Incorporating water table dynamics in climate modeling: 2. Formulation, validation, and soil moisture simulation

In this second part of the two‐part series, we discuss the formulation and implementation of groundwater processes into an existing climate model, by linking a groundwater reservoir and a rivers‐lakes reservoir with its land surface scheme. We present the parameterization and validation of these processes with river flow and soil moisture observations. We use the new scheme as a tool to investigate the role of the groundwater reservoir in controlling the spatial and temporal structure of large‐scale soil moisture fields. We find that where the water table is shallow, the groundwater reservoir is linked to the soil water reservoir through two‐way fluxes. At these locations, the role of the groundwater shifts from being primarily a sink to being primarily a source for the soil, as the season progresses from the wet spring to the dry autumn. Through the two‐way fluxes, groundwater exerts a certain degree of control on the root zone soil moisture fields; there is an apparent spatial correlation between the distribution of shallow water table and wet soil. Since the water table reflects long‐term climatic and topographic forcing and exhibits strong spatial organization, its link to the soil moisture gives the latter a certain degree of spatial organization as well. The slow changing nature of the water table acts to stabilize the temporal variations in soil water, giving the latter stronger seasonal persistence.