The impacts of heavy rains on the vegetation cover in the Limpopo Province of South Africa
Abstract
In arid and semi-arid environments like most parts of South Africa, the state of vegetation cover is an important indicator of the state of the environment. Climate variability coupled with different anthropogenic activities could affect vegetation cover at varying levels. This study aimed at assessing the pattern and magnitude of spatial and temporal vegetation cover changes before and after heavy rains in the Vhembe and Mopani Districts, Limpopo Province, South Africa. Utilising remote sensing methodology, Landsat TM images of 1995, 1997, 2005, 2007, 2010 and OLI-8 2013. Rainfall data for 1961-2011 were used to compute rainfall anomalies. Landsat classification of NDVI density classes for each image was computed for identifying vegetation cover changes. Classification of vegetation density based on NDVI categorised five major classes: non-vegetation (bare land or water), low density, medium density, high density and very high density classes. In addition, a correlation coefficient of heavy rainfall events and vegetation cover was done.
The study established that there have been substantial changes in vegetation densities before and after heavy rainfall has occurred in the area. The study area received above normal rainfall in 1996, 2006 and 2011. Performing vegetation cover change analysis for the above normal rainfall years’, Vhembe and Mopani Districts showed similar patterns in vegetation cover change. This means that when vegetation cover increased in Vhembe District it also increased in Mopani District, though to a different degree. The change analysis showed an increase of 59.45 ha (5504.6%) for Vhembe and 0.81ha (90%) for Mopani in non-vegetation cover from 1995-1997 while, high and very high density decreased. Although some areas recorded a decrease in vegetation cover, there are also areas that had had an increase in vegetation during the study period.
Results of the correlation coefficient revealed a significant weak correlation of (r = 0.44 and 0.18) in 1996, (r = 0.13 and 0.29) in 2006 and (r = 0.04 and – 0.36) in 2011 in Vhembe and Mopani Districts respectively, between NDVI and Mean Average Rainfall (MAR) with a residual of 19.4% (r² = 0.19), 1.7% (r² = 0.002) and 0.2 % (r² = 0.13) in Vhembe District during 1996, 2006 and 2010 respectively. Similarly, Mopani District accounted for 3.2 % (r² = 0.03), 5.3 % (r² = 0.05) and 15.2 % (r² = 0.15) in 1996, 2006 and 2011 respectively, suggesting that other factors influence vegetation cover changes in the study area. Hence, vegetation density cover change could be attributed to socio-economic activities, such as agriculture, veld fires, settlement expansions, and overgrazing.
Land cover mapping and change detection studies are valuable especially for vegetation cover change. From the findings, the study recommended monitoring and analysing Land Use Land Cover Changes (LULCC) in order to understand drivers of the change in the Vhembe and Mopani District. These studies will make significant contribution towards the understanding of socio-economic drivers of vegetation cover change, and the impact on natural and human ecosystems.