Removing alien plants can save water : we measured how much
1
A Rebelo , 2 KJ Ester , 3 M New and
4
PB Holden
NEWS
Current Addresses : 1 Agricultural Research Council , 2 Stellenbosch University and
3 , 4
University of Cape Town Reprinted from : https :// bit . ly / 3z2czBD
Humans ’ relationship with nature is broken . We ’ re transforming the Earth so dramatically that almost one million plant and animals species face extinction . Losing species unravels the tapestry of nature , changing how ecosystems function and , ultimately , damaging society too .
Nature brings huge benefits to people . Some are tangible . In South Africa alone , the value of these benefits to people is estimated at R275 billion annually ( about 7 % of the country ’ s gross domestic product ). That value includes providing the country with crops , wood , water and fuel .
There are other , less immediately tangible benefits , too : air purification , water regulation , water purification , recreation , tourism , and cultural and heritage value .
One of the things that contribute to ecosystem degradation in South Africa is the invasion by alien plants . This is estimated to cost the nation R6.5 billion annually in damages and the government spends over R400 million annually clearing alien trees . Despite this investment , alien tree invasions continue to increase across the country .
Alien trees threaten biodiversity , increase the risk of more intense and frequent wildfires and also guzzle water . This is an important factor in waterscarce regions , like South Africa , that experience drought .
Alien trees are invading mountainous areas across South Africa . These are important water generating regions and the trees threaten water supplies in several cities , among them Cape Town and Gqeberha . Both have experienced water shortages in recent years .
To find out just how much alien trees threaten the water supply , we conducted a hydrological study . The research set up the most fine-scale , detailed models possible to try and estimate how alien trees affect streamflow in four small mountain catchments above some of Cape Town ’ s major dams . This study also used satellite imagery to input accurate information on the types of alien trees and where they are .
Key findings The models predicted that clearing catchment areas fully infested with mature invasive alien trees can increase streamflow by between 15.1 % and 29.5 %. Although the catchments modelled are currently not fully invaded , this presents a strong argument for preventing full invasion .
The study also found that streamflow gains from clearing alien trees from rivers were almost twice as high as clearing the alien trees from the surrounding land . That ’ s because alien trees in rivers have access to an almost endless water supply and so use more .
Another interesting finding was that clearing alien trees seemed to have a greater impact on the mid to low flows – in other words during the dry season when the river flow is low , rather than the high flows – during rainfall events in the wet season when the rivers are full .
This makes sense : during rainfall events , there is so much water that the negative effects of alien vegetation become less evident .
But it is important because it implies that clearing alien trees makes more water available in between rain events , especially in the dry season . This is useful to improve water security during droughts .
The positive effect of clearing alien trees was also predicted to be higher in dry years compared to wet years . This suggests that clearing alien trees is a viable measure to ensure there will be more water when it is most needed .
It is useful to explain what the savings predicted by our models mean to Cape Town ’ s overall water supply – and to consumers . For instance , we found that clearing the current levels of invasion in the catchments above the Berg River Dam ( currently 9 % invaded ) could increase streamflow by over 1 %.
This doesn ’ t sound like a lot , but it could mean an increase in the mean annual volume of as much as 1.5 million m ³ or 4.1 million litres per day .
According to the 1:50 year yield model for the Berg River Dam , this equates to a 0.2 % increase in yield . Putting this into perspective with a quick first-order calculation , from the City of Cape Town ’ s Water Strategy , we have a value of R9 per kilolitre for the operating costs of desalination . If we multiply this by the
Grassroots Vol 22 No 2 July 2022 26