Dams – Latest levels: Winelands and Cape Town

Latest report (updated 22nd January 2018) on the main dams serving

Cape Town City and nearby Boland/Overberg towns

 

Water consumption spiked last week to 12.7 M.cuM, 1.4% of total dam capacity; vs the preceding week’s 9.6 M.cuM. Combined dam levels dropped from 28.4% of capacity to 26.9%.

During the same period in 2017, the main dams dropped by a far larger 18.9 M.cuM, with the dams sinking back then by 2.1% to 39.9% of full capacity.

Read More At Toms Blogs

Day Zero Date – a basis of calculation.

Calculation of likely Day Zero date

Using current and likely future water consumption patterns, projected as at end-February 2018.

2018-02-26-5

 

I comprehend the gradually changing water consumption patterns in my model and therefore my predictions of steadily falling weekly water consumption results in the dotted black line curving away to the right and indicating a “Day Zero” date falling around 30th June at point ‘Y’, depending upon how well households further reduce their water consumption over the coming 4-5 months.

 

Going through it step-by-step.

Starting from the 37.5% total storage at beginning November 2017, the business-as-usual consumption expectation indicated a sharply descending fall in dam levels, originally predicting Day Zero as being during week of 13th February 2018 when total storage would drop to the 13.5% level set by CoCT; that point when they would move to stage-2 of the disaster plan.

Then in November 2017 we enjoyed unseasonal rain in the catchments that lifted dam levels and effectively moved the predicted Day Zero out by two weeks to the last week of February.

My information at that time was that the so-called 10% of ‘inaccessible water’ in the dams was in no ways definite and that if necessary, given some extra filtration, more of that last ‘dead water’ could be accessed. In fact, the dams could be drawn down to virtually zero but not easily in all cases. So I assumed that they would be pulled down to, say, 7.5% (line “D”) thereby making more available for consumption. That in turn allowed a corresponding drop in the Day Zero trigger level from 13.5% to 11% of capacity (line “B”) while still retaining the same stage-2 reserve of 3.5% of capacity “E”). This pushed the point where line “A,B” intersects the 11% line out into March. [In the meantime DWS has announced that they are preparing to pull both Voelvlei and Theewaterskloof dams down to zero if necessary. That would make yet more water available.]

The next major change in November was when DWS imposed new limits and cut agricultural water allocations to just 40% of normal. The effect of this massive reduction in overall water consumption is reflected by the blue-striped zone, labelled “F”the current position being where the actual consumption (denoted by the red ‘X’) now lies. Without further savings then the theoretical Day Zero date would be around end-May 2018, on the disaster stage-2 line (point “G”); by then dam levels could have dropped to 11.0% of capacity.

Although they have been drawing their water allocation fast, in fact agriculture has rapidly reached the 40% limit and the irrigation flows were largely cut off by end-January with most of the rest reaching the limit and closing by end-February. The net effect will be that the agricultural sacrifice has extended the communally available water supply out to late May (point “G”). At that stage agriculture’s contribution should be exhausted and “Day Zero” will have arrived for all the farms.

In February 2018 it was announced that the Groenland Water Users Association (Elgin/Grabou) would release their surplus stored water to be pumped through to Steenbras dam. This donation is expected to add 8-10 M.cuM of water to the SW Cape water supplies and help us to reach a point half way between “G” and “I”.

Now, although rain should start by June this is still too tight and the business and household users in the metro and municipalities must save stringently and meet their reduced allocations. In January the CoCT was running at about 586ML/d but has subsequently dropped down to the current level of 510-520 ML/d.

From 1st February level-6B restrictions require consumers to further reduce to 50L/person/day. Residents now must cut usage in order to save that blue zone labelled “H”. This action would make all the difference and enable us to reach “Success Day”, defined as arriving at 30th June 2018 with 100 M.cuM of fresh water still stored in the dams (point “Y”). Average consumption is still up in the area of 75L/p/d  and every effort must be made to encourage that half of the population that is still using too much water to stop doing so (hopefully the new harsh tariffs will help).

As I write, the point red “X” lies at 23.7% of capacity on 26th February – the small yellow circles with ‘x’ mark the previous positions of “X” over the immediate past. Graphically the task is to make sure that, as position red “X” moves along,  it stays on or to the right of the dotted black line, which will show that we are achieving or doing better than our weekly consumption target. At the moment we are doing very well due to the water being donated from the Palmiet system.

If we do reach mid-year with sufficient water in the dams, then nature should start to help with winter rains and low evaporation commencing late-May, but surely in June (yellow zone “I”).

Last but not least, various augmentation initiatives should be starting to make steadily growing contributions (brown striped zone “J”) and, in the final analysis Cape Town should just skim past the lowest point and out of immediate danger. In fact, hopefully the total available water will recover along the heavy dotted red line as the dams recharge, labelled “K”.

In summary, this outcome depends clearly on three main things:

1) the dams will be drawn down at least into the range of 6% – 7.5% of capacity if needed!

2) agriculture is exhausted, so businesses & households must now reduce consumption to 50L/p/d.

3) at least normal rains needs to arrive to start recharging the dams from July onward.

If rains once more fail and Day Zero arrives, then we will at least know we have done our best!

 

Tom Brown,

retired international businessman and fruit farmer in the Klein Karoo.

26th February, 2018.

 

 



Just how severe is the current drought the City of Cape Town is experiencing?

Are we in the middle of an extreme, unusual climate event that caught authorities napping, or is this purely a political machination of some diabolical state-capturers?  There have been a lot of social media posts stating that contrary to what some scientists have said, we are not facing an extreme climate event.  I came across the results of some analysis done by Anton Sparks, a professional engineer at Aurecon, which support the statements that the last 3 years are in fact very likely to be significantly worse than a 1 in 100 year event.

Comparing the combined inflows over the past three years (Preliminarily estimated from the equation “Monthly Inflow” = “Monthly Increase in Storage” + “Monthly Demands on Dams” which can later be refined using evaporation and rainfall on the dam’s surfaces when the data is available) into the 6 dams that feed Cape Town with potable water (note we’re not looking at rainfall, but the inflows into the storage systems that are the main reservoirs we use) and with similar data for the period from 1928 to 2004 (Derived by DWS as part of the study to assess the availability of water in the Berg River (see http://www.dwa.gov.za/Documents/Other/WMA/19/Reports/BergAssess23Jul10.htm), one can note that:

  1. The annual inflows from Nov 2016 – Oct 2017 was about 100 million cubic metres (1 cubic metre of water = 1 kilolitre) LOWER than the LOWEST of any 1 year period during 1928-2004.
  2. The total flows over the TWO consecutive years from Nov 2015 – Oct 2017 were 80 million m3 LOWER than the LOWEST two consecutive year period during 1928-2004.
  3. The total flows over the THREE consecutive years from Nov 2014 – Oct 2017 were 180 million m3 LOWER than the LOWEST three consecutive year period during 1928-2004.

This can be illustrated graphically using a “Percent Exceedance” graph (below).  To produce a percent exceedance graph for say the annual inflows all that is necessary is to rank and plot the annual inflows in descending order, so that the largest annual flow occurs on the left hand side and the smallest annual flow occurs on the right hand side.  None of the flows are greater than the leftmost flow so it has a percent exceedance probability of 0%, while all the flows are greater than the rightmost flow which has a percent exceedance probability of 100%.  Similarly, the middle flow will have an exceedance probability of 50%.  This graph enables one to visually view the distribution of flows and say to pick out the annual flow which is exceeded say 10% of the time, if that was of interest.

Looking at the first graph then, the far left of the red line shows that the very wettest 1 year period had inflows of about 1300 million m3/a (at 0 percent exceedance) and the driest inflow was 380 million m3/a (at 100 percent exceedance).  The red dot representing the inflow from November 2016 to October 2017 only has inflows of 280 million m3/a, 100 million m3/a less than any record in from 1928 to 2004.  Expanding the sample to 2 year and 3 year consecutive periods shows provides data for two and three year long droughts.

What is very interesting is to then look at the values for the periods 2016, 2015 to 2016, & 2014 to 2016.  In all of these cases, there is NOT ONE equivalent period from 1928 to 2004 where the inflows into the 6 CoCT supply dams are lower.

Aurecon then simulated the probabilistic drawdowns on the dams from November 2014, using a stochastic modeling technique developed by Professor Geoff Pegram of UKZN (http://sahg.ukzn.ac.za/people/) and the hydrology data available up until September 2005 and the actual system water demands from November 2014.  The 2nd graph shows the results of the simulations, with the ACTUAL dam levels superimposed as black dots.  The light green line shows how things would have looked if we’d had average inflows to the dams and average drawdowns – everything just fine, thanks very much.  The dark green line shows a 95% exceedance probability (or 1 in 20) risk of lower storage volumes.  Lines for 98%, 99% and 99.75% exceedance probability are shown.  In October 2015, the dam levels were above the 1:20 risk level, in October 2016 at the 1:20 risk level and in October 2017 below the 1:400 risk level.

Again, Aurecon modelled the probability of different storages going forward from November 2017 which are shown in the third graph.  The graphs show what will happen if we cut our usage, in this case Urban use by 45% and Agricultural use by 60%, both with respect to the average consumption from 2011 to 2015.  The “spreadsheet simulation” is based on the worst case annual inflows from November 2016 to October 2017.  If we cut our usage sufficiently, we could make it through and remain above the “Day Zero” storage level when one would have to queue for water.  If we don’t, and the augmentation projects are delayed, or like Beaufort West, do not produce what is predicted, we’re in big trouble.

The red dots track the current storage.  Note that despite the light rains experienced in early summer which have helped to elevate the storages slightly, the storage below the projected storages because the targets are not being achieved.

We have no choice, we HAVE to ensure the City as a whole uses less water.  Given that the severity of the current drought has deepened because of the last winter we should try to reduce our consumption to 50 litres per person per day or less if at all possible.  This means even less/shorter showers or sharing baths, and utilizing grey water to flush!!  50 litres per person per day is still twice the “Day Zero” allowance.

For drought information and tips and advice on water saving methods consider joining the Water Shedding Western Cape Facebook group :
www.facebook.com/groups/watersheddingsa

My thanks to Anton Sparks of Aurecon for patiently taking me through the graphs and answering questions about them.

Dave Gale
Water Shedding Western Cape
2018-01-15