Good rains have occurred in the northern part of the region while drought conditions continue in the south. This has allowed dam levels to increase rapidly in one part of the region while reaching dangerously low levels in other parts.
Columbia University’s International Research Institute for Climate and Society reports that mid March conditions in the central equatorial Pacific are broadly neutral though tending to warmer in the eastern third and to slightly La Niña conditions in the central and western parts. Despite La Niña conditions not having developed to any significant extent since the recent strong El Niño, the forecast is for El Niño conditions to again develop later in the year. The forecast shown below is one of two produced by the Institute, this one including direct human input based on experience and adjusted for known model biases while the other is what they term the objective model-based probability forecast. Both show quite strong El Niño development (to over 50% probability per the bar chart) but the one below emphasises the chances of more neutral conditions, especially around midyear. (ENSO refers to the El Niño Southern Oscillation and the SST anomaly is that relating to sea surface temperatures.)
There has certainly been very welcome rain in our part of Gauteng with November the wettest since 1998, February the wettest since 2000 and the season to end February (from 1 July 2016) the wettest since 1995/96 except for 2009/10 which was fractionally wetter.
We were not alone in recording high rainfall, with dam levels responding appropriately, especially in the northern provinces of Limpopo, Mpumalanga and Free State, and to some extent in kwaZulu-Natal. The recovery of dams on the Vaal and Orange in just seven weeks was quite remarkable as the table below indicates. Vaaldam increased by more than 1 billion m3 and Gariep dam by more than 2 billion m3 (more than the total capacity of all the Western Cape dams) in just those seven weeks. Water restrictions in parts of the north were quickly relaxed as warnings were issued for those living or working close to the Vaal, as sluice gates were opened.
|Free State total||15 971||87||57|
|W Cape total||1 853||27||38|
|kZN total||4 669||55||47|
|E Cape total||1 826||63||58|
|Limpopo total||1 508||79||64|
|Katse / Mohale (Lesotho)||2 376||56||45|
|Mpumalanga total||2 520||78||66|
|Total RSA and Lesotho||31 913||74||55|
Meanwhile water consumption in Cape Town continues at quite high levels, not meeting usage targets. Emergency measures such as tapping into the Table Mountain group aquifer cannot add significantly to supply. With the last 10% of dam capacity generally not available in practice it will be quite a close-run thing as to whether the roughly 100 days of reserves will be sufficient to last until run off is sufficient from winter rains which may not start in earnest for another two months. This area is likely to encounter continuing problems as population and per capita usage increases, with less than 6% of the SA/Lesotho region’s dam capacity, unless a big contribution is forthcoming from desalination. Western Cape dams are 11% of capacity lower than a year ago, so winter rains are going to have to be very much better than last winter if the crisis is not to continue. Taking Cape Town International airport as a proxy for the region, last year’s figures are as below, indicating that runoff would be minimal until late June after a long dry summer.
Theewaterskloof is the main dam feeding the Cape Town area. The following chart shows how the dam’s % storage trendline for this year is running roughly parallel (albeit at a slightly lower gradient) to last year but at a considerably lower level of capacity, culminating in the current 11% of capacity variance (ie the same as for the Western Cape as a whole). Although the level is surprisingly only shown as Moderately Low, currently, it can be seen how this can easily become Very Low which in turn is only slightly higher than Absolute Minimum. Again, somewhat surprisingly, the chart apparently indicates that a recovery in levels can occur as early as late April though last year the minimum level was in June after only a small uptick in April, followed by some six weeks of declining reserves. [The December blip on the current year line marks our recent holiday in the Cape.]
Interestingly WWF-SA says that of SA’s 490mm average rainfall (40% below the world average) only 9%, or 49 billion m3, is runoff into rivers, 30% of that they say can be allocated “at a high assurance of supply” and 98% of that in turn is already “allocated to users”. Potential evaporation averages out at 1800mm. No doubt this is somewhat misleading in the sense that the highest potential evaporation will be in those areas where there is little soil moisture to evaporate.
Low reserves in the Western Cape inevitably heighten the tensions between agricultural and urban / industrial users. Farmers in the Theewaterskloof area are particularly concerned about the 2018 harvest if little water is available in the current post harvest season.
Sources: As indicated above, plus Department of Water Affairs; Business Day; wunderground.com
© J R B Livesey 2017