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Anatomy of a drought
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By now most people in South Africa will have heard warnings of drought in both the summer and winter rainfall regions for the period 2015-2016.
With this in mind, would you consider consecutive monthly rainfall totals of 193, 299 and 416 mm for May, June and July of this year as constituting a drought in the south-western Cape? Impossible, many of you would say; the annual average rainfall for the whole of South Africa sits at 450 mm, and the sum of these three months falls above the annual average rainfall for about 95% of the country.
These are the rainfall figures that SAEON measured this year at a rather special place in the history of South African meteorology:
In 1945, Dr CL Wicht of the Jonkershoek Forestry Research Station decided that a rain gauge was required at one of the highest easily accessible points in the Jonkershoek range at about 1 200 m above sea level. This was to complement an existing network of rain gauges at various lower elevations in the Jonkershoek valley that were showing that rainfall varied considerably within a short distance. This difference was caused by the influence of mountain topography on rainfall. Rainfall amounts were collected on a monthly basis, on foot, from this 1 200 m rain gauge and went on to provide the highest average annual rainfall measured in South Africa at 3 380 mm/yr over a 45-year period ending in the early 1990s when regular data collection stopped.
The recent rainfall totals were collected by SAEON’s automatic weather station at 1 214 m above sea level, very near Wicht’s original rain gauge (we moved the position slightly so we could get cell phone reception to stream data back to our offices).
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Why are the rainfall figures from the SAEON weather station worrying?
Firstly the long-term average until 1990 for May, June and July are 503, 513 and 535 mm respectively, 1 552 mm compared to 909 mm for 2015. Secondly, this rain is measured at the heart of the mountain catchments that supply the Berg River and Theewaterskloof dams with water. These dams are two of the three most important dams which supply Cape Town and surrounding agricultural areas with water (the third dam is Voëlvlei, which was sitting at an even more alarming level of 40.5% on 16 November 2015).
While the May, June, July rainfalls are disturbing, if we step back and look at the longer record, the situation doesn’t improve. The rainfall for the period April 2014 to March 2015 lies among the five lowest rainfalls recorded in the period 1945-1990.
This year, winter rainfall is lower than any of the historic records for this location. While 2014 was relatively dry, the higher than average rainfall of the previous year probably averted a water shortage. This year we have no such buffer to fall back on. To avoid falling in the range of the five worst rainfall years we will need 1 335 mm of rain this summer, an amount only exceeded in 1976 and 1989. Unlikely…..
Is two years of very low rainfall a sign of climate change?
Too early to say yet. The historic data suggests that years of low rainfall are clustered so this in itself is not too unusual. In terms of rainfall in South Africa, as a whole, showing a climate change response, the evidence suggests no rainfall changes have yet emerged, although temperature is generally rising.
Historic rainfall and streamflow data in South Africa
In the 1930s, concern about the negative impact of plantations of foreign trees on streamflow, and counterarguments that trees brought rain to an area, led to the establishment of the Jonkershoek Forestry Research Centre under the leadership of Dr CL Wicht.
Small catchment experiments were set up where catchments were planted to pines over a time period. Streamflow was monitored in these catchments prior to planting and during the subsequent plantation rotations. These showed unequivocally that catchments carrying the local fynbos vegetation delivered more water, more sustainably, than plantations of alien trees. A network of rain gauges also showed that planting trees did not increase the rainfall of the area.
Similar results emanated from the Cathedral Peak catchment experiments in the Drakensberg, established a decade later in the Grassland Biome. The Jonkershoek Forestry Research Centre later joined the CSIR. With long-term monitoring no longer part of the CSIR’s mandate, SAEON took over the monitoring network in Jonkershoek and is re-establishing the network in Cathedral Peak.
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In the current era, questions about how anthropogenic greenhouse gases are affecting climate have given a new motivation for maintaining monitoring sites with long-term records which provide a means of detecting change in a very variable climate.
SAEON now archives the historic data from the catchment experiments, a small portion of which was used in this article, following agreements with the CSIR.
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Acknowledgements
The CSIR is thanked for providing the historic data. The Department of Environmental Affairs’ Natural Resource Management Directorate is thanked for financial support of the monitoring at Jonkershoek.
