Abstract:
This study assesses the temporal rainfall variability trends, the effects on river discharge and the identification of drought periods across the central and northern Drakensberg (1955 – 2015), as well as at a catchment scale, in the Sterkspruit Catchment. Records from five stations, covering the central and northern Drakensberg, indicate that there is a statistically significant decrease in rainfall over the recording period. The long-term trends in interannual variability show an increase in the variability of annual rainfall over the five stations. The mean annual rainfall in the Drakensberg is highly seasonal and an analysis of the monthly rainfall indicates an increase in the variability of the distribution of monthly rainfall and the strengthening of rainfall seasonality in the Drakensberg, which is shown by a statistically significant decrease in autumn rainfall. The El Nino Southern Oscillation (ENSO) influences summer rainfall variability in the Drakensberg and a strong correlation exists between the summer rainfall and the Southern Oscillation Index, for the preceding periods, suggesting that changes in the intensity and frequency of the ENSO should negatively affect the rainfall in the Drakensberg. A cyclicity of approximately 10 years between dry periods was found, from 1955-2015, and it is noted that the cycles are becoming increasingly variable over time. An analysis conducted across four stations in the Sterkspruit Catchment assessed valley-scale variability and how rainfall variability and drought conditions may affect agriculture in the area. The Sterkspruit Catchment has experienced an increase in rainfall variability and dry conditions in the 21st century. The assessment of meteorological droughts, at both study scales, using the Standard Precipitation Index, displayed a significant decreasing trend, indicating an increase in the number of dry years over time. Hydrological drought was assessed, using the Streamflow Drought Index for two rivers in the Drakensberg. Both drought indices found an increase in the number of dry years experienced over time, and they highlighted 1982, 1992, 1994, 2003, 2007 and 2015 as being the years with the lowest rainfall and experiencing drought conditions. Discharge in the Drakensberg reflects the rainfall and seasonal trends and the Little Tugela River, at the bottom of the Sterkspruit Catchment, shows decreasing discharge trends over time. Farmers in the Sterkspruit Catchment rely on the Bell Park farm dam water to irrigate crops throughout the year because the rainfall is unreliable, especially the early-seasonal and summer rain. However, in 2015, the dam ran dry for the first time since it was built. Thus, more insight into rainfall trends and cyclcity in the Drakensberg could help farmers to understand and plan for the periods with low rainfall conditions.
