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Clearing out alien trees can reduce the impact of climate change on Cape Town’s water supply

Clearing out alien trees can reduce the impact of climate change on Cape Town’s water supply

Aerial shot of the invasive alien pine trees that have grown on the Cape mountains.

Droughts are occurring in water-scarce areas. Climate change caused by humans has contributed to climate change.. While rainfall reduction is the primary driver, temperature and evaporation can also be important.

The catchment area, where rain falls and flows into rivers, is another factor that can influence drought severity. The catchment area’s vegetation and soil characteristics are important factors.

Catchment restoration can be used to offset the effects of climate changes on water resources. It can include revegetation, wetland rehabilitation, gully rehabilitation, alien tree clearing, and wetland restoration. This is just one type of restoration. Nature-based solutionsThese groups are focused on reducing the climate change impacts of working with nature.

But it’s not easy to tell what difference restoration makes on the impacts of Climate change caused by humansWater availability during droughts

In a Recently published analysisWe examined the impact of climate change on catchment conditions in the 2015-2017 period. Cape Town “Day Zero” drought. This drought resulted in one of the most severe water crises that any city has ever faced in recent years. The drought resulted from a prolonged shortage in rainfall, which led to river flows that gradually depleted the reservoir storage in the area.

We wanted to see whether clearing alien trees from mountain catchments where the city’s water comes from would have lessened the impacts of climate change on the drought. This is the first study to examine the potential of nature-based solutions for protecting society from extreme events directly related to climate change.

We found that clearing alien trees before the drought hit could have reduced the impact of climate change on water supply during the “Day Zero” drought. However, it could not have eliminated all the human-driven climate change impacts on water availability.

Water supply and alien trees

South Africa invasive alien tree clearingThis is a well-known method of restoration catchments. These trees have spread to many mountain catchments that are vital for water supply. Alien trees Use significantly more waterBecause they are taller and have more leaf area, and have deeper root systems, they are better than native vegetation. These characteristics increase transpiration, which results in more water being lost to the groundwater, and less water for river flow. During drought periods, when water is most critical, the effects of alien trees on water are especially felt.

Aerial shot of the invasive alien pine trees that have grown on the Cape mountains.
The Cape Mountains are being choked by invading alien pine trees.
Cape Winelands Biosphere Reserve

This is not an isolated South African problem. All over the world, studiesAfforestation (planting trees) has been shown to reduce water yields.

Human influence on rainfall shortages and reduced river flow

Between 2015 and 2017, a very rare drought event – The worst since 1904 – pushed Cape Town into a Water crisis. Researchers previously discovered that climate change caused the reductions in rainfall that triggered this crisis. ThreeTo Six times more likely.

These studies were focused on the problem of rainfall shortage. But Cape Town’s water is dependent on river flows from distant mountainous catchments. These catchments are where the water comes from. Six large dams were used to capture and store the data.. During drought, dam levels fell to They are able to use less than 20% of their potential.

Our AnalyseThis study was done to determine whether removing alien trees from mountain catchments would have lessened the effects of climate change’s lower rainfall on river flows.

We used climate models and a hydrological modeling tool to simulate worlds with and without human-driven global warming and with or without catchment restoration and maintenance. This was done for two mountain catchments, which are typical of those that supply water. Western Cape Water Supply System which feeds Cape Town’s water delivery.

Our results indicate that climate change caused river flows to decrease by 12-29% during droughts, compared to a world where there was no human influence on climate. This was more than the 7-15% decrease in rainfall that we could attribute climate change to these catchments. The climate change impact was amplified by the catchments, which allowed water to move from rainfall to river flows.

Similar to Previous workWe found some influence of climate change on evaporation, but not very large.

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Our analysis revealed that clearing alien trees prior to the drought could have reduced the effect of climate change on river flows during drought. Clearing catchments with moderately infested alien trees (40% coverage) could have helped to reduce river flow reductions caused by climate change by 3-16%. By preventing the spread of alien tree species to cover the catchments, stream flow could be reduced by 10-27% due to climate changes.

Moving forward

Our analysis reveals that clearing all catchments completely would not have fully offset the climate change impacts.

While catchment restoration is a good option for managing drought risk, it does not address climate change’s impact on water resources. In water-scarce regions that are facing a warmer, drier future, catchment restoration must be combined with other adaptation options.

In many cases, however, climate change isn’t the only cause of water scarcity. Poor management, infrastructure, and services can easily override any benefits that nature-based solutions might offer.

Our results suggest that nature-based solutions are possible, but it is important to realize that these solutions must be contextually specific. It is crucial to understand the local conditions before designing any response.

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