Dams – Latest levels: Winelands and Cape Town
Latest report (updated 21st August 2017) on the main dams serving
Cape Town City and nearby Boland/Overberg towns
General rains last week caused all the main dams to show improved water storage levels. Combined net inflows dropped to 12.5 M.cuM for the week (1.4% of capacity) vs 15.3 M.cuM (1.7% capacity) the previous week. Total stored water rose to 32.1% of full capacity.
During this week last year the dams rose by a similar 13.3 M.cuM to reach 56.9% of capacity.
The year-on-year comparison shows the reduction in net water stored in 2017 to be 120.2 M.cuM greater than it was over the same time period in 2016
[Why since 1st November 2016? that was when level 4 restrictions were imposed].
Current status?
There is now 285 M.cuM (200 M.cuM of easily accessible) water in the dams (last year same time this stood at 506 and 421 M.cuM respectively).
The accessible water now equates to just under half of that available at the same time last year.
The weather forecasts predict that only average rainfall is probable through to end-September.
This UCT Climate Systems Analysis Group chart (as at 19th August) shows that cumulative rainfall year-to-date (red line) (measured at CT Airport) remains the lowest across the past 12 years.
How long will the water last?
There are now many “moving parts” in the water equations, so it seemed that the best way to track what is happening from here on is to make a side-by-side comparison with the same periods last year. The new table below does that (it won’t be dead accurate but will give a useful indication):
- Blue are is actuals for 2016/2017
- Yellow is estimate of available water starting 1st Aug and assuming same pattern as last year.
- Brown will progressively show actuals for 2017/18 as the months pass (dates may vary a bit).
For example, in August 2016 the month started with 476.1 M.cuM in storage and ended on 5th September with 535.4 M.cuM (60.2% of capacity); i.e. net inflows during month = 59.3 M.cuM.
If we apply the same to expectation to 2017 – the actual storage level on 1st Aug was 258 M.cuM – and the same rainfall as last year arrives during August, then the combined level stored at the end of August 2017 would be 317 M.cuM of water. As at this report date of 21st August we stood at 285 M.cuM and seem to be reasonably on course to reach the estimated level by month-end.
The model progresses each following month the same way, estimating each month-end level. What it shows is that we would start February 2018 with some 110 M.cuM of water in storage. Assuming that the last 85 M.cuM of water cannot easily be abstracted and the same consumption and rainfall pattern held, then accessible water would be exhausted around mid-February 2018.
Two things can help extend supplies:
- reduced consumption – with the extra restrictions it is sure to be less than last year.
- rain – at present the predictions are rather for less than fell in 2016.
So, these two factors may balance out, pointing to approximately the same “zero water” day.
Clearly something else must and, in fact, is being done done! The CoCT authorities are taking steps to “produce” additional potable water [see below]. As soon as new water is produced and added to the system it will show up in the above table (it will have the same effect as additional rain in slowing down the water drawn off the main dams) and start to stretch out the supplies. Additionally, household water restrictions are likely to be tightened, while DWS is cutting back irrigation water delivered to agriculture.
Will it all be enough? We have to hope so (read here what steps we believe authorities need to take to ensure sufficient potable water is available until winter of 2018). As time passes we will see.
Careful management of the water consumed by visitors will also play an important role.
What ought households to be doing?
Focus on one thing – make your household more water-resilient, meaning;
- Practice getting by using far less water (read my suggestions on how to).
- Install some means to harvest and store water that is under your direct control (see here).
It is great to read all the positive comments made by people who have installed one or other level of rainwater harvesting and storage system in their homes. Their sense of relief and regaining control is obvious. We generally recommend installing 1000 – 2000 litres of storage per person in the household. Use the UCT Climate Groups model here to estimate correct tank sizes for your situation.
What are the authorities doing now?
A week back, Mayor De Lille announced specific interventions to “produce” more potable water to avoid the City running out of water (read the full announcement here). These hinge on:
- drilling and expanding a well field into the Cape Flats Aquifer
- drilling of boreholes into the Table Mountain Group Aquifer.
- installing moveable containerised desalination plants.
- hiring of a shipboard desalination unit.
- water recycling for drinking-use enhancements for water treatment plants.
The announced goal is to ultimately produce a core water supply of 500ML of water per day, thereby making the City sufficiently independent of the main storage dams to be able to easily withstand future droughts and possible water shortages.
Here is a summary of what is planned.
One notable item missing is some mechanism to support households installing storage tanks. Cynically one could say that, firstly, it is simpler to manage and implement large monolithic solutions and, secondly, that centrally controlled water – however produced – can be sold to consumers; whereas household tanks make consumers independent of the grid and reduce paid demand for municipal water. If that is the decisive fear, it is a real pity because extra water storage at a household level is a very practical way to take advantage of Cape Towns topography and rainfall distribution patterns.
The City’s plan is broken down into stages of urgency. At this point in time, tenders are being issued and it will be some months before all these are adjudicated and can be awarded and implemented.
Extreme urgency will be required to implement the first “tranche” if it is to make a useful contribution in time. Papers supplied by one tenderer imply that the land-based containerised desalination plants can be operational by year-end. So also the desalination ship, if available. Groundwater abstraction from the Cape Flats aquifer should also be capable of fast execution.
If this all eventuates as hoped, then additional potable water being introduced into the City’s water supply by year-end could total 150+ML/day. That would effectively add 1 week’s water to the stored supply each month. Seen another way, every month that passed would see the total water supply stretch out for an additional week.
But to see how marginal that is, if such systems implemented and running flat out through January and February 2018 it would simply push the point at which the available water would be exhausted from mid-February out to end-February. It is going to be a very close run thing!
Anyway, at this stage the CoCT cannot give specific operational dates and volumes of water to be produced because they just do not yet know. The tender process has to be followed so it will be a number of weeks before reasonably accurate estimates can be supplied. We are left to trust that the authorities will do everything in their power and budgetary constraints to supply sufficient extra water.
Will the weather help?
The remaining 4 weeks of traditional Cape winter are all important. But this is the time where the weather starts to change from winter to summer patterns. This makes the weather very changeable and, apparently, forecasts beyond 5 days become unreliable.
According to NOAA/CPC’s chart for this week there is a chance for 30-40 mm of rain in total to fall between the 22nd and 24th August.
The rainfall pattern is forecast to spread across all the dam catchments, except for Theewaters dams.
Another burst of rain is predicted to bring 30+ mm of rain across the area between 29th August and 1st September.
What else may we expect?
Looking further out into September, we may expect:
- 30 mm of rain on/about 11th – 12th September.
- 10-15 mm of rain between 26th – 27th September.
Otherwise, only occasional rain is forecast indicating average rainfall across the rest of winter.
One “wild card” is possible emergence of the colder La Niña condition in the SE Pacific. This NOAA/CPC chart shows that it may yet have some destabilising effect on our weather possibly causing late rains.
How much rain has actually fallen in the dam catchment areas? Useful rains have been falling keeping the inflows running into all the dams with exception of Theewaters Dam. DWS is still pumping water from Berg River through to Theewaters dam.
In Summary: continued fresh rains caused net inflows of 12.3 M.cuM lifting the dams levels up from 30.7% to 32.1% of total capacity. More rain is expected over the next 10 days. Dam levels are still far too low and constant water savings is essential.
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 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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