The project gives new perspectives on a long-standing debate of the impact trees have on groundwater resources. A popular perception is that trees are beneficial to water availability which has motivated tree planting projects across the tropics. But the present scientific paradigm until now has been that trees always consumes more water in transpiration and interception (evaporation on leaf surfaces) than other vegetation which has undermined support for these claims.

The study now published in Scientific Reports shows the truth in both perceptions. Based on field measurements in a cultivated woodland in Burkina Faso the team showed that an intermediate density of trees can maximize groundwater recharge in conditions that are common across much of the seasonally dry tropics. Without the trees these tropical soils lose their larger pores and much of the water is not able to enter deep into the soils. Instead water is lost through overland flow and evaporation on the soil surface. But with too high tree cover the trees use more water than is gained by the improved soil properties.

Access the article here or read the abstract below 

Abstract:

Water scarcity contributes to the poverty of around one-third of the world’s people. Despite many benefits, tree planting in dry regions is often discouraged by concerns that trees reduce water availability. Yet relevant studies from the tropics are scarce, and the impacts of intermediate tree cover remain unexplored. We developed and tested an optimum tree cover theory in which groundwater recharge is maximized at an intermediate tree density. Below this optimal tree density the benefits from any additional trees on water percolation exceed their extra water use, leading to increased groundwater recharge, while above the optimum the opposite occurs. Our results, based on groundwater budgets calibrated with measurements of drainage and transpiration in a cultivated woodland in West Africa, demonstrate that groundwater recharge was maximised at intermediate tree densities. In contrast to the prevailing view, we therefore find that moderate tree cover can increase groundwater recharge, and that tree planting and various tree management options can improve groundwater resources. We evaluate the necessary conditions for these results to hold and suggest that they are likely to be common in the seasonally dry tropics, offering potential for widespread tree establishment and increased benefits for hundreds of millions of people.

Read more about the Focali members who have been involved in this study by clicking on their names below.