Lessons learnt from the drought

BETWEEN 2015 and last year, the Western Cape region experienced three of its lowest rainfall years on record.

This led to the depletion of water supply reservoirs and by the summer of 2017/18, there was a real danger that – without drastic reductions in water use – the region, and especially Cape Town, would run out of water.

Droughts close to this magnitude have occurred in the past (for example in the late 1920s, early 1970s, and 2003 to 2004) and led to water shortages in Cape Town.

But are they getting worse?

The reliable yield of the south Western Cape water system has, until now, been calculated under the assumption of a stationary climate.

This is the idea that past rainfall can be used to estimate present day as well as future rainfall, and then also water-system yields.

A water-resource model is used to estimate the frequency of failure under all the known past rainfall conditions – in the case of this region, the last 80 or so years. The water system is then designed to be fairly reliable.

The supply system for Cape Town and surrounding areas was designed to maintain supply without imposing water restrictions 98% of the time, or – on average – 49 out of every 50 years.

Many water-resource planners are taking climate change into account when upgrading or designing water-supply systems.

But has this changing global climate altered the risk of droughts?

We assessed this in a recently published analysis. Using a range of modelling approaches, we estimated the frequency and intensity of threeyear rainfall amounts over the south Western Cape in a world without human-induced warming of the climate. We compared this to drought risk in the world we live in, where greenhouse gases and other pollutants have warmed the planet by about 1ºC.

This means the key assumption of a stable climate has been undermined by climate change, at least for the south Western Cape region.

Our analysis shows that what has been predicted to happen in southern Africa under changing climate in the future is happening, with more dry periods today than, say, 20 or 50 years ago. The water-resource system is stressed more frequently and strongly than had been anticipated.

Our analysis also showed that with a further doubling of global warming over today from 1ºC to 2ºC – likely to happen in the next 50 years – there is a further threefold increase in risk of severe drought.

This means that droughts, which the water resource system is designed to survive, will occur more frequently.

Without adaptation in water supply and demand, events like the 20172018 water shortage could occur once every 15 years, on average, compared to the expected once every 50 years.

It’s been suggested that the Cape Town water crisis was largely because of an erosion of water management capability. We show that another culprit is exacerbating the problem – climate change. Organisations such as the Department of Water and Sanitation and catchment management agencies, who are responsible for working towards a more resilient water-resource system, need to do better than in the past and include estimates of the evolving drought risk. Otherwise they’ll always be underestimating this risk as climate change progresses.

Climate change projections are often taken into account when designing future water-supply systems and other infrastructure. What Cape Town’s drought teaches us is that climate change is happening and impacts us today. We are running out of time to reduce greenhouse gas emissions, and we can no longer postpone taking precautionary and adaptive actions.