Excessive water use is one of the major impacts of invasive alien trees, and it is especially important in arid and semi-arid parts of the world. In eastern Africa, a large proportion of the land receives less than 600 mm mean annual rainfall, and it is in these water-scarce areas that invasive alien Prosopis trees will have the largest impacts on water resources. Prosopis trees are physiologically well adapted to survive and outcompete native vegetation, having deep taproots as well as an extensive network of shallow roots that allows them to effectively utilize both deep groundwater and water deposited on the soil surface by rain. Prosopis trees also use this root network to redistribute water in the soil profile, providing surface water to shallow roots during times of drought and moving water into deep storage in times of excess. A variety of methods have been used to estimate water use by Prosopis trees, including measuring sap flow in individual trees with heat pulse techniques and stand-level evapotranspiration with micrometeorological methods such as the eddy covariance method. Stable isotopes of hydrogen and oxygen in water have also been used to determine the source of water used by trees. These studies have been used to estimate that Prosopis juliflora tree invasions are using between 3.1 and 3.3 billion m(3)/year in the Afar region of Ethiopia alone (around 7 litres of water per tree per day on average). This has significantly contributed to the estimated loss of ecosystem services valued at USD 602 million per year over the past three decades, about four times greater than the annual budget of Afar State. Climate change and an increasing frequency of droughts are set to exacerbate these impacts, due to an increase in evapotranspiration and reductions in groundwater recharge brought about by invading Prosopis trees.