The Western Cape continues to suffer an acute water shortage as rainfall sets record lows for recent years. The (regrettably poor screenprint) graph below shows a solid red line for 2017’s cumulative rainfall to 24 August which is now below any of the 40 earlier years’ lighter lines. The degree to which rainfall has fallen short over the last two years is illustrated by the four driest years to 24 August of this distribution being 2017 at 115mm, 1994 133mm, 2016 189mm and then 1978 at 239mm so only two years have had less than twice as much rain by late August as 2017. As recently as 19 July the 2017 cumulative was ahead of 2016’s but the last five weeks’ rainfall (at Cape Town airport) has been particularly low. The cold fronts that bring winter rain to the Cape continue to pass further south than normal. If this is indicative of a longer term poleward shift of weather systems in response to global warming the Western Cape is going to have to take some extreme measures to rebalance supply and demand.
This leaves Cape Town’s main water supply, Theewaterskloof Dam, at 25.9% capacity on 28 August from 51.4% a year ago. With 2017 to date being even dryer than 2016 the situation has become absolutely critical. Apparently March 2018 has been pencilled in when water can no longer be drawn but an earlier date seems inevitable unless savings increase. The only large (>100 million m3 capacity) dam with a significantly better level is the Berg River Dam at 54%.
The response of authorities in the Cape appears to have been slow. It is well known as a water scarce region and demographic trends point towards much higher water demand. Nevertheless some of the responses now being mooted or implemented at this late stage include:
- FruitLook, an open access online platform, monitors the main farming areas using satellite imagery and provides crop growth, water use and mineral availability information on a field by field basis. Typical increased water use efficiency is 10 to 30%. [In the Northern Cape water scheduling software calculates the exact amount of water needed for optimum crop yields by soil type for 280 enterprises and presumably can be adapted to the Western Cape situation.]
- Cape Town has reduced loss through leaks from 30 to 15% [If this is measured against total water usage, which has declined, then the saving is excellent.]
- Modular desalination plants capable of producing a small but nevertheless useful few million litres a day, which can be deployed in a few months, are under consideration.
- Also being looked at is a R15bn seawater reverse osmosis desalination plant.
The forecasts issued by the International Research Institute for Climate and Society at Columbia University’s Earth Institute indicate a much greater chance of a La Niña phenomenon (that could bring higher rainfalls to Southern Africa) than recently, but from a very low base, and, as the chart shows, the balance of probabilities is still for the relevant sea surface temperatures to be in the neutral zone of normal ±0.5°C with La Niña prospects diminishing around year end. This chart is based on model results as amended by the scientists involved.
Because of the good summer rains in the northern part of the country other provinces are showing increased dam levels compared to a year ago. Importantly, this applies to Free State with over half the country’s capacity (79% of capacity v 55% a year ago). Eastern Cape is the exception (57% v 66%).
Here in Gauteng the winter has continued very mild with only very rare air frosts. Rainfall in July was only 1mm and August also 1mm.
Sources: Business Day; http://www.csag.uct.ac.za; http://www.dwa.gov.za.