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

Modeling root water uptake provides a powerful tool for illustrating the tight linkage of phreatophytic vegetation with spatial and temporal soil water content variation and groundwater level fluctuations. In this study, we develop a new model framework to simulate root water uptake of phreatophytic vegetation using system dynamics approach. The model simulates root water uptake from saturated and unsaturated zones driven by the potential gradients along the groundwater-soil-plant-atmosphere continuum (GSPAC). It incorporates variable soil and plant hydraulic conductivity properties. A new water stress function is introduced; it considers the influence of both soil water and groundwater on phreatophyte water stress. This function is based on the "vulnerability curve" theory that the loss of hydraulic conductance along the soil-plant pathway limits the plant's capability of extracting and transporting water for transpiration. The model calculates energy balance (water potential changes) and water balance (water content changes) in both soil and plant media simultaneously and has the ability to simulate hydraulic redistribution. This model is applied to simulate water uptake of Quercus douglasii (blue oak) in a California savanna; this species has previously been identified as an obligate phreatophyte. The model results show good agreement with the measured ET, soil moisture, and leaf water potential data. The model indicates that the primary water source of blue oak switches from soil water in wet season to groundwater in dry season. In June, July and August, the average groundwater uptake rate is 13 mm/month, which contributes over 90% of dry season transpiration. During the extreme dry period, the rhizosphere resistance increases significantly and becomes the dominant resistance along the GSPAC. Under such conditions, uptake of groundwater becomes more thermodynamically favorable than uptake of soil water. In addition, the model predicts that during the dry season, cavitation will lead to a hydraulic conductivity loss of approximately 85% in the shallow roots, further constraining soil water uptake. The new water stress function shows good performance in simulating daily transpiration with R-squared value of 0.68. The function also reproduces the physiological interactions and feedbacks between transpiration and root and stomatal conductance—root hydraulic conductance loss reduces stomatal conductance and stomatal conductance reduces transpiration rate to regulate plant water potential and prevent plant hydraulic failure. Hydraulic redistribution leads to significant soil water redistribution and promotes over 8% water uptake for annual transpiration. The model illustrates that with the existence of phreatophytes, deep groundwater can hydraulically impact top soil layers through plant water uptake and hydraulic redistribution.