Nature-based solutions in mountain catchments reduce impact of anthropogenic climate change on drought streamflow

Cape Town’s water supply is dependent on streamflow from a relatively small area made up of several mountainous headwater catchments. The streamflow from these catchments is captured and stored in six large dams. During the drought, dam levels dropped to <20% of their capacity which forced Cape Town to prepare for the day when the taps would run dry - Day Zero.

The Du Toits and Upper Berg are upland fynbos-covered catchments feeding into the Berg and Breede Rivers in the southwestern Cape of South Africa. The climate is Mediterranean and characterized by winter rainfall. Geology is typical of the mountains of the Cape Folded Belt of the Table Mountain Group and soils consist largely of sandstone-quartzitic soils which are highly leached and nutrient-poor. Both catchments are extremely mountainous and form part of South Africa’s strategic water source areas providing high natural runoff and supporting the region’s population and economy.

Quantifying how well Nature-based Solutions can offset anthropogenic climate change impacts is important for adaptation planning but has rarely been done. This report aims to show that a widely-applied Nature-based Solution in South Africa – invasive alien tree clearing – reduces the impact of anthropogenic climate change on drought streamflow. Using a multi-model joint-attribution of climate and landscape-vegetation states during the 2015–2017 Cape Town “Day Zero” drought.

AXA Research Fund, the  Danish Ministry of Foreign Affairs (MFA), BNP Paribas Foundation Climate Initiative

Anthropogenic climate change reduced streamflow by 12–29% relative to a counterfactual world with anthropogenic emissions removed. This impact on streamflow was larger than corresponding reductions in rainfall (7–15%) and reference evapotranspiration (1.7–2%). Clearing invasive alien trees could have ameliorated streamflow reductions by 3–16% points for moderate invasions levels. Preventing further invasive alien tree spread avoided potential additional reductions of 10–27% points. Total clearing could not have offset the anthropogenic climate change impact completely.

In terms of NbS, the study focused on a widely implemented and relevant form of restoration in South Africa: invasive alien tree (IAT) clearing. IATs are a major threat to water security especially in the Western Cape of South Africa and for Cape Town’s water supply. They have spread along riparian zones and upslope into mountains and have been shown, through paired-catchment and modeling studies, to use substantially more water in comparison to the native shrublands.

This phenomenon is not unique to South Africa. There have been similar results reported in other water-limited environments and/or winter rainfall-dominated areas through both modeling and paired-catchment studies. For example, in trading water for carbon, it has been repeatedly shown that replacing native grasslands with afforestation reduces streamflow. 

Clearing invasive alien trees could have ameliorated streamflow reductions by 3–16% points for moderate invasions levels. Preventing further invasive alien tree spread avoided potential additional reductions of 10–27% points. Total clearing could not have offset the anthropogenic climate change impact completely. Invasive alien tree clearing is an important form of catchment restoration for managing changing hydroclimatic risk but will need to be combined with other adaptation options as climate change accelerates.

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