How severe is Cape Town's drought? A detailed look at the data

2018-01-23 08:27
(File, Supplied)

(File, Supplied)

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Cape Town – The causes of Cape Town's water crisis are hotly contested. There is a drought, of course.

But there are also other reasons brought up in the public discourse, particularly on social media, such as population and water demand growth, unreported agricultural use, invasive species sucking out water in catchments, poor planning and mismanagement of the water supply system, and lack of foresight in development of new water sources, according to a GroundUp report.

Are these significant contributors to the shortage? It's very difficult to say without dedicated, comprehensive analyses. But facts are few, opinions plenty.

READ: Zille to meet national disaster management on water crisis

Few people question whether there is a drought: What is questioned is its severity. This has important implications.

If the drought has been mild, then it should not have resulted in a major water crisis.

If it has been severe, then it has just been a bad ticket on the climate lottery, and all the other factors would be at most aggravating factors but not the main cause.

The issue is obviously socially tense, creating wedges between authorities and citizens, between those who institute water restrictions and those who have to bear their brunt.

In this article I look at the most up-to-date rainfall data to assess how severe the current drought is.

I have tried to answer that question before. But that article was based on very limited data, and was carried out well before 2017's rainy season had finished. Now I have a more comprehensive dataset, which provides an opportunity to perform a more robust assessment.

It appears that nine stations have data available for recent years, but four are located in the region of the WCWSS dams, and have no significant gaps or systematic errors from 1981 through 2017.

Those stations are: Vogel Vallij, Zacharashoek, Theewaterskloof and Kogel Baai.

Note that the year in the above figures is taken to be between November and October.

This is because October is more or less the end of rainy season, and the last month when an increase in dam levels may be recorded.

Also, because at the time of writing this, data are available only till October 2017.

Putting the end of the year in October allows me to use the 2017 data without estimating rainfall in November and December.

And to account for the fact that the drought is likely of a multi-year character, I created plots showing two-, three- and four-year average rainfall:

These plots tell a consistent story: 2017, as well as the preceding two-, three- and four-year average rainfall in the region are the lowest since 1981.

What it means is that the 2017 drought was at least as rare as once in 36 years.

The rainfall data from the South African Weather Service (SAWS) for some stations in the Western Cape go as far back as the late 1800s.

Unfortunately, there isn't any overlap between the DWS and SAWS rain stations, so we cannot simply extend the record of the four stations used above.

We need to repeat the analyses on the entire SAWS dataset. If available SAWS data are screened in a similar manner as the DWS data were (i.e. for continuity and consistency of record), we get five stations with data covering the period 1933 to 2017. Only three are located in the WCWSS dam region: Vrugbaar, Rustfontein and Nuweberg.

Again, the 2017 rainfall for that station, and importantly the mean of three years prior to 2017 were lower than in any period experienced by this station since 1920.

Here, although relatively bad, 2017 was not – either alone or in combination with preceding years — the driest on record. But this is not the area where most of our water is supplied from.

The statistical analysis I have done to calculate this are explained in my blog article which will be published soon after this article. My findings are that this kind of drought occurs once in 311 years with a 90% confidence that it falls between 105 and 1 280 years.

Importantly, the analyses reveal that the drought was likely less severe in the coastal plains and in Cape Town itself. A possible reason for that might be a weaker penetration of cold fronts that bring winter rainfall to the region into the higher and distant inland regions.

This figure shows a trend in rainfall in the WCWSS region over the last 84 years. That trend is towards lower rainfall and it has a relatively strong magnitude – 17mm per ten years. It is barely statistically significant, though. The important thing is that this trend may be an expression of human-caused climate change, and may be affecting the magnitude of droughts. Simply, if that trend was not there, the 2017 drought would likely be less severe.

In summary – the analyses presented above, based on the best rainfall data available at this time, show that the drought, manifested through low rainfall in 2015 to 2017, was very rare and severe.

Acknowledgements to SAWS and DWS for data, and Chris Jack and Stefaan Conradie (both CSAG/UCT) for valuable discussions.

A more detailed version of this analysis will be available shortly after publication on GroundUp.

Dr Wolski is a researcher with UCT's Climate System Analysis Group.

Read more on:    cape town  |  environment  |  drought  |  water crisis  |  water

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