Cape Drought May 2018

My previous blogs on this subject have relied heavily on the University of Cape Town’s Climate System Analysis Group’s 40 year rainfall figures for Cape Town airport, to highlight the severity of the drought. Hence my disappointment to see that the Group had discontinued using the airport figures due to rainfall events ‘not reported or underreported’ to the extent that the ‘dataset would give a wrong impression of the current conditions’. Instead they are basing their Cape Town rainfall monitor on a number of stations and though the data from these is not consistently good the geographical spread is a considerable bonus.

Pending a review of all the available data it seems not inappropriate to use recent years’ figures for Cape Town’s main feeder dam, Theewaterskloof.  Regular readers may recall that the Cape Town airport recorded rainfall for 2017 was a scant 153mm and followed two other years of well below average rainfall. The corresponding figure for Theewaterskloof was 344mm. Unfortunately there are only records for 2011 onwards at the latter but 2017 was the driest of these followed by 2016 at 394mm and 2011 438mm. The recent drought is not as marked at Theewaterskloof as at Cape Town airport. As regards the year to date (to 27 May) the figure is only 82mm, over a third of which came in January, quite different results to what I was expecting.

Despite the very low rainfall dam levels are higher in the Western Cape than a year ago, 19.0% compared to 18.1% This must be a tribute to the water savings by consumers, though Anthony Turton, one of the best known and authoritative commentators on the subject, is concerned that focus has now been lost and that large industrial users are not sufficiently incentivised.  The situation with the five main dams supplying the Cape Town area is even better, sitting at a weighted average of 23.4% of capacity versus 18.3% a year ago. This of course indicates the dire situation at some of the other Western Cape dams, namely, in order of size, Brandvlei 9.7%, Kwaggaskloof 5.7%, and Clanwilliam 6.4% all of which appear to be at levels where extraction of potable water might be unlikely.

As a footnote, dams in other parts of the region are generally fair to good, leaving South Africa and Lesotho dams in total at 80% compared to 72% a year ago. This figure is still pulled down by the unlikely laggard of the Lesotho dams at only 53%, scarcely changed on a year ago.

As regard the forecast for the winter rainfall area the International Research Institute for Climate and Society of Columbia University’s Earth Institute probability forecast for June through August attributes a higher than normal probability to above average rainfall for the extreme south west of the subcontinent but below for the Garden Route section of the southern Cape, as indicated below. There may be a little winter rainfall for the Highveld areas (with rain already having been recorded as recently as 31 May).

Sources: Climate System Analysis Group, University of Cape Town; International Research Institute for Climate and Society of Columbia University’s Earth Institute



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