Influence of groundwater on plant water use and productivity: Development of an integrated ecosystem - Variably saturated soil water flow model

Plant physiology influences the energy and water balance of the soil-plant-atmosphere continuum. However, impacts of soil water dynamics on plants in shallow groundwater environments are not completely understood, partially due to the limited ability of current models to simulate groundwater-vegetation interactions. In this study, we analyzed the influence of groundwater-induced soil temperature change on plant phenology, and the impact of variable depth to the water table on the net primary productivity (NPP), evapotranspiration and stomatal response, by integrating an advanced dynamic agroecosystem model (Agro-IBIS) and a variably saturated soil water flow model (Hydrus-1D) into a single framework. The model is first evaluated using field observations of soil moisture and temperature as well as annual NPP and weekly LAI measurements collected from three replicated maize plots at the Arlington Agricultural Research Station near Arlington, Wisconsin, USA. Comparisons showed reasonable agreement for each dataset without site-specific prior calibration. We then simulated the influence of groundwater on plant physiological responses as well as the energy, carbon, and water balance at the land surface. The model sensitivity analyses indicated that physiological functions of plants are sensitive to water table depth, and the aridity of a particular production site. For example, shallow groundwater limits water stress during dry years, helping to mitigate decreased NPP associated with water deficits. However, if the water table is persistently too close to the surface during the growing season, photosynthesis is negatively affected through oxygen stress on roots regardless of the aridity. To further explore factors influencing plant physiology other than oxygen stress, we designed simulations without oxygen stress effects. Results showed that under shallow groundwater conditions: (1) higher leaf level relative humidity causes higher water use efficiency because of a lower vapor pressure deficit between the leaf and atmosphere; (2) due to delayed corn plant emergence caused by cooler springtime soil temperatures reduces NPP. Our results suggest that models designed to more mechanistically simulate groundwater-vegetation interactions may lead to a more realistic representation of feedbacks between plant phenology, soil moisture, temperature, anoxia, NPP and ET. However, until critical data are collected to assess simulated feedbacks and advance our understanding of groundwater-vegetation interactions, model confidence will likely remain somewhat limited.