Calculation of likely Day Zero date
Using current and likely future water consumption patterns, projected as at end-February 2018.
I comprehend the gradually changing water consumption patterns in my model and therefore my predictions of steadily falling weekly water consumption results in the dotted black line curving away to the right and indicating a “Day Zero” date falling around 30th June at point ‘Y’, depending upon how well households further reduce their water consumption over the coming 4-5 months.
Going through it step-by-step.
Starting from the 37.5% total storage at beginning November 2017, the business-as-usual consumption expectation indicated a sharply descending fall in dam levels, originally predicting Day Zero as being during week of 13th February 2018 when total storage would drop to the 13.5% level set by CoCT; that point when they would move to stage-2 of the disaster plan.
Then in November 2017 we enjoyed unseasonal rain in the catchments that lifted dam levels and effectively moved the predicted Day Zero out by two weeks to the last week of February.
My information at that time was that the so-called 10% of ‘inaccessible water’ in the dams was in no ways definite and that if necessary, given some extra filtration, more of that last ‘dead water’ could be accessed. In fact, the dams could be drawn down to virtually zero but not easily in all cases. So I assumed that they would be pulled down to, say, 7.5% (line “D”) thereby making more available for consumption. That in turn allowed a corresponding drop in the Day Zero trigger level from 13.5% to 11% of capacity (line “B”) while still retaining the same stage-2 reserve of 3.5% of capacity “E”). This pushed the point where line “A,B” intersects the 11% line out into March. [In the meantime DWS has announced that they are preparing to pull both Voelvlei and Theewaterskloof dams down to zero if necessary. That would make yet more water available.]
The next major change in November was when DWS imposed new limits and cut agricultural water allocations to just 40% of normal. The effect of this massive reduction in overall water consumption is reflected by the blue-striped zone, labelled “F”; the current position being where the actual consumption (denoted by the red ‘X’) now lies. Without further savings then the theoretical Day Zero date would be around end-May 2018, on the disaster stage-2 line (point “G”); by then dam levels could have dropped to 11.0% of capacity.
Although they have been drawing their water allocation fast, in fact agriculture has rapidly reached the 40% limit and the irrigation flows were largely cut off by end-January with most of the rest reaching the limit and closing by end-February. The net effect will be that the agricultural sacrifice has extended the communally available water supply out to late May (point “G”). At that stage agriculture’s contribution should be exhausted and “Day Zero” will have arrived for all the farms.
In February 2018 it was announced that the Groenland Water Users Association (Elgin/Grabou) would release their surplus stored water to be pumped through to Steenbras dam. This donation is expected to add 8-10 M.cuM of water to the SW Cape water supplies and help us to reach a point half way between “G” and “I”.
Now, although rain should start by June this is still too tight and the business and household users in the metro and municipalities must save stringently and meet their reduced allocations. In January the CoCT was running at about 586ML/d but has subsequently dropped down to the current level of 510-520 ML/d.
From 1st February level-6B restrictions require consumers to further reduce to 50L/person/day. Residents now must cut usage in order to save that blue zone labelled “H”. This action would make all the difference and enable us to reach “Success Day”, defined as arriving at 30th June 2018 with 100 M.cuM of fresh water still stored in the dams (point “Y”). Average consumption is still up in the area of 75L/p/d and every effort must be made to encourage that half of the population that is still using too much water to stop doing so (hopefully the new harsh tariffs will help).
As I write, the point red “X” lies at 23.7% of capacity on 26th February – the small yellow circles with ‘x’ mark the previous positions of “X” over the immediate past. Graphically the task is to make sure that, as position red “X” moves along, it stays on or to the right of the dotted black line, which will show that we are achieving or doing better than our weekly consumption target. At the moment we are doing very well due to the water being donated from the Palmiet system.
If we do reach mid-year with sufficient water in the dams, then nature should start to help with winter rains and low evaporation commencing late-May, but surely in June (yellow zone “I”).
Last but not least, various augmentation initiatives should be starting to make steadily growing contributions (brown striped zone “J”) and, in the final analysis Cape Town should just skim past the lowest point and out of immediate danger. In fact, hopefully the total available water will recover along the heavy dotted red line as the dams recharge, labelled “K”.
In summary, this outcome depends clearly on three main things:
1) the dams will be drawn down at least into the range of 6% – 7.5% of capacity if needed!
2) agriculture is exhausted, so businesses & households must now reduce consumption to 50L/p/d.
3) at least normal rains needs to arrive to start recharging the dams from July onward.
If rains once more fail and Day Zero arrives, then we will at least know we have done our best!
retired international businessman and fruit farmer in the Klein Karoo.
26th February, 2018.