Over the past few years South Africa has been experiencing one of the most extreme droughts in recent history. Many experts consider the dry season of 2015/2016 to be the worst drought in almost 25 years, but the 2016/2017 dry season looks likely to be rated even worse. During the 2016/2017 dry season dam levels in most provinces dropped below those of 2015/2016 and some provinces, such as the Western Cape, are still firmly in the grip of the drought.

One of the major causes of the extreme droughts over the past few years has been the impact of El Niño, a weather phenomenon in which the water surface temperature in the eastern pacific is warmer than usual, causing warmer, drier temperatures in South Africa.

El Niño is a cyclical weather phenomenon and so will eventually make way for La Niña, with the opposite effect of El Niño. And while that will bring some relief for the country, it doesn't change the fact that South Africa is a water-scarce country which is extremely vulnerable to climate events.

This is part one of a planned series on South Africa's water supply management and strategy. Here we will look at the country's water infrastructure and some of the effects of the recent drought.

South Africa is a water-scarce country with annual rainfall well below the world average. According to the World Bank's database of annual global rainfall averages South Africa receives an average of just 495mm of rain a year. This is almost half of the global average of 990mm and significantly less than the rain received by Colombia which tops the list at 3,240mm a year.

Receiving so little rain South Africa is heavily reliant on its ability to capture and store water for leaner times. It does this by maintaining a network of storage dams and reservoirs across the county. The Department of Water and Sanitation manages a total of 210 dams that make up the backbone of the country's water management strategy.

The majority of these dams are located in the Eastern and Western Cape but it is the larger dams in provinces such as the Free State that play the biggest role in managing the water risk the country faces.

In the chart below each circle represents one of these 210 dams.

Of the 210 dams it is just a handful that make up the bulk of South Africa's water storage capacity. In the chart below we can see that the five largest dams account for almost 50% of the country's total storage capacity. Their individual contribution to overall storage is also indicated. The other 205 dams account for the remaining 50%.

As part of its monitoring processes the Department of Water and Sanitation releases weekly data on the levels of all listed dams in South Africa. Looking through this data it is possible to build a picture of how the drought rolled through the country. The charts below represent the provincial levels for dams in November 2015, November 2016 and February 2017 (collected 27 February 2017).

Note: These are water levels for dams located in each province, not necessarily the amount of water available to a province. Gauteng, for example, receives the bulk of its water from the Free State. The Vaal dam is in the Free State administration area. So while Gauteng's dams were relatively full throughout the drought, it suffered from the depleted Free State dams.

As you can see from the state of dam storage chart, the 2016/2017 drought season initially affected the inland and more northern provinces. The worst of the drought season for most of those provinces was between November and December 2016. Limpopo and Mpumalanga dam levels were down between 22 and 26% compared with November 2015 and the Free State dams were down 17% compared to the previous year. The low levels of these dams affected not only the province they were located in but also had a major impact on the metropolitan areas in Gauteng. By mid-November 2016 the Vaal dam, the key supplier of water to Gauteng cities, was perilously low at 26% of its full storage capacity.

On 7 November 2016 water authorities began to release water from the Sterkfontein dam to begin replenishing the Vaal dam. At the time the Sterkfontein dam in the Free State was almost 90% full.

Strong rains, and even floods, in the latter months of 2016 and early 2017 have now replenished the major Free State dams and some have even exceeded their full capacity. On 26 February 2017 sluice gates at the Vaal dam were opened and a flood warning was issued to residents in the area as the dam reached full capacity.

Further south, the Western Cape is still in the grip of drought. In November 2016 the province's dams were lower than the same time the previous year but not by as much as those in many other provinces. Western Cape dams were down 9% in November 2016 compared with November 2015. Free State dams, in comparison, were down almost double that at 17% for the same period.

By February, however, the situation was significantly reversed. While the upcountry dams were largely replenished, the Western Cape dams continued to fall. At the time of writing (3 March 2017), Western Cape dam levels were down to just 33% of their storage capacity, almost half of what they were in November 2016 and a third of what they were in November 2015.

The province's largest dam, Theewaterskloof, which accounts for more than 25% of the Western Cape's total water storage now sits at 27%, down from 42% last year.

One of the features of the recent drought, particularly in the centre and the north of the country, was the relatively extreme weather conditions. In early November 2016, while the Vaal dam was nearing its lowest levels and water authorities were releasing water from Sterkfontein to supplement the dam, large parts of Gauteng experienced devastating flooding. And water restrictions remained in place well into the early part of 2017.

The primary reason that parts of the country could have both water restrictions and floods at the same time is related to the various drainage areas (catchment areas) that serve each of the major dams. Looking at the Vaal dam for example:

The drainage region for the Vaal dam is the light pink area. Johannesburg sits on the very northern border of this catchment area, while Sandton and Pretoria sit north of the area. So while heavy rain over these areas might suggest an improvement in the drought situation, the reality is that rains further south and south-west of Johannesburg and Gauteng are what really matter.

Similarly, in the Western Cape the Theewaterskloof dam relies on good rainfall north of the Hottentots Holland mountain range for replenishment. So rain in the Cape Town city centre is not a reliable indicator of general improvement. Although, as you will see later, the province's other major dams are a lot closer to Cape Town than the Theewaterskloof dam.

South Africa is broadly divided into 22 "drainage" regions. They're labelled from A to X and broadly correspond to the countours of the land. The map below shows the ten largest dams in South Africa and their draingage areas. The Bloemhof, Vaal and Sterkfontein dams rely on good rains over most of the Free State. The Vanderkloof and the Gariep dams, the two largest in the country, are largely replenished by rains over the southern Free State, Northern Cape, north-western Eastern Cape and Lesotho.

Looking further south the City of Cape Town's water supply system is built around six primary dams, the largest of which is the Theewaterskloof dam east of Cape Town. The Theewaterskloof dam is dependent on good rains in the Hottentots Holland catchment area while the other dams in the system are in the drainage zone around Cape Town. At the time of writing most of the dams in the Western Cape were between 32% and 38% of their full capacity. The Theewaterskloof dam was at 27%.

A complete water management system and strategy is a lot more than just the dams and the water they are able to hold. In either future versions of this particular piece, or in entirely new versions, I will look at other aspects of the water supply system.

Comments, thoughts, suggestions?
The data for this piece was collected from various sources (see below). The interpretation of that data is entirely mine. Therefore, any errors or ommissions are mine. If you have a suggestion or a comment please let me know via email or via Twitter.

Mobile users: If you're viewing this on a mobile device some of the interactive charts may not work exactly as expected. I have tried to make this compatible with as wide a range of devices as possible but some issues may still surface. Apologies for that.

Sources/credits

• Annual rainfall averages: World Bank
• Cover ostrich photo: Pixabay
• Theewaterskloof dam photos: Greg Gordon
• Dam levels, draingage areas: South African Department of Water and Santation
• Water level gauges: Based on Curtis Bratton's work