The Incomati is a semi-arid trans-boundary river basin in southern Africa,with a high variability of streamflow and competing water demands fromirrigated agriculture, energy, forestry and industries. These sectors competewith environmental flows and basic human water needs, resulting in a"stressed" water resource system. The impacts of these demands, relative tothe natural flow regime, appear significant. However, despite being arelatively well-gauged basin in South Africa, the natural flow regime and itsspatial and temporal variability are poorly understood and remain poorlydescribed, resulting in a limited knowledge base for water resource planningand management decisions. Thus, there is an opportunity to improve watermanagement, if it can be underpinned by a better scientific understanding ofthe drivers of streamflow availability and variability in the catchment.In this study, long-term rainfall and streamflow records were analysed.Statistical analysis, using annual anomalies, was conducted on 20 rainfallstations, for the period 1950–2011. The Spearman test was used to identifytrends in the records on annual and monthly timescales. The variability ofrainfall across the basin was confirmed to be high, both intra- andinter-annually. The statistical analysis of rainfall data revealed nosignificant trend of increase or decrease. Observed flow data from 33 gaugeswere screened and analysed, using the Indicators of Hydrologic Alteration(IHA) approach. Temporal variability was high, with the coefficient ofvariation of annual flows in the range of 1 to 3.6. Significant decliningtrends in October flows, and low flow indicators, were also identified atmost gauging stations of the Komati and Crocodile sub-catchments; however, notrends were evident in the other parameters, including high flows. The trendswere mapped using GIS and were compared with historical and current land use.These results suggest that land use and flow regulation are larger drivers oftemporal changes in streamflow than climatic forces. Indeed, over the past 40years, the areas under commercial forestry and irrigated agriculture haveincreased over 4 times.
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