Dams – Latest levels: Winelands and Cape Town

Latest report (updated 27th March 2017) on the main dams serving

Cape Town City and nearby Boland/Overberg towns

Water stored in the six main Cape dams fell to 26.8% of overall capacity (down from 28.0% last week). Reducing this by 89 McuM (10% of full capacity) for water that is impractical to extract, this means that only some 150 Million cuM of fresh water remain.

Very good news is that the net outflows from the six main dams last week, as reported by DWS, was just 10.9 McuM and evidenced a sharp reduction from the previous weeks, due no doubt to huge savings efforts as well as some possible small run-off from recent light rains. Well done all.

This is a vitally important development. if this new lower consumption rate holds, it means that the presently stored water can then last for around 96 days, i.e. about 14 weeks. And that means it may last until end-June by which time we are far more likely to receive solid relieving rains.

 

The savings reflected in this table of 48.2 McuM versus consumption a year earlier show how very important the restrictions are. At the latest consumption rate this water saved can extend supplies out by more than 4 weeks, which could make all the difference between running out of water and instead being able to hold on until good rains arrive. [This saving is equivalent to having added a new dam about the size of Wemmershoek Dam to the system, without any capital expenditure needed.]

What are the authorities doing?

One issue emerging over the past week is a peculiar taste and smell to some of the tap water, being reported in Cape Flats areas.

Margaret V reports: “I’ve been told it’s caused by algae  but it tastes more like the bottom of the dam. I’m located in Plumstead area. Boiling your drinking water does get rid of the smell and bad taste. When cool store in glass bottles or jugs, not plastic”.

Pam H comments: “I’m located in the Ottery area. The water taste and smell is like rich smelly soil. Boiling water for 3 min and more. The smell and taste goes away. Not very pleasant when in the shower or kitchen”.

Marche P says: “Same in Northern Suburbs, soil smell and taste…”

Substantiating the comments of those of you who have noticed changes as dam levels drop, authorities said geosmin was responsible, a harmless organic compound that affects water’s smell and taste, News24 reports. The nose detects this as a musty, earthy smell or the smell of beetroot.

Geosmin is reportedly present in water from Theewaterskloof Dam, which supplies parts of Cape Town City. The water is now receiving increased doses of powdered activated carbon during the treatment process. Authorities report that water quality is constantly monitored by sample analysis.

This chart shows areas (light brown) served by the Theewaters dam where water might exhibit this musty smell and taste. Residents are assured that geosmin is totally harmless.

What can you do to personally cope with this, and future, water shortages?

The fact is that the SW Cape typically enjoys good rainfall each year varying between 350mm and 1000mm depending upon one’s location. In other words rain that falls in areas right where people live is actually sufficient to sustain the population with little need to draw from the dams.

The problem is that because of the Cape topography, most of this rain falls at very low elevations making it practically impossible to capture this water on a big scale and store it in large dams.

One solution would be to install rain water capture and storage systems in every house that has sufficient space and a large enough roof area. This is exactly what is done in most country areas to supply fresh drinking water to rural households. So why not do it in the cities?

We propose that the provincial government could assist by:

– negotiating economical prices of certain standard tank sizes and piping/connection fittings.

– encourage banks to permit people with bonds to finance the installation through extending bonds.

– help to train and form enterprises of people skilled to install and plumb in such systems.

– provide financial support to low income households to enable them to access rainwater tanks.

We feel extremely strongly that if possible every house should be equipped with a rainwater capture system with at least one tank connected to the house gutters. The rule of thumb for rainwater harvesting is that approximately 0.85 litre is captured per 1sqM of roof per 1mm of rain; i.e. a 10mm shower falling on a 200sqm roof area will capture about 1700 litres of water.

Here is an example of how it could work out for a two-person household who first reduced their overall consumption to 100 litres/day/person; have a 200 sqM roof area; and install a 5000 litre tank. It assumes that the rain showers occur evenly across the month.

They would consume 72,000 litres per year but capture 56,000 litres, only drawing 16,000 litres pa from the system (dams). Theoretically, they would only draw municipal water during 4 months and should never exceed the basic 6000 litre allowance so the cost would be zero. [Note that in survival circumstances with water use cut to 40 litres/day/person, a full 5000 litre tank could supply fresh water for the 2 people for two months.]

Another case would be a 4-person household also using 100 litres/person/day with a bit more roof area (say, 250 sqm) and a 5000 litre tank. They would consume 144,000 litres per year but capture 87,000 litres, only drawing 57,000 litres pa from the dams of which only 14,400 litres pa would be in excess of the basic allowance and be paid. [Note that in survival circumstances with water use cut to 40 litres/day each, a full 5000 litre tank would supply fresh water for the 4 people for a month.]

Or, consider a 3-person household also using 100 litres/person/day with a roof area of, say, 180 sqm and just a 3000 litre tank. They would consume 108,000 litres per year but capture 64,000 litres, only drawing 58,000 litres pa from the dams of which only 5100 litres pa would be in excess of the basic allowance and need to be paid.

And the overall outcome?

A 5,000 litre tank and piping could probably be installed for R7000 or less (I have assumed gravity feed but if a pump is needed then add R1500 or so depending upon its complexity). Assuming households could consistently reduce consumption into this range of 100 litres per person per day, or less, and save water in this manner then the net draw of water from the dams could be reduced to, say, average 30,000 litres of water per annum per household vs the 270,000 litres drawn on average by those households per annum under “normal” circumstances, a net reduction of up to 240,000 litres of water drawn per household.

Lastly, assume that 700,000 households achieved this goal. Savings could total 168 McuM of water per year, equivalent to the output of the desalination plant now under consideration.  Compare:

– cost of 700,000 tank installations @ R7,000 each = R5 billion capex; annual operating cost = nil.

– versus cost of a desalination plant = R15 billion; annual operating cost R1.2 billion.

– and in so doing, water captured and stored in the existing dams would be sufficient for 3+ years.

This could sustainably solve the problem for ever at a fraction of the cost of a desalination plant. Most of the cost would be covered by individual households and effectively be paid for out of future water cost savings (pay-back should be 7-8 years). And installations could start straight away.

Rainwater harvesting by households in SW Cape should be a key part of any solution. 

Read about a D-i-Y household rainwater harvesting and tank system here

Looking ahead.

How much water do we have left stored in the main dams? At this new lower consumption rate, water supplies should now last approximately 96 days  – hopefully deep into June, 2017.

Will the weather help?

The NOAA Climate Centre predicts that southern Africa should enjoy a winter of normal average rainfall. Rain did fall a week back but that expected for end-March seems to have dissipated and been pushed south of the country.

Unstable weather continues to be predicted for around mid-April and end-April with the possibility of some general rain on both occasions. The longer-range forecast indicates a sustained period of unstable weather across the middle of May and, encouragingly, a spell of heavy rain is still being predicted for early in June.

Once more we must warn that these predictions are more or less in line with the normal average rainfall for these periods. There is as yet there is no indication of widespread sustained heavy rains needed to help the dams start to recover.

Read ShowMe’s guide to reducing indoors use of water here.

Read ShowMe’s guide to reducing outdoors use of water here.

In Summary: outflows exceeded inflows last week but less strongly than before. Stored water volume fell from 28.0% to 26.8% of full capacity. At present consumption rates, 96 days of generally accessible fresh water supplies remains in the main dams. 

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.

Guide to reducing outdoors use of water here.

Guide to reducing indoors use of water here.

Read about South Africa’s desalination policies and initiatives.

Details of level-3B water restrictions 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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