Windhoek’s Water Situation: Rain Is Only Half the Story

In July, I wrote about the possible return of El Niño this rainy season and what another prolonged drought could mean for Windhoek.

Since then, predictions of a strong El Niño occurrence have strengthened, prompting a broader question: what does Windhoek’s water history tell us about our ability to withstand another prolonged drought?

Windhoek has more than a century of recorded rainfall. When records from 1901/02 to 2019/20 are plotted, one factor stands out: the rainfall is extraordinarily variable.

There are very wet years, exceptionally dry years, and periods where poor rainfall persists across successive seasons.

The long-term average over the 120-year period is approximately 363 mm a year. But average rainfall tells only part of the story.

For water security, what matters is the enormous variation around that average and particularly what happens when several poor seasons occur in succession.

This leads to an important distinction.

Drought is a natural event over which we have little control.

A water crisis, however, occurs when a water supply system no longer has the resilience to absorb that drought. Unlike the weather, the resilience we build into our water supply system is within our control.

Severe rainfall variability is not new to Windhoek. Our water planning has always had to contend with an arid and highly variable environment.

More importantly, how well prepared is a rapidly growing Windhoek to survive when the inevitable happens?

PROFOUND GROWTH

In 1967, Windhoek had approximately 49 000 residents. The 2023 Population and Housing Census recorded approximately 487 000, an almost exact tenfold increase.

The latest census records annual population growth of approximately 3.8%. If that rate is sustained, the population would mathematically double in approximately 18 to 19 years.

This has profound implications for infrastructure, particularly water supply.

Windhoek’s water system, however, has not stood still. In fact, the city has developed one of the world’s most innovative urban water supply systems.

Groundwater has an estimated safe yield of approximately 1.3 million cubic metres a year (Mm³/a).

The three-dam system has a combined safe yield of approximately 20 Mm³/a, while direct potable reuse added another approximately 5.5 Mm³/a following commissioning of the current reclamation facility in 2002.

Together, these sources provide a safe yield of approximately 26.8 Mm³/a. This represents a substantial engineering achievement and diversification. But the population has continued growing since the last independent source was added.

SHRINKING MARGIN

The best way to understand the shrinking margin is to look at dependable water supply per resident.

When the last independent addition to Windhoek’s water supply was introduced in 2002, the population was around 240 000. The established safe yield of 26.8Mm³ represented approximately 111m³ per resident per year.

By the 2023 census, with approximately 487 000 residents dependent on the same established safe yield, this had fallen to only 55m³ per resident per year.

That is a decline of approximately 50% in sustainable supply capacity per resident in just over 20 years.

This water does not serve households alone. It also supports businesses, schools, hospitals, industry, government and municipal uses, while also accommodating increasing system losses.

Windhoek does have an important strategic buffer: the Windhoek Aquifer.

Its value lies in allowing water to be stored underground during periods of abundance and recovered during drought, avoiding substantial evaporation losses.

But the aquifer does not create additional water. We can only bank water when sufficient water is available and then draw on those savings when drought arrives.

Safe yield is comparable to dependable income. Emergency abstraction from underground reserves is like drawing from savings.

You can temporarily spend more than you earn, but it does not increase your income. It is only possible while you have savings to draw upon.

This creates another important question.

If the next El Niño produces two, three or more consecutive poor rainfall seasons, how much strategic water has Windhoek actually accumulated underground, and for how long could it sustain the city?

Moreover, what additional resilience has been created since the major drought a decade ago when Windhoek came critically close to water security failure?

SOLUTIONS

Windhoek’s diverse water supply system is testimony to the fact that there is no single solution. Additional supply, aquifer recharge, reuse, demand management and responsible development planning all have a role to play.

But the objective should be simple: to rebuild sufficient margin into Windhoek’s water supply system so that the next drought remains a drought and does not become a water crisis.

More than 120 years of rainfall history tells us another prolonged dry period will eventually come.

We cannot control when it arrives. But we can manage resilience and what is done between droughts ultimately determines how well Windhoek survives the next one.

– Pierre van Rensburg, strategic executive for Urban and Transport Planning at the City of Windhoek, writing in his personal capacity

– Note: Water transfer from the Karst aquifer of approximately 4 Mm³/a was not taken into account for this article because of its poor transfer efficiency.

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