Dams – Latest levels: Winelands and Cape Town
Latest report (updated 22nd May 2017) on the main dams serving
Cape Town City and nearby Boland/Overberg towns
Net outflows reduced and the combined level of stored water in the main dams fell by a more reasonable 7.3 M.cuM (0.8% of full capacity) last week.
This usage is shown below. Interestingly, the table now seems to indicate that usage for the same period in 2015/2016 was only just slightly higher than that of 2016/2017. But that is not so; what actually happened is that in April/May, 2016 rainfall run-off had already recharged the dams to the tune of some 35M.cuM. The equivalent rainfall has so far simply not arrived in 2017.
How many days of water is left? Using the water consumption rate of 7.3 M.cuM from last week, arithmetically calculated, the water left will last 85 days, i.e. through to mid-August 2017. Level-4 restrictions should reduce water usage further while steps that can be taken to access another 3 weeks worth of water from the deep end of Theewaters dam can collectively carry us 120 or more days though to end-September. Reasonable expectation is that winter rains should arrive by then.
Further assessment of the current situation!
How are the dams being managed?
Viewers express concern that the Theewaters Dam level is falling fast and could be effectively empty within a month! Much drone footage is shot of the shallow end of this dam and many drive past its increasingly desert-like terrain. Worryingly, charts imply that a large area of the central and east Cape metropole get all their water from Theewaters. So what happens if it runs dry?
In reality, the link between parts of the city and specific dams is notional – that is how the water may flow under gravity and be economically distributed in normal times. However, there is over 10,000km of water distribution piping in the city and water can be pumped from one area to another. So, should one dam empty it does not mean that its connected areas will then have no water. Water would be pumped across from another treatment plant to the area in need.
So the truth is that water in the shallower Voelvlei and Theewaters dams is being deliberately used up faster while the water stored in the deeper dams is being held in reserve.
The following three charts published by DWS demonstrate the varying operational management approach being taken at three different dams. The heavy black lines shows the rate of water consumption for this current 2016-2017 12-month cycle.
- In the case of Theewaters the rate of drop shows that water is being consumed steadily and the normal ‘effectively empty’ point is rapidly approaching. But, as we know, DWS has plans to pull more water out of this dam, so its level will then sink even further.
- For the Berg River Dam the fall, initially steadily downwards, has since April been reduced to hold water back in the deeper section of this dam.
- Wemmershoek Dam, as previously reported, has been untouched since February (the blip between March and April was an incorrect reading) and is reportedly held as an “iron reserve” by City of Cape Town, which owns this dam.
Why is this? DWS control software constantly recalculates prevailing weather factors and suggests from which dam water should next be drawn. At the moment the shallow dams with large surface areas (Theewaters and Voelvlei) are being drawn down fastest to minimise evaporation losses.
These charts also show something else that is very relevant right now. Please look at them carefully and see if you can see what that relevant aspect is. All SW Cape dams exhibit the same characteristic! Then click here for the answer to see if you got it right.
What ought you to be doing?
Apart from continuing to help with all the water savings that you can manage, you would be well advised to install the biggest water tank system that you can afford and/or accommodate on your property, located as high as possible and capturing rainfall off your house gutters. If you have already done so, good! If not, then seriously consider borrowing if you need to but find some way to install a rainwater harvesting and storage system at your home.
This will not only give you greater water security but it will also save you money (remember that CoCT will soon also be charging for the first 6000 litres each month – nothing will be free any more). Importantly, by capturing and using your own water supply, you will also reduce your draw of water from the municipal system and so help conserve supplies in the dams.
I will be writing more on this subject next week. But, if you are curious as to just how much water you could save I recommend that you visit the UCT site and do some quick calculations through their easy-to-use “Water Harvesting Tool”. All it requires is for you to simply:
- indicate on the Google map where you are physically located
- input the area of the roof that would capture water and select a tank size
- input approximately how much water your household uses at present.
And then explore the results seeing how much of you water your would likely supply yourself and the substantially reduced amount that you would still need to draw from the municipal supply.
The UCT team are to be congratulated for making such a useful planning tool available. Thank you!
What are the authorities doing now?
The drought is believed to be the worst since 1904. Accordingly, the Western Cape has now been declared a Drought Disaster Area. This will last for three months but can be extended.
Why do this? “The disaster declaration will accelerate the Western Cape Disaster Management Centre’s Project ‘Avoiding Day Zero’, the Province’s strategy to ensure that taps do not run dry,” Premier Zille stressed. The Disaster Management Act empowers the provincial government to protect key front-line service delivery points by re-prioritising funding. “Funding will be re-prioritised provincially and, should further assistance be needed, the province will approach National Treasury and the National Department of Water and Sanitation,” the Premier added.
One initiative to be undertaken will be the urgent drilling of boreholes in school grounds so that these may be used as emergency water supply points if needed.
Progress is being made with actions to start tapping into the SW Cape underground water supplies as well as to reprocess waste water.
Level-4 restrictions will be introduced from 1st June in an effort to further reduce water consumption. Consumers will be urged to cut consumption to maximum 100L of water per person per day. And any external use of potable water will be forbidden.
Read mayor De Lille’s recent statement regarding Cape Town water supply planning here.
Will the weather help?
What we should see this week is the penetration through to the land of several cold fronts. However, as can be seen from the Weather SA synoptic chart below, the front making landfall on 23rd May is weak; the lines of pressure (“isobars” – the closer these lines are the deeper the low pressure, the stronger the accompanying winds and the higher the likelihood of heavier rains) are far apart indicating a “gentle pressure slope”. Nonetheless it has slid around the high pressure cell “hill” and will bring some light rain and drizzle to the Cape Peninsula and Agulhas areas.
Another cold front will make landfall on Friday 26th May bringing more general rain of 7-15mm across many areas into Saturday; this one looks strong enough to displace the high pressure.
But, unfortunately, it seems that total rainfall for May could still end up below average, further delaying any recharging of the dams.
June is starting to look more promising, as one would expect. There are at least four weather fronts forecast, each with the capability to bring 15-20mm of widespread rain. The front towards the end of June could be particularly strong as it is predicted to include thunderstorms. We shall see closer to the time if these fronts are able to displace that hovering high pressure off the west coast.
So what may we expect over the coming months?
NOAA/CPC, recorded that surface temperatures in the Pacific have declined and their models now predict that any El Niño may remain neutral through our winter period and therefore not influence our winter rainfall. We shall see.
The important final element is how much rain has fallen in the dam catchment areas. For May month to date the result is very poor and the rains have so far been less than that which fell in April.
At the moment it is only the two Steenbras dams that are benefiting from rain systems just clipping the southern point of the SW Cape.
In Summary: net outflows continued last week and stored water reportedly fell to 20.0% of full capacity. Stored water volumes indicate that water is only being withdrawn from the Volelvlei and Theewaters dams while water in the other smaller dams is being held back. Rainfall has been poor.
Here are the levels of the six main dams that serve the Cape Town/Overberg/Boland area. Also shown is the total water storage level of all the dams in the whole greater Western Cape.
Read about a D-i-Y household rainwater harvesting and tank system here
Our water-wise landscaping and water-wise gardening suggestions.
Read about South Africa’s desalination policies and initiatives.
Details of level-3B water restrictions here.
Draft services pricing plan for 2017/2018 and proposed water tariff structure here.
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).
Note: To calculate number of days of water left we first reduce reported water stored volume by 10% of capacity (i.e. 89 Million cuM) to allow for water that usually can’t be extracted or processed and then divide the remainder by the net outflow rate during the previous week.
Tom.
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