Dams – Latest levels: Winelands and Cape Town
Latest report (updated 11h September 2017) on the main dams serving
Cape Town City and nearby Boland/Overberg towns
This was a good week. Sturdy net inflows resulted in combined water stored rising by 18.5 M.cuM (2.1% of capacity) vs 7.5 M.cuM (0.8% capacity) during the prior week. Total stored water rose to 36.8% of full capacity (vs 34.7%).
During this week in 2016 combined level of the dams rose by 2.5 M.cuM to reach 60.2% of capacity.
[Why since 1st November 2016? that was when level 3 restrictions were imposed].
Current status?
The dams now contain 327 M.cuM (242 M.cuM of easily accessible) water (last year this stood at 536 and 451 M.cuM respectively).
This week I have included an interesting graph prepared by the Fynbos Node of the South African Environmental Observation Network (SAEON). Unlike the graphs of rainfall at CT Airport compiled by the Climate Group at UCT, this SAEON graph shows the rainfall records of the weather station located in the Dwarsberg Mountains above Jonkershoek.
Their weather station is at 1214 metres above sea level on the boundary of the catchments of the Eerste, Berg and Sonderend rivers and is a good indicator of rainfall feeding the Berg and Theewaterskloof dams.
And this gets directly to the nub of the matter. The Berg River and TWK dams together store 606 M.cuM of water; 68% of the combined storage capacity of the 6 main dams. Therefore, the rainfall in the catchment around this weather station is closely indicative of what might be flowing into these two important dams.
Unsurprisingly, the graph indicates that 2015 and 2016 are the worst rainfall years on record since 1945; and that 2014 is not far behind. The fear, of course, is that the poor rainfall of 2017 may yet eclipse them all.
In addition to rainfall, the SAEON website “presents the record of stream flow rates for the Langrivier catchment and rainfall from the Dwarsberg weather station in the Jonkershoek Valley for the period January 1961 to the end of August 2017“. Their analysis is particularly interesting in that it indicates the amount of rainfall that is necessary to achieve useful water run-off rates, from which one can derive a sense of how much rainfall might typically be needed to fill the dams.
What seems clear from their analysis (and is also intuitive) is that the same given amount of rain produces less run-off in summer than it does in winter. This is no doubt due to the greater saturation of the ground in the rainy season coupled with the fact that evaporation is greater in summer. Stated differently, one benefits more from rain received in the rainy season than from the same amount of rain received unseasonally in summer.
This also questions the “common wisdom” that snowfall is somehow better than rain. The fact seems to be that in our environment of only occasional light snows, rain is better because more of it will run off into the dams in winter. On the other hand gradual snowmelt is more likely to sink down into the (less saturated?) ground and therefore not run off as efficiently into the dams. [This is no doubt different to areas with deep, long-lasting snows where the gradual release of meltwater extends streamflows over longer periods reducing potential losses from dams that might otherwise overflow].
I recommend that you visit the SAEON website here and read more about their interesting findings. Also take a look at their weather stations.
How long will the water last?
The next table compares progressive estimates for 2017 with what actually happened in 2016.
From this it can be seen that the combined level of the dams (309 M.cuM) on 5th September did not reach the level previously expected of 317 M.cuM, meaning that we did not receive the same rainfall in August 2017 as we did in 2016. Then, by applying what happened last year, the model recalculated that we would stand at 325 M.cuM of water stored by 3rd October.
The good inflows last week have already lifted us to 327 M.cuM indicating that the target of 325 M.cuM may well be exceeded at September month-end. However, unless considerably more rain arrives and/or the planned interventions to produce fresh water meet all expectations, the model predicts that easily accessible water (i.e. that greater than the last combined 85 M.cuM – the pink zone) in the 6 main dams water would be exhausted by the third week of February 2018.
The situation remains very grave.
What ought households to be doing?
I recommend that households calculate present water usage using the handy calculator on the Cape Town City website here and use the results to plan ways in which the household might further reduce its consumption of water. Also read our suggestions on ways to save water.
Then install some means to harvest and store water under your direct control. Use the UCT Climate Group’s model here to estimate correct storage tank sizes for your household.
What are the authorities doing now?
There has been no worthwhile news from the authorities this past week.
We await CoCT’s promised operational plan for implementation in the event Zero Water Day arrives and for specific news about execution of planned interventions to produce more fresh water.
Will the weather help?
There was some useful rain in the catchments this past week, especially in Villiersdorp in the Theewaters Dam area where 35mm fell. The Steenbras dams also benefited, with the Upper Steenbras dam now overflowing and spilling into the Lower Steenbras Dam.
Some occasional rain is still expected between 12th and 14th with a little more on 16th. Between 24th and 26th of September 20-30mm of rain may fall in the mountain catchments.
No major unstable weather systems are foreseen for October and November at this time.
The sea temperatures in the Pacific once more turned down toward lower La Niña ranges. Its just possible that this trend could contribute toward weather instability that could bring late rains to the SW Cape.
The chart below shows the rain that has actually fallen over the dams since the winter began. Clearly, the Berg River Dam and the two Steenbras Dams once more enjoyed good inflows. It is interesting that although very little rain fell on the Theewaters Dam itself, inflows resulting from the 35 mm that fell in the nearby Villiersdorp area contributed to the very welcome 2% rise in the level of TWK Dam last week. Rainfall at the Berg River Dam was not reported by DWS for last week but it must have been substantial given the 4% increase in water stored in that dam.
In Summary: fresh rains caused net inflows of a good 18.5 M.cuM. Dam levels lifted from 34.7% to 36.8% of total capacity. Dam levels remain critically low.
This table shows the levels of the six main dams that serve the Cape Town/Overberg/Boland area as reported by Dept Water and Sanitation. It also shows total water storage levels of all dams in the greater Western Cape area.
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
Read about South Africa’s desalination policies and initiatives.
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).
Tom.
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