Above average rainfall to break drought in Cape Town 'highly unlikely'

2018-05-02 17:36
The City of Cape Town's largest storage dam, the Theewaterskloof Dam. (Supplied)

The City of Cape Town's largest storage dam, the Theewaterskloof Dam. (Supplied)

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It is highly unlikely that Cape Town’s catchment area will get the above-average rainfall it desperately needs this year to break one of the city’s worst droughts on record.

Experts have said that the catchment area needs at least four consecutive months of above average rainfall in 2018 to fill the dams sufficiently to see Cape Town and surrounds through next summer and to avoid a Day Zero in 2019.

But it looks as if there is less than a 5% chance of this happening.

SPECIAL REPORT: Water Crisis

Climate scientists Peter Johnston and Piotr Wolski from UCT’s Climate System Analysis Group have worked out that the cumulative rainfall up to the end of April is a good indicator of whether there will be a wetter than normal year.

After analysing rainfall figures from 1930 for Cape Town’s catchment area – the mountains that feed rainfall into the dams – they found that if a year’s cumulative rainfall recorded by the end of April was above normal, there was a 70% chance that we would have above normal rainfall for the rest of the year.

However, Johnston said on Monday that while he had not received all the official rainfall data for April, it was safe to say that the provisional figures suggested that there had definitely not been “anything approaching above normal” rainfall to date.

“This means that it is highly unlikely – only a 5% chance – that we will receive a substantially above normal total for the year,” Johnston said.

READ: Iceberg proposal as new water source for Cape Town

Above average rainfall unlikely

Above average is defined as rainfall figures that fall into the wettest one third of all the years recorded.

Johnston said they could not tell from the rainfall data to date whether the rest of the year would be average or drier than average. For that they would have to wait for additional rainfall data later in the year.

“We will make a further assessment at the end of May,” Johnston said.

He said if by the end of May the cumulative year’s rainfall was below average, they could say that it was likely that we will have a drier than normal year.

He wrote earlier this year that if the rains were above average, the city should be able to avoid Day Zero next year, but if they were below average, it would be very hard to do so.

These assessments would help give some answer to help government, farmers and residents make some decisions about what to do in 2019, he said.

Nicky Allsopp, manager of the fynbos node at the SA Environmental Observation Network (SAEON) has said because soils had dried out after three years of drought, they would act like sponges and absorb a lot of rainwater, reducing run-off into dams.

She estimated that we would need at least four consecutive months of above normal rainfall this year to fill the Cape dams enough to supply water into 2019.

'Great commitment from residents'

The city council said water consumption in Cape Town last week had reached a record low of 505 million litres a day, and the late April rains had resulted in the dam levels rising slightly, from 20% to 20.9%.

“The record savings are encouraging in the light of recent rainfall, which could possibly have led to some of us relaxing our efforts,” the City said in a statement.

However, although this showed “great commitment from residents”, Cape Town residents still had not reached the target consumption set by the Department of Water and Sanitation of 450 million litres a day.

* This story has been changed to reflect that if the cumulative rainfall is below average this year, it would be hard to avoid Day Zero in 2019.

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Read more on:    cape town  |  climate change  |  drought  |  water crisis

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