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What do long-term data reveal about Cape Town’s water shortage?

By Margaret Koopman and Abri de Buys, SAEON Fynbos Node
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The origins of the settlement that became known as Cape Town are intimately tied to water.

As the area’s indigenous inhabitants had undoubtedly done, one of the VOC’s (Vereenigde Oost-Indische Compagnie's) main reasons for choosing this area to settle was the ample supply of fresh water, running from a network of springs and streams around Table Mountain.

It did not take long, however, before the settlement started experiencing water shortages and pollution problems. As early as 1655 instructions were broadcast to minimise the pollution of a water supply that was used both by residents and by ships calling at Table Bay.

A comprehensive history of the water supply of Cape Town was researched for the book Rivers and Wetlands of Cape Town1, where it was demonstrated that the growing population of Cape Town had to constantly develop new solutions for water supply. By the 1850s, the first major interventions had been built in the form of two reservoirs, still in use, that can be seen in De Waal Park.

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The R1.5 billion Berg River Dam constructed in 2009 was approved by the South African Government with the proviso that the City reduce water consumption by 20% by the year 2020 (Picture: Daniel Saaiman)

When the Molteno Reservoir was built in the 1880s, the mountain springs were insufficient to fill it, resulting in the construction of a tunnel to bring water from the Disa River in Hout Bay2. The combined capacity of these three reservoirs was soon insufficient to provide water for the growing city and in 1894 the first of the Table Mountain reservoirs, the Woodhead, was built.

Looking back over the history of Cape Town and the surrounds, one can see a series of management interventions evolving into the modern-day intricate network of water supply called the Western Cape Water Supply System (WCWSS) 3. Over time, the little VOC refreshment station with its network of grachts (canals) developed into a complex regional network that integrates the City of Cape Town, the West Coast District Municipality as well as the Swartland, Saldanha Bay, Bergrivier and Stellenbosch Local Municipalities4.

All of these are either fully dependent or have their supply augmented by the major dams3&5. The infrastructure capacity history, in a nutshell, is half the supply side of the story. The other half of the supply side history that may better represent what Cape Town and the surrounds actually have in storage is rainfall, to which we will return a bit later.

On the demand side, it is estimated that approximately 70% of water in the WCWSS gets allocated to urban and industrial users and 30% to agriculture5. To fully understand water demand in relation to supply, would require an understanding of the evolution of the regional economy and population growth in addition to understanding the development and integration of the WCWSS against the backdrop of historical rainfall in the catchments. However, it is generally understood that water demand increases as populations grow.

It is a well-known fact that there are seasonal changes in water demand. This is mainly related to agriculture as well as urban aesthetic (gardens) and recreational (sports fields, swimming pool top-ups) use. The WCWSS has to balance water availability seamlessly to meet demand through the wet and the dry season5 - no mean feat!

The relationship between water demand and population growth is mediated by how efficiently we use water, both for our economic activities and at home. To use population numbers as a proxy for water demand may be an oversimplification, but it does yield some interesting results. Ideally one would need cumulative population numbers that include populations adjacent to Cape Town as their water supply systems became integrated with the Cape Town water supply system to form the WCWSS.

In this article we present an estimate of the population numbers obtained from a range of historical and contemporary census documents6. Population growth of cities can be due to births, migration and urbanisation, and, in the case of Cape Town, a change in municipal boundaries. By 1996 Cape Town consisted of six municipalities with a central administration that were consolidated in December 2000 to become a metropolitan municipality with a far higher combined population7.

We therefore inflated the “Cape Town” population from the 1960s onwards by adding populations from the neighbouring towns, where these were easily available, because these populations had by that time been integrated into the water supply system. Despite this, we regard the population numbers presented as conservative estimates because of the difficulty we had piecing them together for the WCWSS.

Storage capacity and population

If we accept that water demand increases with population growth, one may plot population numbers against cumulative storage capacity over time. Figure 1 shows the population of greater Cape Town alongside cumulative dam capacity for the years 1841 to 2016. The graph shows that as the population of Cape Town grew, major additions to water supply had to be made by the Lower Steenbras Dam in 1921 and the Wemmershoek Dam in 1957.

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Figure 1. Population and water storage capacity (in megalitres) growth of greater Cape Town

Cape Town’s average annual population growth during the 26-year period 1970 to 1996 was 2.97%8, but because of proactive planning by the Government, dam capacity had been increased with the addition of the Voëlvlei Dam in 1971 and the Theewaterskloof Dam in 1980. During the 20 years from 1996 to 2016, the annual population increase of Cape Town was approximately 2.8%, with the introduction of one more dam, the Berg River Dam in 2009.

After the major storage capacity added in the late 1970s to early 1980s, population growth has increased fast over the last three decades despite the addition of the Berg River Dam (Figure 2). We divided storage capacity by population to create a per capita storage capacity index, understanding however that not all people use the same amount of water and that there are many water users we have not accounted for. This suggests that current storage capacity on a per capita basis is approximately half of what it was 37 years ago at the completion of the Theewaterskloof Dam.

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Figure 2. Per capita water storage capacity (storage capacity divided by population) of greater Cape Town

The uncontrollable half of water supply

We previously alluded to the fact that dam storage capacity is only half of the supply-side story of the water issue. The other half is rainfall runoff that ends up in our dams and as we are all too aware, Cape Town has been experiencing a low rainfall period since 2014, with dams currently at only a fraction of their capacity - insufficient to supply the water demands of metropolitan Cape Town9.

This seems to be one of the worst droughts in a century according to many media reports. What do the long-term rainfall records say? Is this low rainfall exceptional in the history of Cape Town’s water supply?

The SAEON Fynbos Node examined three long-term rainfall data sets that were originally collected at the Royal Observatory weather station (1841-1965), the Stellenbosch Gaol weather station (1878-1965) and the Jonkersnek rain gauge at Dwarsberg, Jonkershoek (1945-2013). These data sets were brought up to date with rainfall data from the South African Weather Service and the SAEON automatic weather station at Dwarsberg, Jonkershoek. The historical rainfall data accessed from the Council for Scientific and Industrial Research (CSIR) and Cape Nature were brought up to date with rainfall data from the South African Weather Service and the SAEON automatic weather station at Dwarsberg, Jonkershoek.

With continuous long-term rainfall records being relatively hard to find and access, we used these time series firstly to eyeball whether they agreed about the timing of wet and dry years over the last century. Figure 3 suggests there is broad agreement in the two longest rainfall records we examined for this article. The pattern is similar for our “younger” gauge at Jonkersnek (Dwarsberg), Jonkershoek.

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Figure 3. Annual rainfall totals and five-year moving average rainfall for Royal Observatory and Stellenbosch tronk (gaol) rain gauges 1880-1965

Having confirmed that the rain gauges agree, we had a closer look at the record for the Jonkersnek (Dwarsberg) station that SAEON assumed monitoring in 2013. This station is situated at a location near the headwaters of rivers that supply most of Cape Town's major supply reservoirs (Theewaterskloof and Berg River dams) and therefore offers a rare and valuable glimpse into rainfall conditions high up in the catchments.

Unfortunately, the record for Dwarsberg only stretches back to 1945. However, having established that the different rain gauges examined broadly agree on the timing of droughts for the time series examined, we compared Dwarsberg with the Royal Observatory record (the oldest we examined) more closely. We plotted five-year moving averages for both time series to illustrate the extent to which they agree for the period of overlap.

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Figure 4. Actual and five-year moving average rainfall for Royal Observatory compared with Dwarsberg (1945-2016)

Figure 4 suggests that while 2014-current is a severe drought, it is by no means the driest in terms of rainfall input in recorded history. Indeed, one does not have to look very far back to find a three-year succession with similarly low rainfall than the 2014-16 period (2004-2006 most recently and 1971-1973 before that).

If we delve further into the past, the Royal Observatory record illustrates a 176-year history of droughts (Figures 5 & 6). Based on our Dwarsberg vs Royal Observatory five-year moving average comparison, one might make an educated guess that rainfall in the catchments may have tracked the Royal Observatory record further back in time. A drought of much greater magnitude than we are experiencing today can be seen from ~1927-1936.

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Figure 5. Dwarsberg five-year moving average in perspective (a comparison against Royal Observatory)

Looking at positive and negative deviations from the norm puts the current drought into long-term perspective.

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Figure 6. Annual rainfall anomalies for Royal Observatory

If the long-term rainfall records we examined are representative of general trends in our catchments, statements like "worst drought in history" or "worst drought in a century" appear to be unsubstantiated. The 1920s-30s drought (to name one) was dryer and lasted longer than the current drought (to date).

If Cape Town has experienced previous droughts of similar magnitude, the reasons why this particular drought is having such a dire impact seem to be first and foremost related to water demand, not rainfall.

This leaves us with questions about why Cape Town faces a dire water crisis within the next year if rainfall does not bring relief. Is it because temperatures have been increasing, leading to excessive evaporation from the catchments? Is it because our catchments are invaded by thirsty alien vegetation which, despite our best efforts, seem to be spreading? We know that access to water for domestic use in the 1920s-30s would have been far more limited and rudimentary than it is in 2017. Is it because demand was easier to manage during previous severe droughts? Or are there other reasons?

What can we do?

The R1.5 billion Berg River Dam constructed in 2009 was approved by the South African Government with the proviso that the City reduce water consumption by 20% by the year 202010. Reducing water consumption by 20% is a challenging target to achieve.

Managing water consumption in urban areas is a critical component of ensuring that there is sufficient water for essential needs. On the consumption management side the City is reducing water pressure, which in turn minimises leaks from the system. In the minority of households (16% in 2014) where water management devices are installed, the council can cut off water supply to an individual house as soon as it reaches a prescribed maximum, currently set at at 350 litres per day11.

On the supply side, many proposals are in the "pipeline" for increasing Cape Town's water supply, including tapping the Table Mountain Group aquifer and siting desalination plants at key points on the coast.

As our history has shown, demand always catches up with supply, but global weather patterns are very challenging to predict and can’t be managed. We can manage our water use and this includes solutions on both the demand and supply side of the equation.

  1. Brown, C. & Magoba, R. (eds.) 2009. Rivers and Wetlands of Cape Town: caring for our rich aquatic heritage. Water Research Commission Report No TT376/08. ISBN 9781770057777 Read more
  2. Timoney, T.W. 1993. Cape Town’s early water supplies. Read more
  3. City of Cape Town. 2016. Annual Water Service Development Plan Performance and Water Services Audit Report. Read more
  4. Breede-Gouritz and Berg-Olifants water management areas limiting the use of water in terms of item 6 of schedule 3 of the National Water Act of 1998 for urban, irrigation and industrial (including mining) purpose in the catchment areas of the dams supplying the Western Cape Water Supply System and from the System. 2016. Government Gazette Notice. Read more
  5. Guidelines for Water Supply Systems Operation and Management Plans during normal and drought conditions (RSA C000/00/2305), Appendix A. The Western Cape Water Supply System – Pilot Study, October 2006. Read more
  6. Batson, E. 1941. Growth of population. Social Survey of Cape Town, Report No. SS1. University of Cape Town, Department of Social Science. Read more ;
    City Planner's Department Technical Management Services. 1988. The distribution of population and its characteristics for the Western Cape RSC area based on the 1985 Census. Report no 1. ISBN 0947439080;
    Stats SA. 1962. Population census 1960. Volume 1. RP 62/1953;
    Stats SA. 1962. Population census Report 02-02-01;
    Stats SA. 1991. 1991 population census results after adjustment. Volume 03-01-01;
    City of Cape Town. 2012. Trends and Change – 10 years: Census 2001 – Census 2011. Read more
  7. Local Government Business Network. 2013. City of Cape Town Metropolitan Municipality. Read more
  8. Todes, A. et al. 2008. Contemporary South African urbanisation dynamics. Read more
  9. Department of Water & Sanitation, RSA. Western Cape Province State of Dams on 2017-09-04 Read more
  10. SA News. 2015. R1.5bil Berg River Dam to supply 20% of Cape Town’s water. Read more
  11. Water-management-devices
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