Aquifer-atmosphere interactions within watersheds on annual to decadal timescales

The primary goal of this work is to improve our understanding of the interaction between groundwater and the atmosphere. In the past, in quantitative studies, hydrologists have tended to exclude feedbacks with the atmosphere while climate scientists have tended to exclude feedbacks with groundwater. In this thesis, I argue that to understand watersheds we must view the complete system of aquifers, the unsaturated zone, and the atmosphere. Considering the interactions between each component of the hydrologic cycle will allow us to better understand the effects of potential climate change on water resources.; In this work, a coupled atmosphere-land surface-groundwater numerical model was developed to examine aquifer-atmosphere interactions in watersheds on annual to decadal timescales. The two studies presented here estimate the contribution of groundwater to evapotranspiration in two watersheds. The first study compares simulations with and without an aquifer represented in the model, as a means of distinguishing the effects of groundwater on annual evapotranspiration. The second study examines the effect of aquifer hydraulic conductivity on evapotranspiration, streamflow, and lake levels, under variable climatic forcing.; Quantitative results demonstrate that when the water table is high enough to encroach on plant roots and the soil zone, it acts as a source of water for evapotranspiration, on annual to decadal timescales. During periods of drought, a high water table in the lowlands supplies more water for evapotranspiration than would otherwise be available through precipitation and soil moisture. The hydraulic conductivity of the aquifer affects how readily water can flow from the uplands to raise the water table in the lowlands. In the first study, compared to a run without an aquifer represented, annual evapotranspiration was about 5% (wet year) to 20% (dry year) greater for the fully coupled simulation incorporating an aquifer. In the second study, during a drought period, annual evapotranspiration increased by as much as 7% when the hydraulic conductivity of the aquifer was increased by two orders of magnitude. Also, during a drought period, streamflow was greater and lake level fluctuations were smaller, with increased aquifer hydraulic conductivity.