Plant transpiration and groundwater dynamics in water-limited climates: Impacts of hydraulic redistribution

The role of groundwater in sustaining plant transpiration constitutes an important but not well-understood aspect of the interactions between groundwater, vegetation, the land surface, and the atmosphere. The effect of the hydraulic redistribution (HR) process by plant roots on the interplay between plant transpiration and groundwater dynamics under water-limited climates is investigated by using the Variable Infiltration Capacity Plus (VIC+) land surface model. Numerical experiments, with or without explicitly considering HR, are conducted on soil columns over a range of groundwater table depths (GWTDs) under different vegetative land covers, soil types, and precipitation conditions. When HR is not included, this study obtains transpiration-GWTD relationships consistent with those from watershed studies that do not include HR. When HR is included, the transpiration-GWTD relationships are modified. The modification introduced by HR is manifested in the soil moisture of the root zone. The mechanism of HR is explained by detailing the roles of the hydraulically redistributed water, the upward diffusion of soil water, and the daytime root uptake. We have found that HR is particularly important in water-limited climates under which plants have high transpiration demand. At the beginning stage of a dry period, HR modulates the severe impacts that climate has on plant transpiration. Only after a prolonged dry period, impacts of HR are lessened when the groundwater table drops below the depth of water uptake by roots and are diminished when plant transpiration is decoupled from groundwater dynamics.