Cape Water and Dams Report

(Steenbras Upper Dam. Now 98% full. Photo : Brandon Spear)

 Dams – Latest levels: Winelands and Cape Town

Report nr. 116 (updated 6th August 2018) on the main dams serving

Cape Town City and nearby Boland/Overberg towns

A shortage of rain in the catchments caused a slow-down in run-off and the inflows to the main dams narrowly exceeded outflows causing total combined levels to rise by just 1.7M.cuM (0.2% of capacity). Overall water in storage climbed from 56.5% of capacity to 56.7%.

In the same week of 2017 the dam levels lifted by 13.2 M.cuM (1.6%) to 29.0% of capacity.

What could the 2018/2019 use of water look like? will it last to next winter?

Our goal should remain to at least cross 60% of capacity storage and to hold above that level through to end-October 2018. At this stage it seems that the forecast rains in August should see us past his target.

This model has now been restated. The previous 2016/17 water usage has been replaced by that from the immediately past 2017/18 period (now shaded blue) and the on-coming 2018/19 prediction has replaced it in the yellow shaded section.

What I have assumed for now is that level-6B restrictions will remain in place until end-October 2018 and will then be relaxed back to level-5 (87 L/p/d) and combined this increased consumption with a 20 M.cuM per month normal outflow for agricultural irrigation purposes across the summer season. Lastly I have assumed a slightly below-average annual rainfall pattern.

(Here is our model predicting the amount of water likely to be stored in the dams each month) 

Putting it into context!

The trend of continued reduction in use of water is encouraging as are the benefits of the pressure reduction programs and water loss prevention and leak repairs. Together the effect is that the Metro is now close to regularly meeting its consumption targets of 450 ML/day agreed with DWS.

What progress is Cape Town making towards augmented ‘resilience’? 

Latest indication is that the contribution of augmentation stands at 45 ML/day. This is likely to edge up slowly but steadily as various groundwater initiatives progressively come into production. The next ‘step-function’ increase in the augmentation contribution should happen in November when the pilot re-use plant at Zandvliet adds a planned 10 ML/d to the system.

This graph shows gross potable water ‘produced’ by the Metro this week from ‘all sources’ (505 ML/d) vs the part (460 ML/d) drawn from the dams; i.e. 45 ML/d of ‘extra’ water.

The two illustrative trend lines added to the graph show that since April [why since April? That was when the “Day Zero” frenzy started to calm down] gross water consumption has declined only slightly, whereas the amount of water drawn from the dams has declined more steeply as additional water produced by augmentation initiatives has replaced use of dam water. This is precisely the kind of outcome one would wish for.

(Graph tracking overall use and production of extra water through the augmentation efforts)

Read full report on status of Cape Town’s augmentation efforts here.

How well are we actually doing overall?

The CoCT graph of the combined level of the 6 main dams shows how the recharge rate slowed sharply across July, the problem simply being a shortage of rain in the dam catchments during July. Hopefully the general rain now falling as well as that forecast for the rest of August should once again give the dam levels a lift. We cannot consider ourselves to be completely safe until the combined level of the dams exceeds 60% of capacity.

Happily, of the three large inland Western Cape irrigation dams, Brandvlei and Kwaggaskloof now both have risen up to 39%, whereas the Clanwilliam dam is full.

This additional graph (below) from DWS of the rainfall pattern across the Western Cape shows that 2018 is so far proving to be a rather pedestrian year with rainfall to date barely different to that of 2015/16, and certainly below-average for this time of year. In fact despite the excellent start to winter, once the under-performance in July is brought into account 2017/18 has the potential to turn into one of the worst of the most recent years. [It is not known where DWS measures this rainfall].

Here is year-to-date rainfall recorded by UCT’s CSAG at the different dams.

The pair of Steenbras dams, despite solid rainfall in their area,  are producing a below-average result.

Wemmershoek Dam is running comfortably above-average. If this dam continues to receive good inflows then there is a strong possibility that it might start to spill before end of winter.

Seeing that the rainfall records for Berg River Dam are not posted by UCT CSAG, I have taken the nearby Dwarsberg weather station as a reasonable proxy for rain in this dam’s catchment. Although water is being transferred from BRD to TWK, this dam could also spill by end of winter.

Voelvlei Dam’s catchment received particularly solid rains during the second half of June so it has since then been maintaining an above-average position.

Theewaterskloof Dam is still the ‘problem child’. Despite transfers from BRD it remains below-average. And it alone holds almost half our water, its under-performance means that the combined level of storage across the whole system is unsatisfactory.

Will the weather help?

Except for Berg River Dam, which received some 12 mm of rain, none of the other dams had any significant rain last week. That changed yesterday and today with general rains of 20-30 mm due across the whole SW Cape, with all the dams including Theewaterskloof likely to benefit. In the process temperatures in the mountains will fall below freezing and snow and sleet will accumulate in the higher elevations.

From the following chart it can be seen that the four smaller dams have in fact received good rainfall year-to-date resulting in them all standing at good levels (all above 80%) of capacity. The two large dams are certainly far better than the same time in 2017, with Voelvlei now approaching 60% of capacity.

Click here to see ShowMe Winelands Weather forecast of possible near-term rain.

What rain ought we to now expect?

1) At the moment a cold front has skirted the S. Atlantic high, is working its way across the whole southern Cape and is expected to bring 15-30 mm rain to many areas. 

2)  The Antarctic Oscillation is predicted to remain negative out beyond the middle of August and, if so, the High Pressure may move away sufficiently to permit several approaching cold fronts to make landfall. Sunday 16th is expected to see sharp falls of 30-40 mm of rain, starting in the early hours and then clearing away by nightfall.

3) On the 17th/18th a broad system is expected to bring general rains to the eastern and interior part of the Cape, including the Klein Karoo, as this chart from NOAA CPC indicates.

This system could also bring relief for the areas along the south coast up to P.E.

4) Two waves of unstable weather accompanied by thundershowers should bring a useful 25-30 mm to the catchments over 22-25th and again on 29-30th August. Not much rain is forecast at this time for September, although this is sure to change.

5) ENSO has fallen back to the fully neutral level as the  NOAA/CPC chart shows

Regarding matters of climate change explained in my last report, climate-hydrologist Piotr Wolski has chided me for being perhaps too cautious regarding the effect that humans are having on the climate. He points out that since 1955 there have been no significant natural causes that could account for the ever-accelerating increase in average temperatures and accumulation of greenhouse gasses. One is left to conclude that humankind and its activities have become, at least for now, the primary cause for the recent manifest warming of the planet.

What are the authorities doing?

There was considerable confusion last week when a sloppy, incendiary report from IOL implied wrongly that the City of Cape Town had inappropriately gathered surplus cash through the higher tariffs and then failed to spend this on the intended augmentation processes. As I reported separately at that time this IOL report was totally wrong and misleading.

The situation was actually simple. CoCT received authority to spend up to R1.4bn capex and up to R2.9bn Opex (over 3 years) IF NEEDED to ensure water supply continuity to the Metro. The Capex would have to be raised in the form of interest-bearing loans. (I.e. It had nothing to do with tariffs whatsoever at this stage). Some small amounts were spent (mainly on pressure management systems to reduce demand and make Point-of-Distribution preparations should Day Zero have arrived) but when the emergency calmed in April/May the bulk of the anticipated expenditure could be avoided. Thus CoCT never raised the loans and never had the cash and didn’t misspend or under-spend anything, as had been alleged in the IOL and Argus articles.

Long-term utility-scale augmentation initiatives are continuing (see my report here) and have their own separate capex budgets that are fully disclosed in the Metro’s published budget.

The Metro has announced that all three temporary desalination plants are now running and together delivering the expected 15+ ML/d of fresh water.

Summary: Combined levels of the six main reservoirs serving the Cape Town/Overberg/Boland area rose from 56.5% to 56.7% of capacity. Some useful rain is expected during August.

Watch me answer the often asked question: “why do you write this report?“

Read the full City Cape Town release about Water By-law amendments here.

Read here about Cape Town’s reduced water and sanitation tariff increases.

Plan ahead for possible water outages; here are points that could help you.

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

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 Brown.  Retired international businessman with a background in Finance and IT; and now a fruit farmer, with a passion for the weather and climate.

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