Cape Water and Dams Report
Dams – Latest levels: Winelands and Cape Town
Report nr. 98 (updated 16th April 2018) on the main dams serving
Cape Town City and nearby Boland/Overberg towns
Net outflows from the main dams rose to 5.1 M.cuM, or 0.6% of capacity (vs the preceding week’s 3.1 M.cuM), and combined storage dropped from 21.1% to 20.5% of total capacity. Clearly the beneficial effects of the recent transfer of water from the Groenland Scheme has ended.
During the same week in 2017 the dams dropped by 11.2 M.cuM (1.3%) to 23.5% of full capacity.
How long will the water last?
This week I have once more made a mid-month correction, reducing the expected net outflow from the main dams from model-predicted 25.7 M.cuM to 19.8 M.cuM for the month of April. If this lower consumption does materialise then we will be relatively sure to reach end-June with more than 120 M.cuM of water stored in the dams (this is the cut-off level below which the authorities will have to introduce collection/rationing; so it is vital to stay above this level).
(Here is our model predicting the amount of water likely to be stored in the dams by mid-year.)
Putting it in context!
Average daily draw from the dams by the Metro as at 16th April jumped to 515 ML/d while water used from “all sources” lifted (from 516 ML/d) to 542 ML/d. This implies that the augmentation contribution once more fell to around 28 ML/d.
The authorities are apparently struggling to bring extra water production on line and the temporary desalination inflows at V&A and Monwabisi are not yet generating significant additions to the system. All-in-all it seems that the augmentation initiatives have slipped some 4 weeks behind target. This makes it all the more important that water consumption is reduced and not allowed to creep up.
(Graph tracking overall use and production of extra water through the augmentation efforts).
So we now need to be exceedingly careful and absolutely must continue to strive to use less than 50 L/p/d if we are to be relatively safe until winter 2019; and that also assumes rainfall in winter 2018 of more or less the same as was received last year. If the 2018 rainfall is less than last year then it will only be our water savings that can carry us through without striking “Day Zero”.
Should the water consumption once again climb from current levels, as it is presently tending to do, then unless substantial rains arrive, we could fall towards failure of system in late summer or autumn of 2019, as shown in here.
Our present consumption at 520-540 ML/d lies between these two forecast lines; far too marginal.
On our water consumption tracker chart we are at point red ‘X’ (now 20.5% of capacity on 16th April), still above but arcing ever more rapidly back towards the minimum performance level indicated by the dotted black line.
(see here for a detailed explanation of how the chart is to be read).
We are now entering a very critical phase.
The coming 8 weeks will determine whether we have succeeded or failed. At present we have built up a reserve of some 20 M.cuM but that will not carry us through if the early winter rains fail.
The CoCT chart below sets out what the combined dam level patterns have been over the past 20 years and how rainfall affects this.
What can clearly be seen are three things:
1) the 2018 water consumption line is very flat demonstrating the importance of the water savings being achieved in that, despite a far lower rainfall, we are approaching a similar level of water stored as we did at the same time in 2017. While an outstanding overall performance, this came at the expense of a substantial crash in agricultural performance due to irrigation water withheld (figures just released show that apricot/peach/pear tonnage dropped by 16% and that smaller fruit resulted in lower prices being achieved – possibly an overall revenue drop in excess of 20% for these farmers).
2) As recently statistically demonstrated by Pytor Wolski, good rains in May (weeks 18-22) tend to point to a good rainfall year (see 2013, 2014 below) while the biggest lift in dam levels usually tends to occur most often in June month (weeks 23-26) (see 2011 and 2012 below).
3) As we all know, we have not experienced total dam storage as low as this in recent times (see 2018 line) which underscores how very fragile the situation is; obviously still very dangerous!
How is the City’s long-term water augmentation plan developing?
Other than a brief announcement that the V&A desalination plant is now in production (2 ML/d) there has been no useful news.
See updated details of Cape Town’s planned interventions to produce fresh water here.
Learn about water savings efforts by other Cape municipalities – read more here.
Do visit the City of Cape Town website to see additional graphically presented information regarding the water situation – it is very helpful. See the full water status presentation here.
What ought you to be doing?
Apart from trying to reduce water usage to 50 L/p/d or less, it is time to start thinking about winter. Purely statistically, there is a growing possibility that we will enjoy a normal to strong rainfall season this year. So it makes sense to now make suitable preparations in case we do.
Installing tanks to capture rainfall would be ideal. However if your budget does not run to that then you could consider digging and building a water-tight pond into which you can lead water flowing from gutter down-pipes. A 2m x 1m x 1.1m deep pond can act as a catchment sump storing 2000L of water; 10 days supply for a family. From May through to September, normal rainfall could fill such a pond 2-3 times per month, a welcome supply of extra water for flushing, general cleaning purposes and outside use. If a pond is covered then the water should last well. And when normal rains return it can be converted to ornamental use with some water plants and used to cool off in summer. Here is a sump that we built to catch water tank overflow and storm water.
Will the weather help?
Rainfall across April month-to-date has been minimal and currently lies below average. During the past week nothing significant fell in the catchments. What may we expect for the rest of the month?
However!
At last some very interesting weather patterns are currently forming and the coming two weeks promise some very useful rain.
1) over the next two days, 18th – 19th, some 15-20 mm of rain is expected in the dam catchments.
2) then between 22nd – 25th April possibly heavy rain may fall in the catchments, with 60-80 mm possible in the mountain dams, 40+ mm reaching Voelvlei and 10-20 mm even reaching TWK Dam.
Then during the last two weeks of May, unstable weather with possible thunderstorms accompanied by good rains is also predicted.
Why is this happening?
Is this the start of a return to normal winter rainfall patterns?
This SAWS synoptic chart holds a clue – the front arriving tomorrow although not particularly deep, has not been “knocked on its back” by the high pressure and therefore is approaching the land in an ‘upright’ position. This in turn means that strong winds carrying the rain will approach more normally from the NW and NNW, meaning that the rain can better penetrate to the large dams located behind the mountains.
The NOAA Climate Prediction Centre chart that tracks the South Atlantic Mode indicates that at this time the high pressure cell will lie more to the north and allow these on-coming two cold fronts to develop in the normal manner, and hopefully the heavy rain that has for years been falling south of the country may this time strike the land full-on, with 40-60 mm of rain across the whole SW Cape.
In Summary: outflows exceeded inflows by 5.1 M.cuM. Combined levels fell to 20.5% of capacity.
Here are the levels of the six main dams serving the Cape Town/Overberg/Boland area.
Plan ahead for possible water outages; here are points that could help you.
Read our Indoors Water Savings Ideas here.
Read our Outdoors Water Savings Ideas here
Read about a D-i-Y household rainwater harvesting and tank system here
Read about South Africa’s desalination policies and initiatives.
Click here to see Winelands Weather forecast of possible near-term rain.
Report instances of piping failures and incorrect use to municipalities.
In Cape Town the contact details of the 24-hour Technical Operation Centre are: Email: waterTOC@capetown.gov.za Telephone: 0860103089 (choose option2: water-related faults) or SMS: 31373 (max 160 characters).
Tom Brown.
Retired international businessman with a background in Finance and IT; and now a fruit farmer, with a passion for the weather and climate.
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