Senegal’s Faidherbia trees draw water from deep underground; scientists found their roots may help sustain groundwater recharge at 26% of rainfall

A tree growing across Senegal’s dry agroforestry landscapes may help move water deeper into the soil. Scientists studied ‘Faidherbia albida’ to understand how its roots influence soil water movement and groundwater recharge in an arid environment. The study combined several years of soil moisture and sap-flow measurements with hydrological modelling to examine what happens to water below the ground. The study published in the Journal of Hydrology notes that the tree’s roots can locally alter soil hydraulic behaviour, helping water move towards deeper layers. Researchers estimated that sustained groundwater recharge accounted for about 26% of annual precipitation.How Faidherbia albida influences soil water and groundwater rechargeA study in Senegal has examined how Faidherbia albida trees affect water movement in dry agroforestry landscapes. The research focused on the tree’s role in the water balance, an area that is difficult to assess because underground water flows are hard to measure. Researchers combined soil moisture and sap-flow measurements with hydrological modelling. Their results reveal that the tree can change soil hydraulic behaviour around its roots, helping water move deeper into the soil. The study estimated that groundwater recharge beneath these systems may remain substantial, indicating that Faidherbia can have a positive effect on water balance in arid areas.How the study measured water movement around the treesFaidherbia albida is widely used in Sahelian agroforestry parklands because it provides several benefits and follows a reverse seasonal growth pattern. Yet its influence on water availability in these systems has been difficult to establish. The researchers addressed this uncertainty by studying an agroforestry parkland in Senegal and examining water movement beneath the trees.They used multiannual observations of soil moisture at different depths alongside measurements of sap flow. These observations showed how water was taken up by the trees and how it moved through the soil. The approach allowed researchers to examine processes that are difficult to capture accurately through direct field measurements.Researchers used two models to track water beneath the treeThe researchers combined two hydrological models with Bayesian inference to analyse the system. HYDRUS-1D provided a lower-cost, one-dimensional representation, while a kriging-based correction helped reproduce the response of the more detailed HYDRUS-2D model. The resulting multifidelity modelling approach was calibrated using observed soil moisture data and sap-flow measurements. The model reproduced observations with good accuracy and limited uncertainty during both calibration and validation. The analysis also confirmed the tree’s phreatophytic behaviour, that means its roots reach down to the water table and draw directly on groundwater, not just moisture held in upper soil layers.How does the root system affect water flow below the treeThe modelling showed that the tree’s roots can influence how water moves through the surrounding soil. The estimated soil hydraulic parameters indicated that the roots may change soil properties in a way that resembles preferential or macropore flow. As a result, water may move more easily into deeper soil layers around the root system. The researchers linked these conditions to sustained recharge fluxes of about 26% of annual precipitation. A sensitivity analysis found that fully compensated root water uptake could reduce groundwater recharge by 13%. These results indicate that although the tree draws water from the soil, its roots may also improve the downward movement of water.How Faidherbia albida may support water balance in arid regionsThe findings indicate that Faidherbia albida may have a net positive effect on the water balance in arid areas, even though its roots take up water from the soil. The study indicates that changes in soil hydraulic behaviour around the roots can help water move towards deeper layers and contribute to groundwater recharge.This means the tree’s role in the water cycle may involve more than simply using scarce soil water. Its overall effect depends on both the amount of water absorbed by the roots and how the root system influences water movement. The researchers used field observations and probabilistic modelling to assess these processes and their contribution to recharge.Catch the latest World News and Live updates. Download the TOI app.