The hydrological benefits of restoration: A modelling study of alien tree clearing in four mountain catchments in South Africa

Abstract Ecological restoration efforts at scale have been shown to play an important role in reducing human impact on the environment, improving climate change adaptation and halting extinctions globally. Upscaling restoration efforts needs funding, and therefore evidence of the benefits of restoration is needed. The aims of this study are firstly to improve the evidence base of the water-related benefits of restoration using a fine-scale modelling approach (with field work and remote-sensing inputs, and well-validated models), and secondly to explicitly quantify uncertainties to understand the general applicability of the principles discovered. We model the impacts of restoration (in this study the clearing of alien trees) on streamflow in four strategic water providing catchments using the fully-distributed MIKE-SHE modelling tool. We find that the benefits of clearing mature infestations of alien trees, such as pines, from naturally tree-less ecosystems increases available water resources by 15.1-29.5%. Clearing riparian invasions has a 1.7 times greater impact compared to terrestrial (non-riparian) invasions. The largest impacts of passive restoration on streamflow are in the mid to low flows, and this impact is greater in dry years relative to wet years. The findings are novel in that they shed light on the spatial uncertainties in the modelled gains. These findings have implications for leveraging investment to upscale restoration efforts in water scarce regions, as they suggest improved water security during the dry season and droughts. Upscaling efforts is essential if the degradation of ecosystems globally is to be prevented, halted and reversed, as proposed by the United Nations Decade on Ecosystem Restoration.