Clearing Alien Trees Boosts Water Supply #AfricaClimateCrisis

Droughts in water scarce regions are being worsened by human-driven climate change. While rainfall reduction is the main driver, increased temperature and evaporation can also play a role, write a group of scientists in The Conversation.

One way of offsetting the impacts of climate change on water resources is catchment restoration, which can involve revegetation, wetland restoration, gully rehabilitation, and alien tree clearing. This is one of several types of nature-based solutions, which focus on reducing climate change impacts through working with nature.

In a recently published analysis, the scientists took a closer look at the role of climate change and catchment conditions during the 2015-2017 Cape Town "Day Zero" drought. This drought led to one of the most serious water crises experienced by any city in the world in recent times. The drought arose from a prolonged shortage of rainfall and, consequently, river flows, which progressively depleted reservoir storage in the area.

The scientists 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. But it could not have removed all of the human-driven climate change impact on water availability.

Day Zero refers to a period of severe water shortage in the Western Cape region, most notably affecting the City of Cape Town. While dam water levels had been declining since 2015, the Cape Town water crisis peaked during mid-2017 to mid-2018 when water levels hovered between 15 and 30 percent of total dam capacity.

In South Africa, invasive alien tree clearing is a well-known form of catchment restoration. These trees have spread across many mountain catchments critical for water supply. Alien trees use substantially more water than native vegetation because they are taller, have greater leaf areas, and have deeper root systems. These characteristics increase the amount of water lost due to transpiration from the tree, compared to native plants, leaving less water for river flow and groundwater recharge. Impacts from alien trees on water are felt especially during drought periods when water is most needed.