Groundwater control on land surface fluxes, boundary layer structure, and precipitation

Current regional climate models use simple representations of surface and sub-surface hydrology, which precludes full interaction between the atmosphere and the land and the co-variability between the two intimately connected systems. Our hypothesis is that the groundwater reservoir can play a significant role in controlling soil moisture and the subsequent exchange of water and energy between the two systems. This in turn influences the boundary layer growth and structure, including instability, atmospheric water recycling, as well as spatial and temporal variability of precipitation. To test the hypothesis, we apply RAMS-Hydro, a recently developed, coupled regional climate and hydrology model, to investigate how a dynamic water table modulates land-atmosphere coupling. We perform three numerical experiments over North America, during May-October, 1997. The control experiment includes a dynamic water table, the second allows free drain at the bottom of the soil (a common approach in climate models), and the third allows no drain (used in standard RAMS). We find that the three different drainage schemes can lead to different land-atmosphere coupling. That is, including the groundwater reservoir can alter the simulated land-atmosphere feedbacks, the degree of the alteration depending on the hydro-climatic conditions, water and energy limitations, and whether the water table is either shallow or deep.