Understanding Groundwater Uptake by Phreatophytic Vegetation Using a System Dynamics Modeling Approach

Modeling groundwater uptake provides a powerful tool for estimating the groundwater consumption of phreatophytic vegetation. We developed a physically-based model framework to simulate groundwater uptake of phreatophytic vegetation using a system dynamics approach. The model simulated root water uptake from saturated and unsaturated zones and hydraulic distribution driven by the potential gradients along the groundwater-soil-plant-atmosphere continuum (GSPAC). A new water stress function based on the “vulnerability curve” theory was introduced; it integrated the influence of both soil water and groundwater on transpiration. The model was applied to simulate groundwater uptake of Quercus douglasii (blue oak) in a California savanna, and showed good agreement with the measured ET, soil moisture, and leaf water potential data. The model indicated that the primary water source of blue oak switches from soil water in wet season to groundwater in dry season. The annual groundwater consumption of blue oak is approximately 80 to 100 mm. The blue oak mainly relied on groundwater during the dry period that nearly 85% of dry season transpiration came from groundwater. The model also revealed that the dry shallow soil layers received water from groundwater at night through hydraulic redistribution; thus deep groundwater recharged lateral roots in top dry soil layers to maintain the lateral root survival during the drought. Groundwater uptake supported a large portion of plant water demand and served as a protection mechanism to mitigate the impacts of drought on plant subsistence. With the existence of phreatophytes, deep groundwater (e.g., 9 m in our case study) also can influence the surface energy and mass fluxes through plant water uptake and hydraulic redistribution. The proposed model framework can be further incorporated into the coupled groundwater, land surface and climate models to study how much groundwater is consumed by phreatophytes and how the groundwater depletion impacts phreatophyte survival.