Surface- and groundwater interactions and their potential roles in modulating land-atmosphere interactions and regional climate under prolonged drought in the Southwest United States

IPCC Fourth Assessment predicted warmer and dryer conditions in the subtropical regions in the future. Particularly, climate models projected an imminent transition to warmer and more arid climate in the Southwest United States in the 21st century. Terrestrial response and its feedback to the atmosphere could be particularly important in determining the severity of droughts. In this study, we evaluate the response of the terrestrial hydrologic processes to future drought and their interactions with the atmosphere using a modified version of the Community Land Model (CLM4), in which the interactions between groundwater table, vertical distribution of soil moisture, and plant roots are represented by implementing parameterizations on surface and groundwater interactions from the Variable Infiltration Capacity (VIC) land surface model into CLM. The modified version of CLM4 (i.e., CLM4VICGROUND) is applied at small catchment and regional scales over the Southwest United States driven by observed and perturbed climate conditions to evaluate potential land surface responses and feedbacks in projected future Mega-drought. Our study suggests that surface and groundwater interactions, as an important aspect in terrestrial processes, might have profound impacts on different stages of droughts through its modification on root zone soil moisture, plant physiological responses, and therefore land and atmospheric interactions in the climate system. At the initial stage of drought, diffusion processes in the soil would help tap moisture in the deeper soil layers or groundwater table, and through stomatal response to maintain transpiration. However, when the groundwater table falls below a critical level as the land continues to dry, droughts could be intensified or prolonged as plants and surface processes become decoupled to subsurface processes. Simulations using CLM4VICGROUND coupled with the Weather Research and Forecasting model will be performed to evaluate the impacts of the feedback mechanisms on land-atmosphere interactions and regional climate.