Incorporating water table dynamics in climate modeling: 3. Simulated groundwater influence on coupled land‐atmosphere variability

Abstract

Using a coupled regional climate‐hydrologic modeling system, RAMS‐Hydro, we investigate the role of the water table dynamics in controlling soil moisture, evapotranspiration (ET), boundary layer dynamics, and precipitation. In an earlier study we showed that a shallow water table can primarily exist in two types of hydrologic settings in North America: the humid river valleys and coastal regions in the east and the arid or semiarid intermountain valleys in the west. We also showed that the shallow water table in these settings can lead to significantly wetter soils than would exist without the presence of the water table. Here, we show that the water table–induced wetter soil directly maps into enhanced ET in the western setting, where soil water is a strong limiting factor of ET flux, but it is less likely to be the case in the more humid eastern setting where soil water is not limiting in general. We also ask whether any resulting enhanced ET will directly map into enhanced precipitation. Our hypothesis is that this can occur through two primary mechanisms: local, ET‐driven enhancement of convective precipitation and enhanced regional or lateral moisture convergence caused by altered soil moisture fields, and hence altered ET, far from the region of concern. We find that, indeed, water table–induced higher ET in the arid west results in greater convective precipitation and that ET‐precipitation coupling is primarily through local feedback pathways and precipitation recycling, with the main role of large‐scale moisture convergence as an initiator of convection following dry periods. Transitioning to the more humid regions farther east, the greater atmospheric (relative to surface) control of precipitation progressively obscures any potential effects of the water table, and the effects of large‐scale moisture convergence tend to dominate.