The amount of rainfall Cape Town receives in April can indicate whether rainfall will be high for the rest of the year, according to research from the University of Cape Town (UCT).

Scientists Peter Johnston and Piotr Wolski at UCT’s Climate System Analysis Group have analysed rainfall data from 1930 to 2017. They found that the amount of rainfall in April indicates what rainfall we can expect for the rest of the year. If the total rainfall in April is above normal, then it is likely that the rest of the year will also see above average rainfall.

“If the cumulative total is above normal by the end of April, we can say there is a 70% chance of it being wetter than normal by the end of the year,” Johnston said to News24.

But if April doesn’t deliver above average rainfall, then we have to look to May and July for more predictions. If rainfall in May is below average, then the rest of year probably will be too. And if July rainfall is normal, then the total for the year likely will be too.

“So based on statistics, if it is wetter than normal at the end of April, we are likely to have a wetter than normal year; if rainfall is normal at the end of July, we are likely to have normal rainfall and if it is drier than average at the end of May, we are likely to have a drier than average year,” Johnston said.

But rainfall has to be consistent in order for dams to recover, according to Nicky Allsopp, manager of the Fynbos Node at the SA Environmental Observation Network (SAEON).

Thanks to three years of drought, the soil is very dry. Initial rain will just be soaked up into the ground like a sponge, instead of running off the catchment areas into dams.

“It looks like we would need at least four consecutive wetter than normal months in the catchment to get enough runoff into the dams,” Allsopp said. “Every time it rains more than 20mm, we are likely to get some surface flow because the soils can’t absorb it all. But rainfall of 10mm or less will just wet the soil.”

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