An Investigation of Interactions Between Plants and Hydrological Processes with Emphasis on Droughts Using A New Coupled Ecohydrological and Biogeochemical Model

Understanding interactions between plants and soil water is of paramount importance for constructing a comprehensive picture of how climate change could affect the hydrological response and how plants adapt to these changes. This study focuses on drought impacts since it is one type of extreme weather brought about by the climate change with devastating effects: how the drought affects plants and, conversely, how plants’ responses, in turn, affect the drought’s impacts. This study investigates impacts of droughts on plant growth associated with hydraulic redistribution, re-infiltration, surface and groundwater interactions, and other hydrological processes governed by the mechanism of plant optimality—plants optimize their use of available resources. This is achieved by coupling VIC+, an extended version of the Three-Layer Variable Infiltration Capacity model, with a terrestrial biogeochemical model--the Carnegie Ames Stanford Approach model of carbon, nitrogen, and phosphorus (CASA-CNP), and with a multiscale routing model as well. The VIC+ model is extended in this work to include the optimality theory in representing the behavior of plant stomatal conductance which plays a pivotal role in the soil-plant-atmosphere system. Specifically, the modeling strategy presented in Luo et al. (2013) of simultaneously using multiple plausible expressions, derived from different perspectives, in representing the same processes is employed to represent the plants’ stomatal conductance and enforcing them together with an optimality rule and a semi-empirical expression. Results in this study illustrate that such a modeling approach makes the new coupled model capable of representing the tight interactions between plants and hydrological processes with much less uncertainties. The coupled routing model makes it possible to consider the re-infiltration process along the flow routing paths. By integrating CASA-CNP into VIC+, one can investigate how drought affects the various interacting processes simultaneously. Using the new coupled ecohydrological and biogeochemical model, various scenario simulations have been conducted with different temporal distributions of precipitation intensities and durations but all with the same total precipitation. Results obtained demonstrate that: (1) the re-infiltration and plant optimality have significant impacts to the plant and water, energy budgets; and (2) the soil moisture is critical for plant mortality and plant regrowth