Cape Water and Dams Report
Dams – Latest levels: Winelands and Cape Town
Report nr. 94 (updated 19th March 2018) on the main dams serving
Cape Town City and nearby Boland/Overberg towns
The net outflow from the main dams was 3.2 M.cuM, or 0.4% of capacity (vs the preceding week’s 4.7 M.cuM), and combined storage dropped from 22.7% to 22.3% of capacity.
Inward transfers from Groenland Water Users continue to reduce the apparent water consumption by around 2 M.cuM per week. This effect should fade away by end-March 2018.
During the same week in 2017 the dams dropped by 12.6 M.cuM (1.5%) to 28.0% of full capacity.
How long will the water last?
This week we make a small mid-month downward correction to fully comprehend the compensating inflows from the Groenland system by dropping the predicted March month water consumption from 23.1 M.cuM of water to 19.3 M.cuM.
The model now shows that water usage in the SW Cape has been reduced to the extent that the likely combined storage volume will be 136+ M.cuM of water at the end of May which, even in the complete absence of meaningful rain, points to our likely attaining our “Success Day” target of at least 100 M.cuM of water stored in the system at the end of June; and we will have reached winter.
(Here is our model predicting the amount of water likely to be stored in the dams by mid-year.)
Putting it in context!
However, make no mistake, we are still at severe risk: although statements have been made that “Day Zero” will not happen in 2018 this is wholly dependent upon good rains falling in the dam catchments before end-June.
While it is reasonable to expect that the SW Cape will receive rain in June, if good rains do not fall in the catchments and we reach end-June with less than 120 M.cuM of water (13.5% of total capacity) stored in the total system, then the authorities would have little option but to implement stage-2 collection/rationing (“Day Zero”) already during July 2018.
Another concerning fact is that even though the net outflows from the dams has reduced, water consumption in the Metro has actually increased – the most recent daily average consumption as at 19th March has leapt to 565 ML/d from that of the previous week of 506 ML/d. This is far too high compared with the goal of 450 ML/d. More worrying is that the reported amount of water drawn from the dams climbed sharply to 535 ML/d. Every effort must be made to keep reducing water use to the level of 50L/p/d in order to achieve the safe levels that have been forecast
Our altered CoCT tracker chart shows the progress. We are today at point red ‘X’ (22.3% of capacity on 19th March) and remain comfortably ahead of the minimum target usage (defined by the dotted black line which gets us to a minimum 100 M.cuM of water stored at end-June).
While the overall storage situation remains positive, when the inflows from Groenland stop and also if these spells of higher consumption continue then we will see a possibly accelerated rate of decline in the levels of the dams. This would be a time of considerable danger of the level of the dams once more dropping too low.
The hope is, of course, that if rains commence in May and June and extra water production through augmentation initiatives starts to grow (more about that below), that the total volume of water stored in the dams will start to level out and follow the upward trajectory described by the dotted red line, gradually climbing away from the survival level; then water will continue to flow through the taps.
(see here for a detailed explanation of how the chart is to be read).
What are the authorities doing now?
1) The average amount of extra fresh water being produced is holding steady at 30-35ML/d. This is expected to start growing steadily from April as augmentation efforts come into production. The worrying aspect is that the graph shows that the underlying total water consumed has lifted sharply.
(Graph tracking overall use and production of extra water through the augmentation efforts).
2) It has been announced that as planned some 10 ML/d of recycled water will be available inexpensively if collected from the City’s various water treatment plants. You can apply to obtain some of this water. People have installed small bladder type flexible tanks in the boots and back of their vehicles and collect recycled water for non-drinking purposes around the home and garden.
3) The groundwater extraction program is proceeding well.
Last week I joined a group inspecting the work being done at Steenbras (seen here at an exploratory borehole site above Steenbras Upper Dam). This is accessing the Table Mountain Group (TMG) geological structure which in this area offers two water-bearing layers – the shallower unconfined Skurweberg aquifer and the deeper confined Peninsula strata. It was obvious that extremely careful, deeply scientific work is being done to establish the sustainable yields while protecting the environment.
What is not generally known is that the TMG aquifer is one of the largest of its type in the world. In simple terms this means that is contains enormous reserves of underground water. Also interesting is that its structure, running south-west from the mountains means that it recharges est.1%-5% annually due to the strong rains that tend to fall in the mountain area and then flow down to exit the aquifer via springs and underwater seeps in False Bay. The TMG aquifer is potentially hugely productive and some test boreholes individually produce 1.5 ML/d.
Research into these aquifers has been in progress since 2002 and resulted in findings in 2012 which now form the basis of accelerated abstraction work that is being done. Ground water does unquestionably present an extremely important source of large volumes of water for the SW Cape. It is sustainable and also closely fits together with the existing dam and water piping systems, thereby permitting good economies of operation to be achieved; as a result of which the cost of abstracting groundwater is expected to lie between the low cost rainwater and high cost desalination water.
At present the estimated fully sustainable amounts of ground water that may be obtained comprise Atlantis aquifer 25 ML/d, Cape Flats aquifer 80-100 ML/d, Steenbras Skurweberg aquifer 10 ML/d, other TMG aquifer at SteenBras/Theewaterskloof/Wemmershoek/Voelvlei dams providing another 50-80 ML/d; all totalling in excess of 200 ML/d with much more potential for later. The beauty of ground water is that it can flow directly to the dams and from there be economically processed and distributed by the existing water treatment and reticulation system. Lastly, groundwater production can relatively inexpensively be scaled down in times of good rains filling the dams and then variably scaled up again in drier times; i.e. it costs relatively little when not in use.
3) The City reports that it is struggling with theft of components from the Cape Flats underground water extraction borehole installations as well as from the new desalination plants. This senseless and selfish activity threatens to both increase the costs of the intervention activities and delay their coming into production; a tragic outcome if it prevents the vital extra water from being produced on time. Also water is being wasted. Additional security costs will have to be incurred. Report any theft or vandalism to the City’s call centre on 0860 103089.
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?
1) It is vitally important that we all maintain efforts to reduce water usage. Water consumption has been increasing during March. It may be that recent statements by political leadership that “Day Zero” will not occur in 2018 has lulled folks into a false sense of security. This pre-supposes that everyone reduces consumption to 50 L/p/d, and at the moment this is not happening! It will be cruelly unfair to the farming and rural communities who have sacrificed so much to make their water available to the towns and the City, if town dwellers demonstrate ingratitude by relaxing and starting to waste water once more.
2) Be careful about basic hygiene. Stomach-related problems are rising. Do wash all raw food carefully before use to avoid any spread of sickness through contaminated food.
3) Consider collecting and using recycled water for non-potable purposes around your home.
Will the weather help?
La Niña condition is still in force according to NOAA/CPC. It appears to be reverting towards neutral range where it’s expected to remain through the second half of the year, with a continuing bias to below-average temperatures which still may help bring rainfall this winter.
Cape weather for the coming two weeks is generally forecast to be warm and fine with occasional spots of overcast and light drizzle – typical March weather.
However, there is a most interesting weather pattern potentially developing for early April.
According to NOAA/CPC, for the first time since late-November, the Antarctic Oscillation (AAO) is becoming negative and is being forecast to be so again in early April. Why is this important for the Cape? Well, when this AAO is positive it implies a pole-wards shift of this “southern circulation”) that broadly equates to the South Atlantic high pressure cell pushing the low pressure fronts to the south away from the land, as has been happening for some time. But when this AAO value becomes negative, it implies the opposite with the high pressure shifting equator-wards permitting cold fronts to more directly approach the Cape.
Now, as it happens NOAA reports that there is a very deep depression in this circumpolar circulation currently passing south of Australia. If that depression continues to exist it should get here about two and a half weeks from now. As it happens, the long-range forecasts are starting to pick this up and beginning to show unstable, thundery weather reaching the Cape between 6-10th April, bringing chances of some useful rain. Could this turn out to be the first major rainy weather system to strike the Cape this year? Rainfall of the order of 30-40mm is being forecast for the Paarl area, a good proxy for the mountain dam catchments.
In an unrelated but very relevant work, Peter Johnston and Piotr Wolski of UCT Climate Research Group have analysed past weather patterns and developed a possible basis for anticipating likely annual rainfall outcomes (read more about this interesting analysis here). Intriguingly, they propose that April is the key month that may indicate if above-average rainfall could occur in that year. So if a strong weather system does indeed bring good early rains in April, their research implies that this could point to an above-average rainfall year in 2018. Won’t that be gratifying!
Our chart of rainfall for the dam catchments shows that some rain penetrated into the mountains last week reaching Steenbras and Wemmershoek dams. This could partly account for the very small drop in the level of the Wemmershoek dam last week. Additionally, Steenbras continues to benefit from the inward transfer of water from the Groenland system.
In Summary: outflow exceeded inflow by 3.2 M.cuM. Combined dam levels fell to 22.3% 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.
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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