Is the drought over when most of the dams are almost full?

By Abri de Buys, Technical Officer, SAEON Fynbos Node Tweet

Accurate rainfall records rely on regularly checked and calibrated instruments. In the picture above, Fynbos Node technician Abri de Buys is calibrating a rain gauge in Jonkershoek.

A brief look at social media will show that water issues in the Western Cape remain hotly debated.

This is the case even after the Western Cape Water Supply System’s (WCWSS) main dams made a remarkable recovery over the 2018 wet season, with several of the main supply dams filling to above 90% of capacity and the Berg River Dam even spilling over¹.

At the end of the wet season the largest of the lot, Theewaterskloof Dam, is just under 60% full (up from a little over 10%) and the system as a whole has peaked at 76%¹.

The City of Cape Town has lowered water restrictions to Level 5 in response², bringing a bit of relief from the 50 litres-a-day and punitive³ water tariffs known as Level 6B. The public debate rages on over whether water restrictions should remain in place at their most severe levels or be relaxed further.

Some self-proclaimed proud “water warriors” say the drought and restrictions have taught them the worth of water, they’ve made the necessary adaptations to their homes and habits and will continue to live on 50 litres a day irrespective of what restrictions are in place. On the other hand, a very valid question may be asked: “What restrictions were in place the last time our dams were 76% full and why are the current restriction levels not the same?”

Apprehension about water seems to have become part of the local culture after four years of drought, yet there is no denying we are currently at least as well off as we were at the start of the 2015/16 dry season in terms of supply in storage.

Does the rainfall record suggest the drought is over?

Public perception, understandably after three consecutive very dry years, is that the 2018 rainy season “saved us” from the dreaded “Day Zero”, when strict water rationing would have had to be implemented.

Yet according to many rain gauges across the region, the 2018 rainy season was average at best³. This is also reflected in the records from SAEON’s catchment monitoring network at Jonkershoek.

Figure 1. Cumulative annual (calendar year) rainfall from nine rain gauges in the Jonkershoek catchment-monitoring network

The 2018 record is obviously not complete, but the historical record suggests little is added between November and the end of the year. Indeed, dam levels have slowly started declining as we move into the dry season.

Comparing the 2018 rainy season (defined as April–September in Figure 2 below) against the long-term average does not fill one with confidence that the drought is over – that is if we define drought in terms of rainfall only.

Figure 2. Seasonal rainfall totals measured at Dwarsberg high-elevation rain gauge in the Jonkershoek catchment-monitoring network5. Note that “Summer” on this graph is October to March and “Winter” is April to September.

Comparing 2018’s rainfall against the long-term average and feeling mildly relieved but mostly uncertain, is perhaps not the most effective use of long-term rainfall data. What if we looked at some of the longest continuous records we have and tried to see how often droughts occur that exceed the duration of the current one?

Below is one of the longest continuous rainfall records in South Africa, from the Royal Observatory in Cape Town, starting in 1841 (Figure 3). Here we’ve subtracted the long-term average from the annual totals to produce a time series of above- and below-average deviations (anomalies).

Figure 3. Annual rainfall anomalies (percentage deviation from average) for the Royal Observatory rain gauge in Cape Town.

If we define a drought simply as a period of below-average rainfall, we can ask “how frequently has this gauge experienced drought events that equalled or exceeded four consecutive years in the past?” Note that this is a very loose definition since any below-average year counts as a “drought” year. The answer for this specific station is eight times in 176 years.

These “droughts” occurred as follows:

At this specific station we see that, historically, the longest “drought” events generally lasted 4–5 years, with the exception of the 1926–37 drought. Granted, extrapolating across the WCWSS catchments from a single sample point in the City is not the most scientifically sound approach. However, droughts tend to be regional phenomena even if we do not present the data here to demonstrate this. It is worth considering that not all “droughts” (defined as above) are equal and by many accounts the drought of 2014–2018 was particularly severe.

It seems appropriate to ask then: After four years of severe drought, with parched catchments, many of which are invaded by thirsty alien vegetation⁶, is an average rainy season really all we need to bounce back from 22% to 76% of storage capacity filled?

The demand side of the story and what we can learn from it

This requires a more holistic view and adapting our hitherto (in this article) definition of the term “drought”. Expanding the definition of drought to include potential of risk to the wellbeing of people may be more appropriate. This recognises our dependence on water for our livelihoods and, ultimately, our lives.

More importantly, it helps us realise that how we manage catchments and use the water that flows from them matters. We become aware that we’re all dependent on a catchment somewhere and that those of us who own, manage or live in areas that supply water to others have a responsibility to society. Conversely, society has a collective interest in how catchments, river ecosystems and built water infrastructure are managed and maintained.

We’ve also learnt that a coordinated public response can achieve great results! Data show City of Cape Town’s water consumption demands were reduced by more than half compared with the summer of 2015 levels¹. This is a remarkable feat, thanks to citizens’ water-saving efforts and the City’s pressure reduction to reduce water lost from leaks in the distribution network³. Many a hard-earned rand from private pockets has been invested in becoming more water-wise.

Severe sacrifices have had to be made by agricultural and industrial users dependent on the broader WCWSS. Consequences of the dramatic forced reduction in water use in the agricultural sector have amounted to a cumulative loss of billions of Rands, more than 30 000 jobs and a significant percentage of output⁷.

That it was possible to cut the city’s demand in half does raise questions about how the “other half” was used before. How much of it was saved by pressure reduction? How much by suburban gardens getting to experience, perhaps for the first time, ambient conditions consistent with the fynbos biome we inhabit? How much can we save before incurring the severe costs mentioned above?

The case study of the 2014–2018 drought has reiterated the importance of long-term continuous environmental observation. It has confirmed that publicly engaged scientists working with well-supported research infrastructure can make valuable, real-world contributions to the public debate and to policy. It has also been a learning opportunity that will inform what research we do and how better to communicate our findings in future.

We would like to acknowledge Dr Ian Glass from the South African Astronomical Observatory for sharing the complete Royal Observatory data set.

References  

1. City of Cape Town: Dam Levels Report, 22 October 2018 https://resource.capetown.gov.za/documentcentre/Documents/City%20research%20reports%20and%20review/damlevels.pdf, accessed on 26 October 2018.
2. City of Cape Town Level 5 water restriction guidelines https://resource.capetown.gov.za/documentcentre/Documents/Procedures,%20guidelines%20and%20regulations/Amended%20Level%205%20water%20restriction%20guidelines.pdf, accessed on 26 October 2018.
3. City of Cape Town : Alternative water installation guidelines & drought crisis update, 20 March 2018, https://www.green-cape.co.za/assets/Uploads/Alternative-water-workshop-for-plumbers-20-March-2018.pdf, accessed on 26 October 2018.
4. Current season’s rainfall in Cape Town, http://www.csag.uct.ac.za/current-seasons-rainfall-in-cape-town/, accessed on 26 October 2018.
5. Drought monitor for the Jonkershoek valley, Western Cape, South Africa, http://www.ecologi.st/post/2017-04-01-langrivier/, accessed on 26 October 2018.
6. Aliens are the greatest threat to Cape Town’s water security, Groundup, 29 May 2018, https://www.groundup.org.za/article/aliens-are-greatest-threat-cape-towns-water-security/, accessed on 26 October 2018.
7. Western Cape farmers call for easing of water restrictions to repair drought damage, https://www.news24.com/SouthAfrica/News/western-cape-farmers-call-for-easing-of-water-restrictions-to-repair-drought-damage-20180917, accessed 26 October 2018.