How much water is left?

Water levels at the Theewaterskloof dam near Villiersdorp are at an alarmingly low level. Photo: Ian Landsberg

Every Monday, the City of Cape Town publishes its water dashboard on its website. In garish burgundy letters and numbers, it tells the chilling tale of the current state of our water resources.

Last Monday’s dashboard screams Dam storage (%) – 39.2, Weekly dam level change(%) – -1.3, Average daily production in mega litres per day (Ml/d) – 807, the last figure is the amount of purified water which is delivered to users each day, against a target of 800 Ml/d. Taken together, these figures paint a bleak picture of our water future. Just how many days water do we have left, before the pumps start sucking up mud?

The City’s mayoral committee member for utilities, Xanthea Limberg, says that once a dam drops to 10 percent of its capacity, it is no longer possible to abstract water for purification, which means that as of Monday, the City has only ? percent of the combined capacity of its six major supply dams – Berg River, Steenbras Lower, Steenbras Upper,Theewaterskloof, Voëlvlei, Wemmershoek – to abstract, before we run out of water.

Aside from Cape Town, other municipalities and the agricultural sector abstract water from these six dams. Trouble is, we don’t know how much other abstractors take, and it looks like the City doesn’t know either.

This emerged in an email exchange, in which the City was asked to supply a list of other abstractors, as well as what their proportionate abstractions are.

The concluding response from the City reads as follows: “National government is the custodian of the dams from which the City of Cape Town gets water, and would monitor usage from the different municipalities and agriculture for planning purposes. The City estimates the abstraction rates, but the department is uncomfortable providing an estimate due to concerns about accuracy.”

Which can only mean, that the City does not have a clear picture of what our water position actually is, nor how it is likely to change over time, if we get no more rain before the dam levels hit 10 percent on average.

Ergo, the weekly reduction in dam levels – ?% last week – is the only metric the City can use to estimate how much water remains, because it takes into consideration all abstractions from the dams, as well as losses through evaporation. Using that metric in the following calculation ((ave dam level % – 10%) / last week’s % decrease,) x 7 days), translates into ((39.2%-10%)/1.3%) x 7) = 157 days of water left as at last Monday.

All well and good you might say, because in theory that should tide us over until July 5, by when it is hoped the rains will have come, but the weather experts are saying that our rains are probably going to come late, and they are probably not going to be terribly good.

I stress probably, because if you press a weather forecaster, they will retreat – and rightfully so – into probabilities.

The further you move into the future, the more uncertain becomes the accuracy of any weather forecast.

But even if nothing else changes, and weekly abstraction remains at two percent, whether or not the remaining water will last is far from clear.

What we don’t know now, and what the City is also not divulging, is how the remaining volumes in the dams are calculated, whether in percentage terms or in mega litres.

South Africa has a major problem with siltation of dams. The older the dam, the more silt builds up, and the less water it can contain.

It is possible to remove silt from a dam, which will restore some of its original capacity, and that is something which increasingly people are saying must happen, but that will do nothing to solve the current problem.

Making the dams deeper won’t make it rain any sooner.

The process of bathymetry – determining the depth of water in a dam by means of a series of transacts using an accurate GPS and a bottom reflecting radar – together with a series of calculations, will give an accurate measure of how much water a dam currently holds, and by definition, how much it can hold when full, compared with its original design capacity.

This bathymetric survey process ought to be conducted every three to five years on all water supply dams.

When last were the City’s supply dams surveyed, and what did that survey show?

Are the average dam levels, and the consequent estimates of remaining water volume in each, based on the original design capacity of each dam, or on an adjusted capacity based on bathymetric survey?

If it is the latter, then the figures quoted to us by the City of Cape Town are probably accurate.

If it is the former, then we’re in far more trouble than we thought, because we actually have no clue how much water is left. At the time of going to print, a comprehensive media enquiry to the office of Anton Bredell, MEC for Local Government, Environmental Affairs and Development Planning, asking when last the City’s six supply dams were surveyed, and what were the results of that survey, remains unanswered.