The amount of rainfall in certain parts of South Africa has increased and become
heavier over the past 50 years or so, meteorology experts at the University of Pretoria have found.
With climate change said to be affecting the intensity of rainfall, the experts investigated if there were observable changes in the probability of significant to extreme daily rainfall across the country. Many areas are prone to flooding, they said.
The results of the study suggest that extreme rainfall events are likely to become “more intense and are to become a feature of climate change over South Africa”.
The investigations were part of a study led by Charlotte McBride of the South African Weather Service, who is a PhD candidate in the university’s department of geography, geoinformatics and meteorology.
One of the consequences of human-induced climate change is the increase in frequency and/or intensity of some weather and climate extremes. These can include heavy rainfall, heatwaves, floods, droughts and tropical cyclones.
South Africa is focusing on flooding, as this is what is happening at the moment, McBride said. “However, during 2018 and the Day Zero debate, when Cape Town was set to run out of water, the public’s attention was focused on drought. South Africa has a variable climate, with droughts and floods a common feature of this variability.”
Highly regionalised
As surface temperatures increase because of climate change, so the water content of the atmosphere changes. These increases in the water-holding capacity of the atmosphere equate to about 7% per degree of warming. With more moisture available, the nature of rainfall events is likely to become more intense, with increased rainfall rates.
But changes in extreme rainfall patterns are thought to be highly regionalised. South Africa’s annual rainfall distribution is diverse and increases from below 200mm in the west to above 1 200mm a year in the east.
To probe if rainfall patterns are changing, the researchers analysed the daily time series of 70 manual rainfall stations between 1921 and 2020. This was divided into an early period (1921 to 1970) and a later period (1971 to 2020).
After comparing the rainfall figures in these two periods, it became evident that most rainfall stations showed an increase in their 1 to 50 and 1 to 100-year return period values.
The two periods had more or less the same number of rainy days (more than 1mm) but the rainfall amounts on any given rainy day for certain parts of the country had increased or became more extreme in the latter period.
“We then mapped the change between the two periods as a ratio for each station,” McBride explained. “This gave us an idea of where areas are experiencing higher or lower rainfall values for the specific return periods.”
More extreme rainfall
Some stations over the eastern parts of the country showed increases of more than 100mm in the later period compared to the early period, when considering the 1-50 and 1-100 year return period values. For example, the Letaba district rainfall station in Limpopo experienced an increase of more than 35%. Another is Hlobane in KwaZulu-Natal, where the 1-50 and 1-100 year values have essentially doubled.
This means that these areas and others highlighted in the research, such as the western interior and southern parts of the country, are likely to experience more extreme rainfall, which is probably a “feature of climate change over those areas”, according to the researchers.
South Africa is also projected to become warmer and experience an increase in the occurrence of droughts, said McBride.
“However, this does not mean that the risk of severe storms — including tropical cyclones and intense thunderstorms — will not be expected to occur. With the atmosphere heating, it can hold more water vapour. More water vapour means more rainfall. So, we can expect the intensity of rainfall to increase.”
Damage to agriculture and infrastructure
The study describes how, although the number of rainy days has remained near constant over the 1921 to 2020 period, the probability of experiencing significant and extreme daily rainfall has increased for most regions in South Africa.
“This is of concern as rainfall of this nature can have serious consequences in terms of flooding, erosion and damage to agriculture and infrastructure,” it said.
It notes how, if structures are not designed to take into account the potential extreme events in a changing climate, “the loss of life and the economic impact could be significant”.
This was witnessed recently in KwaZulu-Natal coastal areas where over 40 000 people were affected by floods, with some areas receiving record daily rainfall amounts.
Sudden disasters
Although flooding in South Africa may not be as frequent, or affect as large areas as drought, it can be sudden, which has consequences for human life and settlements, water management, the built environment in general and agriculture, the study said.
The severe flooding in April 2022 that devastated KwaZulu-Natal is an example of how heavy rainfall “can be the cause of severe impact” with 443 human casualties and more than 40 000 people displaced, it said. The loss of and damage to infrastructure ran into billions.
Other recent extreme events include flooding in the Cape Town area in June 2021 where an estimated 6 300 people were affected and more than 3 250 buildings were damaged. In January 2021, tropical cyclone Eloise caused severe flooding over large areas of Limpopo. In February 2000, tropical cyclone Eline caused extensive damage to infrastructure and loss of life.
“One of the most disastrous flooding events in living memory occurred on 25 January 1981 when intense rainfall over the Laingsburg area, where 425mm fell in 24 hours, causing widespread destruction, with 102 people losing their lives.
“The very heavy rainfall which occurred during these events was associated with well-organised synoptic scale weather systems. However, localised extreme convective rainfall also occurs over South Africa such as on 9 November 2016 when 90mm of rain fell in an hour near OR Tambo International Airport in Gauteng.”
The researchers said that understanding the likelihood of extreme rainfall is critical for strategic planning, town and city planning and adaptation of the built environment “to accommodate any possible increases in extremes”.
Liesl Dyson, associate professor in meteorology, said the value of the research is that it uses real, observed data of rainfall over South Africa for an extended period.
“These results are based on what we know has happened over the past century, and show that, in general, rainfall extremes are becoming more probable and therefore increasing in South Africa.”