Dams – Latest levels: Winelands and Cape Town
Latest report (updated 4h September 2017) on the main dams serving
Cape Town City and nearby Boland/Overberg towns
Net inflows into all the dams caused combined water stored to rise by a reduced 7.5 M.cuM (0.8% of capacity) vs 15.9 M.cuM (1.8% capacity) the previous week. Total stored water rose to 34.7% of full capacity.
During this week last year the dams rose by a strong 16.3 M.cuM to reach 60% of capacity.
[Why since 1st November 2016? that was when level 3 restrictions were imposed].
Current status?
The dams now contain 309 M.cuM (224 M.cuM of easily accessible) water (last year this stood at 533 and 448 M.cuM respectively).
The UCT Climate Systems Analysis Group chart (as at 2nd September) shows cumulative rainfall year-to-date (red line) (measured at CT Airport). To throw it into perspective I have highlighted 2017, 2016, 2015 which shows how exceptionally low the rainfall has been over the past three winters.
Dr. Piotr Wolski, from UCT’s Climate System Analysis Group, has analysed just how severe this current drought has been and I encourage you to read his full analysis “How severe is this drought, really?” here. It is an intriguing paper that shows just how very extreme this drought is turning out to be. A quick glance at the chart above will show that 2016 and 2017 together are likely to be the worst two successive years on record.
Dr. Wolski records the mean annual rainfall at Cape Town Airport as 506 mm. In 2016 it was 218 mm and he estimates that in 2017, without further exceptional weather, it could be as low as 153 mm.
He further calculates that we are statistically unlikely to see another two such dry successive years for another 1150 years.
Against this background Dr. Wolski encourages some empathy for the Western Cape Government and City of Cape Town Council. He explains “Water supply systems are usually designed with an assurance rate of 97%, which means that in worst case they may fail only 3% of time. The conditions we experience now seem to be well beyond what one usually plans for. If my calculations are right, for the water supply system to go through this drought unscathed, its assurance level (would need to) be 99.9%. Surely one can implement a system like that, but firstly, who is going to foot the bill, and secondly, even this might be difficult if the climate goalposts are being shifted.”
Why has this happened? No one knows for sure but it does match the lower rainfall scenarios that global climate models seem to predict. So it is perfectly possible that, statistically unlikely as it may be, the drought might deepen further! All the more reason to proceed as cautiously as possible until the situation normalises again – hope for the best but prepare for the worst.
How long will the water last?
The table below compares the situation this year with what actually happened in 2016 (In 2016, the dams reached 55% of capacity, which saw us through to following winter.
It is done it this way because there are now many “moving parts” as CoCT starts to implement its promised interventions. It will not be practical to isolate each effect and anyway, it is the overall situation that really affects us. Simply stated,
- firstly; if we receive the same rainfall in the catchments as last year then, given the tougher restrictions now in force, we should use less water and the dam levels should fall more slowly, month by month. If they don’t then we will surely have received even less rain than over the same period last year.
- secondly; once the promised CoCT interventions – such as desalination – start to contribute additional fresh water after the end of winter, then (absent very unusual late rains) the dam levels should also fall more slowly than they did last year.
In a nutshell, if the level of water collectively stored in the main dams continues to fall at the same rate as last year, or faster, then it will mean that a combination of rainfall plus contributions from interventions have not offset the on-going consumption of fresh water.
In the table below, it can be seen that the combined level of the dams (309 M.cuM) did not quite reach the level previously expected of 317 M.cuM, meaning that we did not receive the same rainfall in August 2017 as we did last year.
The model has therefore recalculated, once more assuming that the months from September onward will see the same levels of rainfall and consumption as last year. Accordingly, it now predicts that easily accessible water (where the estimated amount of water falls below 85 M.cuM) in the 6 main dams water will be exhausted during second week of February 2018.
At this stage September 2017 is forecast to certainly bring some more rain so one hopes that the goal of combined storage of 325 M.cuM by 3rd October will still be reached.
The situation is now very grave.
In reality the only water that we can rely on for the coming eight months is what we now have stored together with that which is still to run-off in the catchments.
So it is now up to us, collectively:
- what we can save – the authorities cannot take a risk on new water production and should therefore encourage a further very sharp reduction in all forms of water consumption across the board. The safe plan should now be to assume that all planned interventions will fail and that the existing water must last until next winter. The only sure way to do that is set lower targets (50L/person/day), reduce pressure and introduce rolling supply cuts [I understand that it will cause extra leaks and piping failures, but that inconvenience must be borne].
- what additional water we can produce – decisions have been taken and every means possible should be used to speed implementation of mobile desalination, recycling waste water and tapping into the aquifers. If these interventions work as hoped on time then that will be great and restrictions can be gradually eased as additional water production increases. But if delays occur and water production targets prove to be impossible to reach then at least with greater restrictions existing stored water will last for longer, giving more time to sort out the new water production.
What ought households to be doing?
Make your household more water-resilient, as soon as you can;
- practice using less water (read suggestions)
- install some means to harvest and store water under your direct control.
Use the UCT Climate Group’s model here to estimate correct storage tank sizes for your situation.
What are the authorities doing now?
City of Cape Town has announced implementation of level 5 water restrictions, aimed at reaching the overall target consumption of 500ML/day. Specifically:
- personal targets of 87 L/person/day remain unchanged. Use this to set the monthly maximum consumption target for your household.
- households usage limits have been set at 20,000 L/month. This caters for large families and allows for 7 people to use 87 L/person/month [if you are fewer that 7 people then do not use this much water]. If any household usage goes above this then the City will install water flow restricting devices and may also impose fines of up to R10,000.
- the key new target this time is for all commercial property users to reduce their water consumption by 20% compared with what they used during the same period in 2016.
Our belief is that this is too lenient. By our calculations, personal water consumption limits should be reduced to 50 L/per person/day and all commercial and agricultural users should be obliged to reduce consumption by 35% compared to 2016. Otherwise the goal of making the existing water last until winter 2018 is unlikely to be achieved.
The City is processing tenders for its planned interventions. We can expect announcements in this regard to be made over coming weeks giving more accurate dates and water production estimates.
Will the weather help?
September should see some bursts of rain with 10+mm falling later this week, 10-20mm falling between 12th and 14th, and a hopefully heavier spell of 15-20mm between 24th and 26th.
Unfortunately it seems that the weather systems will be weak and may not penetrate far into the main dam catchments; perhaps most favouring the two Steenbras dams.
At this time of year, forecasts are subject to change so we may yet hope for heavier rain in places.
Long-range forecasts of October and November do not predict any heavy rain.
Possible arrival of a colder La Niña condition seems to be fading, and with it the chances of this influencing our late winter weather. Pity, but it seems we may expect no help from that quarter.
The chart below shows the rain that has actually fallen in the dam catchment areas since the winter began. Last week the Berg River Dam and the two Steenbras Dams enjoyed reasonable rainfall with nothing falling in the other catchments. Upper Steenbras Dam is now full and is spilling into Lower Steenbras Dam.
Given the low starting levels of the dams, far more rain was needed to bring them back up to desired levels; but those rains have simply not arrived.
In Summary: fresh rains caused net inflows of a modest 7.5 M.cuM. Dam levels lifted from 33.9% to 34.7% of total capacity. Dam levels are now critically low.
This table shows the levels of the six main dams that serve the Cape Town/Overberg/Boland area as reported by Dept Water and Sanitation. It also shows total water storage levels of all dams in the greater Western Cape area.
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.
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