Water crisis: Floods needed to fill dams

2020-05-21 06:01
Here is a graphic of the rainfall pattern of Nelson Mandela Bay Metropole from 1900 to 2019.    Graphic:SUPPLIED

Here is a graphic of the rainfall pattern of Nelson Mandela Bay Metropole from 1900 to 2019. Graphic:SUPPLIED

Multimedia   ·   User Galleries   ·   News in Pictures Send us your pictures  ·  Send us your stories

REGULAR mini-floods are needed in the catchment areas of the Nelson Mandela Bay Metropole dams to fill them and save residents from a severe water crisis.

This is according to Garth Sampson from the South African Weather Service in Port Elizabeth, who wrote an in-depth article with insight concerning the drought and water crisis in the metropole and surrounding areas.

“We have been warned that Day Zero could fast be approaching. All are crying out for answers as to when the rains will come, but we must realise that rainfall alone is not going to change the water scarcity situation,” said Sampson.

Considering that a water crisis (water scarcity) is the lack of freshwater resources to meet demand, population growth is putting more demands on industry and agriculture, thus in turn on the available water supply.

“With a record hot summer season, and an increased demand on water, caused inter alia other by the Covid-19 pandemic, water usage has increased to 300 megalitres (ML) per day, up from a figure of 250 ML (what should be used per day).

“This has caused dam levels to drop to a level of around 20% of total capacity. This has caused alarm bells to go off as most are aware that the last few percent of capacity is dead capacity,” said Sampson.

“Water supply is finite with climatic fluctuations. As was highlighted in Biblical times, there are seven years of great abundance followed by seven lean years (famine).

“This holds ground meteorologically, as there are generally seven years between a peak and a trough in all weather elements. That is why a climate norm is always taken over 30 years to have two peaks and troughs in its calculations.”

The metro gets most of its water from the Langkloof catchment, as well as the Patensie/Baviaans catchment. “The rainfall for these regions is between 450 and 600mm. So essentially this is a dry region. Historic figures show us that as far as rainfall is concerned, the abnormal is the normal for this region. We receive either too much rain or too little rain.

According to Professor Mike Muller, Climate Specialists, Hydrologists and Disaster Management Specialists generally distinguish between three different types of drought, namely:

. A meteorological drought occurs when the rainfall is less than the average rainfall, for a significant period.

. An agricultural drought occurs when a lack of rainfall leads to a decline in soil moisture affecting pastures and rain-fed crops.

. A hydrological drought occurs when a meteorological drought significantly reduces the availability of water resources in rivers, lakes, dams and even underground water.

“From the above we can see that we don’t necessary experience all types at once. This depends in what form the rain falls, be it one or two severe events or many small falls.

“The metro dam’s catchment area cannot rely on normal or even above normal rainfall to fill its dams.

“Regular mini-floods (falls of more than 50 mm in 24 hours) over our catchment followed up by regular follow up rains of more than 10mm in 24 hours are required to provide some run off,” said Sampson.

“The weather system responsible for this, in 99% of cases is a cut-off low in the upper atmosphere, or a severe thunderstorm. Naturally the cut-off low must be perfectly positioned and accompanied by the necessary surface conditions. This was responsible for the Great 1902 Flood, the Face Changing 1968 Flood and the Devastation 1981 Flood.

“Since November 2017, we have not had one 50mm event. September 2018 gave us a lifeline with a 40mm event, but since then there have been no significant falls fill to make a marked increase in dam levels.

The big question is thus what is the outlook for the rest of the year?

Sampson said the seasonal forecast, which has been spot on for the last three years shows a tendency towards normal to above normal rainfall for the autumn winter months.

“However, this has not been the case thus far in 2020.

“Considering that winter is the time of year we receive most of our rainfall at a rate of around 50mm per month, the 14mm in April and 24mm thus far in May is far below the norm.

“As the seasonal forecast works on the total rainfall over a three-month period, each day without rain means that we need a larger event for the forecast to be correct.”

Adding to this, history shows us that all droughts in this region are broken by floods.

“Thus, we must realise that we are going to have a flood that will temporarily break this drought. Exactly when is the million-dollar question.”

Sampson said the best advice to the public is that they must be prepared for the worst as far as the drought is concerned and use water very sparingly.

“They must also be aware that a flood will occur some time in the future and be prepared by taking the necessary precautions beforehand.

“Finally, when that extreme event has occurred and the drought temporarily broken and water restrictions lifted, start preparing for the next water crisis.

“As the advocates of climate change say, extreme events will be worse, therefore the next drought could be even worse,” said Sampson.


Inside News24

Lockdown For
Traffic Alerts
There are new stories on the homepage. Click here to see them.


Create Profile

Creating your profile will enable you to submit photos and stories to get published on News24.

Please provide a username for your profile page:

This username must be unique, cannot be edited and will be used in the URL to your profile page across the entire 24.com network.