How severe is the drought?

31 August 2017 | Story Piotr Wolski. Illustrations Supplied.
Dr Piotr Wolski, from UCT’s Climate System Analysis Group, uses the Cape Town rainfall monitor to ascertain just how severe the current drought is. Photo <a href="" target="blank">Steve Dorman via Flickr</a>.
Dr Piotr Wolski, from UCT’s Climate System Analysis Group, uses the Cape Town rainfall monitor to ascertain just how severe the current drought is. Photo Steve Dorman via Flickr.

For the past couple of weeks, we’ve all been watching the rain fall and its annual total rise, hoping for some reprieve from running around bathrooms with buckets, and frantic purchases of ridiculously overpriced rainwater tanks. It hasn’t happened. Rainfall has been meagre.

The rainy season is about to finish and dam levels are at 34%, compared to last year’s 58%. Even if there are good late rains, our predicament is not going to change. Hopefully, the planned transformation of Cape Town’s water supply system will bear fruit, promises of the city will stand true and there will be no ʻday zeroʼ. But we will only know this next June/July.

Rainfall that Cape Town is receiving can be tracked with the Cape Town rainfall monitor, which was created by UCT’s Climate System Analysis Group (CSAG/UCT) earlier this year for exactly this purpose. Below is a snippet:

Similar figures can be plotted for a number of other stations in the Western Cape for which rainfall monitoring data is available:

As these figures illustrate, the total rainfall in the region is so far below that experienced in the same part of the year during the past couple of years, and in the case of Cape Town airport (CTA), during the past 40 years.

Of course, in any 41 years, there must be a year that has the lowest rainfall, so why not now? Indeed. Droughts happen. But even without much climatological knowledge, one could guess that there are strong droughts and mild droughts, and that strong droughts happen less frequently than mild ones.

African lore includes “cattle-killing droughts” that happen every now and then, “goat-killing droughts” that happen every decade or so (goats, being more sturdy than cows, die only if conditions are more extreme), and “man-killing droughts” that happen once in a generation.

So is the Cape’s drought a cattle-, goat- or man-killing one? Is it a super-rare event, or is it something that we can expect to occur every now and then?

Complex concept

Drought is a relatively complex concept, particularly hydrological drought. And it is the hydrological drought that affects the Cape’s dams. A hydrological drought is a result of an intricate interplay of characteristics of weather such as timing, magnitude and intensity of rainfall events, and other factors affecting runoff and dam storage.

Here, however, for simplicity, and due to lack of data, we focus on the most basic approach available to characterise the drought: total annual rainfall. Which is okay, because the current drought does not appear to be intricate. Rather, it is strong and obvious.

In simple terms, to assess the expected frequency of occurrence of the current drought, we will determine how frequently years with annual total rainfall of various magnitudes happened in the past. There should be a relationship between how frequently a given annual rainfall occurred, and its magnitude, and we will use that relationship to describe how frequently the rainfall observed in 2017 is expected to happen ʻin generalʼ.

The analyses are limited by the lack of appropriate data (long term, but extending until a couple of days ago). The Altydgedacht station located in Bellville, near Cape Town airport, has only three years missing in the 1923–2015 record available to us at the moment, its rainfall regime is very similar, and the long-term mean (502 mm/year) is almost identical to that of CTA (506 mm/year).

We have thus merged the long-term Altydgedacht and recent CTA records (the latter extrapolated to estimate the entire year’s rainfall) to generate a long-term annual rainfall time series that looks like this:

Return interval

Based on frequency analysis of Altydgedacht’s rainfall, the 2017 total is expected to occur once in 325 years on average. In other words, the return interval of the 2017 rainfall level is 325 years. The uncertainty of this estimate is rather large – there is a 95% probability that the ʻtrue’ value of the return interval of 2017 rainfall is more than 126 years.

Pretty rare. Pretty strong. Pretty scary. But that’s not the end of the story.

Firstly: trend

There is a long-term trend in rainfall at Altydgedacht, and that trend might partly be responsible for the low rainfall of 2017. We can assess the effect of that trend on the return interval of the current drought by simply repeating the above analysis on detrended data. The detrending process is simply removing the long-term trend like so:

For the detrended data, the estimate of the return interval of 2017 rainfall is 135 years, and there is a 95% probability that its ʻtrueʼ value is more than 63 years. It thus appears that the trend has more than doubled the frequency of low-rainfall years. What drives the trend? We’re not sure. It could be anthropogenic (human originated) climate change, but we cannot confirm that at this stage. Long-term trends in rainfall in the Western Cape vary depending on location, and are potentially affected by a multitude of factors, so one cannot be sure without a more in-depth analysis.

Secondly: multiple years

We can and should take a look at a multi-year drought. This is because it’s not only 2017 that matters. Had the low rainfall of 2017 followed a wet year, the water situation in the Cape wouldn’t be that dire. It has, however, followed a relatively dry year, and we started the 2017 season with already low dam storages. Figures below illustrate the time series of the two- to five-year mean annual rainfall at Altydgedacht:

The figures indicate that the total rainfall delivered over the past five years was not exceptionally low. But the totals as low as those over the past two, three and four years were unprecedented.

Calculating the frequency of occurrence of multi-year droughts requires some special effort because of autocorrelation. Simply, the year-on-year sequencing of rainfall is not totally random. Wet years are likely to follow wet years. Dry years are likely to follow dry years. That’s autocorrelation. We have to account for that in our calculations if the autocorrelation is strong, and in Altydgedacht data it is strong (and statistically significant).

Using frequency analysis methods that account for autocorrelation gives these return intervals of the cumulative rainfall in the past two, three, four and five years:

  • two-year rainfall (2016–2017): 1150 years
  • three-year rainfall (2015–2017): 628 years
  • four-year rainfall (2014–2017): 63 years
  • five-year rainfall (2013–2017): 10 years.

The low rainfall total of the two-year event (2016–2017) and three-year event (2015–2017) are very, very rare. Calculations on the detrended time series reveal, similarly to those performed on one-year data, that the trend has decreased the return interval, ie increased the frequency, or in other words the risk, of observed events. However, unlike for an individual year, that risk has increased by a factor of three to four.

So what?

Well, two things.

Firstly, the above results should make us think hard about anthropogenic climate change as a possible driver of the trend.

Secondly, the results somewhat exonerate the Cape’s government, as well as the water engineers designing Cape Town’s water supply system, from blame for the current water crisis. Water supply systems are usually designed with an assurance rate of 97%, which means that in the worst case they may fail only 3% of time.

The conditions we are experiencing now seem to be well beyond what one usually plans for. If the calculations are right, for the water supply system to go through this drought unscathed, its assurance level would have to be 99.9%.

One could implement a system like that, but this might be difficult if the climate goalposts are being shifted. And who is going to foot the bill?

This article is an abridged version of a blog post published on the Climate System Analysis Group website. It forms a part of a series of research activities that are intended to make climate and weather information available for use in order to facilitate a better understanding of drought among researchers, decision-makers and the general public. Other outcomes include the Cape Town Rainfall Monitor and the Water Harvesting Tool.

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