Modeling Plant-Scale Root Zone Water Dynamics in an Oak Savanna

Study of water exchange between soil, plants, and the atmosphere in response to seasonal or periodic droughts is critical to modeling the hydrologic cycle and biogeochemical processes in water-controlled ecosystems. The difficulties in such studies arise from insufficient understanding of the complex interactions between the various processes and their scale-dependence. The purpose of our study is to establish and calibrate a plant biophysical model that couples plant-soil and plant-atmospheric interactions to calculate the water exchange through the soil-plant-atmosphere continuum at a plant scale (~10 m2), with the regulation of root water uptake and evaporative fluxes by water deficits and climatic conditions explicitly considered. The complexity required for modeling water dynamics at the plant scale is investigated in this study. We start with coupling a big-leaf biophysical model with a bucket soil water balance model, with soil water loss regulated by soil water availability in a linear fashion. The alternative biophysical models with increasing complexities include the dual-source model that divide the canopy into shaded and sunlit parts and a multi-layer 1-D model with sophisticated radiation transfer and energy balance modules. The level of detail in subsurface water dynamics is adjusted by changing the dimensionality of the Richard's equation. The impact of soil water availability on water loss is modified to a nonlinear pattern as desired. The models are calibrated and compared using a cluster of measurements collected on single trees, which includes multiple soil moisture probes that monitor soil moisture profile vertically and laterally and sap flow sensors at different tree heights for measuring tree transpiration. This study forms the basis for scaling up the water dynamics to a stand scale (~100 to ~10000 m2) or other larger scales.