Cape Water and Dams – monthly report (127)

 ShowMe Winelands

Cape Water and Dams – monthly report nr. 127

as at 4th December 2018

 [Voelvlei Dam, 89.6% full. Photo DWS]

Here are the predicted levels of the total amount of water that could be stored collectively in the six main SW Cape dams at the end of each month through to next winter (green shaded area), following the actual levels for the year-to-date (brown shading), and compared with the actuals from the periods of the prior year (blue shading).

Our assumptions of normal average weather and annual rainfall patterns, implementation of level-3 restrictions and the virtually full allocation of irrigation water to the agricultural sector, hold true.

The model currently indicates that combined dam levels could drop to around 400 M.cuM of water (from the present level of 620 M.cuM) by early May, 2019 and that thereafter replenishment of the dams should commence as winter rains arrive.

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The following chart produced by City of Cape Town has been moved forward 12 months and now reflects their anticipated outcome (blue line) for the coming hydrological year assuming normal average weather and rainfall. From this it can be seen that:

• stored water levels are way above danger levels and are likely to stay that way.
• the actual consumption (black dotted line) has not yet accelerated to the extent expected and so levels are actually falling slightly more slowly than the City model anticipates.

My feeling is that the closing position shown for October 2018 is possibly too optimistic as it assumes a net recharge of the dams that will be even greater than what happened this year. In reality a net recharge of, say 350 M.cuM (more in line with recent past annual averages), would lift the total storage back up to a similarly safe 75-80% level. But, who knows; we shall see what transpires.

 [click on the graph to see an enlarged version].

While it is still too early to tell for sure, it seems that domestic users are continuing their water saving practices. This is great to see but the municipalities will have set their lower tariffs assuming a higher minimum of water sales.

Thus, if water consumption indeed does not increase into the higher range that has been budgeted (e.g. 650 ML/day in the Metro), then weirdly the munis could suffer revenue shortfalls. So I am sure that the authorities will be watching this daily consumption figure closely and may need to later ease the limitations on how water can be used to encourage more consumption.

Here is our revised chart of augmentation contribution. It has been re-scaled to reflect the Metro’s increased target of 650 ML/d. I have also removed the trend lines as they became meaningless with the targets changing so often.

Regarding augmentation, there is some confusion as to why the amount of extra water being produced appears to have fallen. Why has it declined from levels of 50 ML/d to just 20 ML/d? (Note that our definition of extra water produced is the difference between the water drawn by the Metro from all sources, i.e. including augmentation, and that drawn only from the 6 main dams).

The reasons, of course, are not hard to discern.

1) Back in May when the risk was highest, the Metro was operating in emergency mode and trying to establish as many facilities as possible to produce extra water AND to actually bring them on line, with that water flowing into the system. Of course, operating in emergency mode tends to be more expensive and carries a higher risk of mistakes being made. So once the dams started to recharge in May/June, the emergency program could be throttled back to progress at a more normal, and more economical rate. This was done.

2) Once the dams were filling there was no need to continue actually producing extra water, because that attracts extra cost. So, in exactly the same way as augmentation facilities will operate in the future, those that were not needed were shut down. The facilities will still be there but why operate them when that extra water thereby produced is not needed?

And this also proves the core concept of water augmentation. It is best to have a mix of facilities that only need to be started up and operated to that extent needed to make up a shortfall in water. If rain is sufficient and the dams fill then no extra water is needed and the facilities can be switched off. As soon as a water shortage emerges then the extra facilities can be started up but only to the extent needed to make up the water shortfall. This is particularly practical with boreholes and recycling plants. On the other hand it underscores the big weakness of utility-scale desalination. Such plants are best operated continuously 24/7. Expensively! That is one good reason why a large desalination plant is at the back of the list for implementation. It needs to wait until all other less expensive methods of water production and saving are implemented before the very costly step of desalination is undertaken.

In summary then regarding augmentation, work continues as planned, establishing extra water production facilities. However, as there is sufficient water stored in the dams at present, some newly established facilities (e.g. groundwater) have not and will not be brought into emergency production until needed –  additional water is not required at the moment therefore there is no point in incurring the additional cost of fast-tracking what would just be surplus water.

Weather-wise, El Nino continues to develop. Another month with temperatures more than 0.5degC above Pacific average surface temperatures is needed to be classified El Nino. But it’s on its way and seems sure to cross the bar.

Why is this significant? El Nino doesn’t influence the Cape summer rain, as there normally is no rain. But if still present, an El Nino may negatively affect our autumn and early winter rains. This chart NOAA CPC chart shows the ensemble forecast for this El Nino and, although predicted to be mild (i.e. of low effect), it is expected to persist into our winter and that may turn out to be bad news.

My last comment is about Eskom, the “elephant in the room” – there is the danger that power outages could start to affect water distribution as well. For example, a few days back water users in the Grabou area were asked to use water very sparingly as the reservoir levels had fallen very low due to power interruptions. Similarly, Beaufort West which is currently so dependent upon borehole water, was experiencing water supply problems due to power outages.

Regarding power continuity in Cape Town, I understand that back in 2015, the City installed generators at critical pump stations, so that while some problems may be experienced depending upon how severe the power outages become, the Metro W&S department believes that it is fairly well prepared to cope with power interruptions. Good forethought and sensible planning!

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Our weekly Cape Dams Levels report shows latest water consumption rates.

• See the latest weekly report on Dams Levels and changes in water used here.

Items that you should read quarterly to remain informed.

• To see what the Cape Metro augmentation water production program is achieving click here. This chart will be updated mid-December.
• To be informed of climate issues affecting our lives check the noticeboard of the UCT Climate S A Group (CSAG) here for their latest papers on climate change and related matters.

Prevailing water restrictions 

• Since 1st Dec. 2018, level-3 (105 litres/person/day in Cape Town; 120 L/p/d in Drakenstein).
• A summary of Cape Town’s level-3 restrictions is given here.