Dams – Latest levels: Winelands and Cape Town
Latest report (updated 30th May 2017) on the main dams serving
Cape Town City and nearby Boland/Overberg towns
Net outflows apparently reduced and the combined level of stored water in the main dams reportedly fell by a relatively small 5.0 M.cuM (0.6% of full capacity) last week.
It strikes me that the dam readings level are becoming less precise as water levels drop below the gauge plates. This may be contributing to the readings jumping around so much. The past 5 weeks’ water consumption has averaged 6.2M.cuM per week, trending down.
So, to iron out these water level reading inconsistencies I am going to apply a 5-week moving average of water consumption when estimating the amount of usable water that is left. This average should drift down as level-4 restrictions take effect. Doing so should also stop the odd effect of the number of days apparently going up and down.
How many days of water is left? Using the average water consumption rate of 6.2 M.cuM from the last 5 weeks and absent any useful intervening rainfall, arithmetically calculated, the usable water left will last 13.5 weeks, i.e. 95 days, through to 27th August 2017.
New Level-4 restrictions should further reduce water usage. Also steps being planned to access another 3-4 weeks worth of water from the deep end of Theewaters dam as well as other possible interventions can collectively carry us though to end-September. Winter rains should arrive by then.
Further assessment of the current situation!
Much is made of dams that appear to be falling fast and little is said about those that are not. In fact, diagrams that appear to indicate large swathes of the metropole perhaps shortly being without water if/when the Theewaters Dam empties are dangerously misleading, could cause needless anxiety and perhaps even spark unrest.
The Cape Town water distribution system is heavily networked with thousands of km of piping. While there may be some extra pumping involved, water can be re-routed anywhere in the system. Watching individual dam levels may be fun but it is the overall level of water stored in the whole system that actually matters.
Because of this precise management capability, DWS can elect hour-by-hour which dams it will draw water from. So Voelvlei and Theewaters are deliberately being run down, while water is being held back in the other 4 mountain dams in order to help reduce non-consumption water losses.
To maintain proper context, note also that simply comparing % total storage levels year-on-year can deceive. The important raw fact to appreciate is that, versus the same time in 2016, this year we have less than half (47%) of the the amount of fresh water still available. Put another way, last year we had the equivalent of two full dams (Lower Steenbras and Wemmershoek) more water available; water that this year just doesn’t exist.
Last week I suggested that readers look at the charts for the Cape dams to discern a particular pattern. As several hundred of you picked up, the dams historically all reach their low point around end-April and then, during May rainfall run-off usually starts to recharge the dams. This latest chart of Theewaters Dam echoes that of all the main dams – it shows that in 2017 the May rains have simply not arrived and the dam levels are, therefore, not recharging as they historically have tended to do. The critical point was passed around mid-May and this drought is now really starting to bite hard.
What ought you to be doing?
Last week I advised that one should install the biggest water tank system that you can afford and/or accommodate on your property.
Why?
- Clearly “water scarcity may from now on be the new normal” in SW Cape. The dams will surely fill and empty again but the trend (due to climate change?) now seems to be towards diminishing average annual rainfall. So water shortages will happen again and again; the only question is when? Therefore, households should change water consumption habits, actively practice water re-use and creatively conserve and store water.
- A peculiarity about the topography of the SW Cape argues for a different approach. Most of the population lives in low-lying areas near the sea but are reliant on fresh water from dams located in the distant mountains. Interestingly, however, more than enough rain falls each year in the low-lying areas to satisfy most needs; but it is not practical to collect and store this rainfall run-off in large dams. So every year, most of this water runs unused into the sea.
So consider this: if one takes these two aspects together – reduced consumption and topgraphically available water – it becomes apparent that a potent solution lies within the hands of most households. And what’s that? Obviously, install appropriate rainfall water capture/storage systems in every home.
The simple but critically important outcome would be that fresh water is then captured precisely where it is needed. With care, no filtration is needed and in many cases by locating the tanks properly, little or no pumping is required. Careful calculations are needed but desk-testing many variations shows that most households can capture 60%-70% of their reasonable annual water needs.
The only vulnerability is the dry summers where very little rainfall run-off is available. However, the central fresh water system supplied by the dams would bridge this summer period.
Hypothetically then, if every household supplied at least 50% of its own fresh water needs and practised continued re-use of grey water, then the draw on the main dams could be reduced by, say, 30%-40%. This in turn means that the water stored in the main dams could last for 3+ years (instead of the present two years) sufficient surely to cope with any ‘normal’ drought. Last but not least, other water production interventions such as water recycling and tapping into underground water would add extra sources that could be variably used from time to time to supplement supplies in times of greater shortage.
The possible outcome?
- Many homes could be transformed into “mini damlets”. At a household level, the cost of doing so should typically be paid back within 3-5 years through saving the costs of water no longer taken from the taps. Savings would naturally continue indefinitely thereafter – suppliers indicate such systems and storage tanks should have a 20+ year lifetime.
- At a city level, it would mean the creation of a water capture and storage system equivalent to building a new dam larger than Wemmershoek for a fraction of what that would otherwise cost. And this collective reduced need for central investment would proportionately grow as the population expanded.
- Many citizens would personally take responsibility for their own water usage and would directly contribute to the management and conservation of water resources. This would support and free the authorities more to concentrate on creating and managing several additional levels of communal water storage, production and distribution.
In so doing greater water security, at both individual household and collective community levels, would be achieved.
I notice that many of you visited the UCT site to make calculations of possible rainwater harvesting installations using their easy-to-use “Water Harvesting Tool”. I would be interested to receive comment back from those who did.
Do take a look a practical example of rainwater harvesting calculated using a real-life property. Then try it for your own property.
Read about a D-i-Y household rainwater harvesting and tank system here
What are the authorities doing now?
Previously declared actions to augment water supplies are being followed.
Cape Town Mayor Patricia De Lille has announced change to level-4 water restrictions from 1st June 2017. Read the mayor’s statement here.
Read main details of level-4 water restrictions here.
Will the weather help?
The rainfall received in May was well below average, as one weak system after another was deflected south of the country by the prevailing high pressure system. Very disappointing.
However, between 7th and 9th June a large depression with pressures as low as 988mb, presently gathering over the Atlantic, is scheduled to strike the Cape.
With luck this may bring the first really heavy rains across the SW Cape and up into the dam catchment areas. Forecasts are presently for cumulative falls of 30mm-60mm being possible (here is NOAA/CPC’s forecast chart for that week).
More rain is forecast at other times in June with another strong system arriving towards month-end.
NOAA/CPC, recorded an up-tick in surface temperatures in the Pacific. However, their models continue to predict that any El Niño may remain neutral through our winter period and therefore not influence our winter rainfall.
How much rain has fallen in the dam catchment areas? May month to date rainfall in the dam catchments has been very low and could end up less than that which fell in April.
Only the two Steenbras dams continue to benefit a bit from rain systems touching the southern point.
In Summary: net outflows continued last week and stored water reportedly fell to 19.4% of full capacity. Indications are that water is being withdrawn mainly from the Voelvlei and Theewaters dams while being held back in the other smaller dams. No significant recharge from rain has occurred.
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.
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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