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City’s model shows how we can avoid Day Zero

Getting household consumption down is key

Photo of Theewaterskloof dam
Theewaterskloof Dam on 11 May 2017. Photo: Ashraf Hendricks

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Day Zero can just be avoided — at least in 2018 — if households and farms keep water use down, according to a model by technical staff at the City of Cape Town.

The model shows what will happen as various things change: water use in Cape Town and in smaller municipalities, agricultural use, evaporation from the dams, rainfall, and new water sources.

The good news is that the water consumption levels we need to reach are attainable, and last week households made reasonably good progress getting there. But there is no room for complacency. If we don’t make more progress towards the City’s target of 50 litres per day each, or the winter rains don’t come on time, Day Zero is likely.

You can download the model here.

This detailed model takes six variables into account: consumption in Cape Town, consumption in smaller municipalities, agricultural consumption, evaporation from the dams, rainfall, and new water sources (augmentation). Rainfall and evaporation are out of everyone’s hands. But it appears that if Cape Town’s residents can collectively improve on our new water-saving habits we are likely to avoid Day Zero this year.

The model predicted that if household consumption dropped to 513 million litres daily, consumption in small municipalities served by the same dams as Cape Town dropped to 57 million litres daily, and agricultural use dropped 60% from what farms would normally consume at this time of year, the dam levels would drop to 26.2% by the end of last week.

As it happens, Monday’s reading published by the City showed dam levels at 26.3%, a negligible difference with the model.

This was in spite of the fact that, according to City estimates, last week’s household consumption was 580 million litres daily (of which 556 million litres daily was from the large dams) - higher than the model assumption. But not too much should be read into this, and to avoid Day Zero (when the dams reach 13.5% and water is cut to most parts of the city), the model requires the city’s consumption to drop further.

There are a few important things to note:

  • No model predicts reality with precision. There are too many unknowns. Models must be used merely as guides.
  • The model was developed by technical staff working for, or contracted by, the City. These are experts who very likely are motivated by the best interests of the city and the province.

  • Both farmers and households have brought their consumption down remarkably from past years. For example, the model estimates that “baseline unrestricted demand” for this time of the year is 932 million litres daily by the city, 104 million litres daily by other municipalities, and over a billion litres daily by agriculture. All three sectors have dropped far below that.

  • It makes sense for the City to pressurise households to drop their consumption. Farms are critical to our economy and employ tens of thousands of people; they have already made considerable sacrifices. As most of us have discovered, it is not that hard for most people living in formal houses to dramatically lower water consumption. For most of us it’s a minor lifestyle change. This really is the best way to avoid Day Zero.

  • The model ends its forecasts in October 2018. While rainfall cannot be predicted with much confidence, if the drought continues it will be even harder to avoid Day Zero in 2019 than 2018. We have to be in water-saving mode for the long haul. It is vital to the city’s future that households that can catch rain off their roofs make plans to do so (perhaps with state subsidies for, at least, lower-income households) and that all new buildings have rainwater catching mechanisms. Civil society activists may wish to consider campaigning for lower costs for buying and installing rain tanks, and the consequent plumbing required to connect them to cisterns, washing machines, basins and showers.

We asked the City’s media department to comment on the model. We were provided with technical notes and an explanatory document which have helped us improve this article.

Technical notes

  • The model is a spreadsheet consisting of three sheets, of which only the first two are discussed here. It is designed so that users can explore different scenarios under different levels of consumption. Here is an explanatory document.

  • The first sheet, titled User Input, contains a green section designed for user input. If you enter household savings to 0% for “CCT” (City of Cape Town) and “Other Municipalities” Day Zero arrives in the first half of April and continues until September. The good news is we (the residents of Cape Town) are doing way better than in the past. In fact if we improve our current consumption a bit, we are likely to avoid Day Zero in 2018 — but only just (other models we’ve seen are less optimistic). Also remember, it’s a model, and we can’t predict rain or even evaporation with much precision.

  • Judging by the name of the spreadsheet “2018-01-24 SImplified PUBLIC dam level model”, which we haven’t changed, it is a simplification of a more complex one used by the city. The name of the model in the explanatory document is Dam Draw-Down Model.

  • The model is the only time-varying one we are aware of (e.g. it gives different consumption figures each week based on historical data). Part of the problem for other modellers is that no state body publishes how much water goes to agriculture each week. But this model contains week-by-week estimates of agricultural use, though we do not know how accurate these estimates are.

  • Although the model is capable of including augmentation, the values in the augmentation column have all been set to zero. As new water comes online, you can enter it to see its effect on the dam levels.

  • We have been informed: “Baseline values are the five-year average of the unrestricted water usage of the various user groups as per the National Department of Water and Sanitation Regulation.”

  • You can fine-tune the model on the second sheet, by looking at baseline values and adjusting the future consumption values accordingly. Note that Column E in the second sheet appears to have been set to the actual dam levels as of 21 January.

Also read this very interesting article.

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