New research has revealed how an invasion of the alien evergreen tree Prosopis juliflora seriously diminishes water resources in the Afar region of Ethiopia, consuming enough of this already scarce resource to irrigate cotton and sugarcane generating some $320 million and $470 million net benefits per year.

A team of Ethiopian, South African and Swiss scientists, including lead author Dr. Hailu Shiferaw, Dr. Tena Alamirew and Dr. Gete Zeleke from the Water & Land Resource Centre of Addis Ababa University, Ethiopia, Dr. Sebinasi Dzikiti from Stellenbosch University, South Africa, and Dr. Urs Schaffner, head of ecosystems management at the Centre for Agriculture and Biosciences International (CABI), have been assessing the water use of Prosopis and its impacts on catchment water budget and rural livelihoods in the dry Afar region since 2015, as part of a long-term collaboration in the framework of the CABI-led Woody Weeds project.

Their new study, published in Scientific Reports, provides evidence that this alien tree, which has invaded both the floodplains of the Awash River and the surrounding dryland habitats, uses excessive amounts of water by consuming approximately 3.1-3.3 billion m3 of water per year in the Afar region.

Shiferaw said, "We found that single trees of the evergreen Prosopis consume between 1-36 liters of water per day, depending on stem diameter and site conditions. Prosopis trees not only use water throughout the year, but they also consume more water during the dry season, when almost all native plants have shed their leaves. The high sap flow of Prosopis in the drylands throughout the year may be due to exceptionally deep roots that penetrate up to 50 m below the surface, where they tap into groundwater that cannot be used by native trees with shorter roots."

In the context of climate change and an increasing frequency of drought events in dry regions of sub-Saharan Africa, the report concludes that this invasive tree is likely to have serious consequences for sustainable livelihoods in the region unless its spread is contained and its density reduced.

Schaffner, the senior author of the study, said, "Since its introduction in the Afar region in the 1980s, Prosopis has invaded 1.2 million [hectares] of land. Thus, unless the spread of Prosopis is contained and the density reduced in areas where it has become established, this invasive tree is likely to have serious consequences for sustainable livelihoods in the region. The estimated net benefits from water savings alone would strongly justify the implementation of a coordinated control program."

The report clearly supports findings from work undertaken in South Africa on water use by invasive tree species. Professor Brian van Wilgen of Stellenbosch University, previous scientific advisor to the Working for Water program in South Africa and partner of the Woody Weeds project, said, "In South Africa, invasive alien trees are estimated to reduce surface water runoff by between 1.5 and 2.5 billion [cubic meters per year], and this could increase substantially as the invasions continue to spread. In addition, invasive trees in drier parts of the country have substantially reduced water in groundwater aquifers on which local farmers and towns are totally reliant."

He further explained that, "These losses have serious consequences for a country where water scarcity limits economic activity and growth. The government in South Africa has responded by creating a multimillion dollar, national-scale program, dubbed Working for Water, to control invasive alien trees, and has also passed legislation preventing further propagation of invasive alien trees and requiring landowners to control them."

Additional information

Find more information about Prosopis juliflora on CABI’s Invasive Species Compendium.

Find more information about CABI’s work on woody weeds on the Woody Weeds project page.

Find more information about CABI’s work on invasive species at Action on Invasives.

Full paper reference

Hailu Shiferaw, Tena Alamirew, Sebinasi Dzikiti, Woldeamlak Bewket, Gete Zeleke and Urs Schaffner, "Water use of Prosopis juliflora and its impacts on catchment water budget and rural livelihoods in Afar Region, Ethiopia," Scientific Reports, 29 January 2021, DOI: 10.1038/s41598-021-81776-6.

This paper is available for open access view from 10 a.m. GMT, 20 January, 2021.