The hydrogeological role of trees in water-limited environments

Field experiments have already proven that many tree species in water-limited environments (WLE) depend on groundwater. Typically, such trees survive dry seasons and droughts by uptake of water, directly from the groundwater body or from the capillary fringe, by rooting systems that may extend to several tens of meters depth. Such trees are also very efficient in finding soil moisture in the unsaturated zone, reducing groundwater recharge. Considering that WLE are typically characterized by low recharge, and that trees may use a significant amount of groundwater, this groundwater "consumption" should not be neglected in groundwater balancing, modeling and resources management. In practice, groundwater uptake by trees in WLE is either underestimated or disregarded because of limited knowledge about that phenomenon. This review discusses the current understanding of the hydrogeological role of trees in water-limited environments, the partitioning of tree transpiration into groundwater and unsaturated zone contributions and the integration of that partitioning in numerical groundwater models. Problems involved in this research are highlighted and possible future research directions are discussed.