TM 1989 and OLI-8 2013 (autumn) and TM 1996 and TM 2005 (summer) were classified using Maximum Likelihood algorithm. Classification of vegetation density categorised three major classes: low vegetation density (< 0.1), medium vegetation density (>0.10.5) and high vegetation density (>0.5) based in Normalized Difference Vegetation Index (NDVI). For purposes of interpreting the changes identified, ancillary long-term data on anthropogenic factors (human population, number of houses, household use of wood as energy source, livestock populations) were obtained from state sources. The overall accuracy of vegetation density maps generated from post-classification change detection methods and evaluated using field data was 88% with a Kappa coefficient (K hat ) value of 83%. Results indicated a growth in built up area from 3% in 1989 to 16% in 2013 with a simultaneous decrease of medium vegetation density within 5 km of human settlements. Low vegetation density revealed an increasing trend whereas high and medium vegetation density areas have been declining during the study period. In addition, a significant negative correlation between human population and area of cover by medium vegetation density (r = -0.960, P < 0.01) was obtained. The decline in medium vegetation density in the Upper Molopo River Catchment is of ecological concern. There is therefore, a need for short-term and long-term strategies to ensure sustainable land management in order to preserve vegetation density and biodiversity within the catchment.