CONNECTING EVAPOTRANSPIRATION AND GROUNDWATER FLUXES IN THE CRITICAL ZONE

The critical zone—an open system extending from the top of the canopy to the base of groundwater—can be a highly dynamic and heterogeneous environment. To investigate the spatial and temporal controls on water partitioning in the critical zone a research framework that incorporates an understanding of the pedological, geological and ecological variables is necessary to advance understanding of catchment hydrology. Quantifying vegetation-subsurface connections is difficult due to the heterogeneous nature of plant roots, evapotranspiration (ET), and soil moisture.

Here, we aim to quantify the connection between vegetation and subsurface water storage at the hillslope scale within a forested watershed. Previous analysis of stream hydrographs, water table records, and stable isotopes have found that ET and groundwater are coupled under certain temporal and spatial regimes at this site, as well as other forested watersheds. Two conceptual models have been developed to describe the connection between groundwater and ET: 1) riparian interception and 2) hydraulic redistribution. In riparian interception, vegetation captures water moving laterally, whereas in hydraulic redistribution, ET drives soil matric potential gradients towards the surface resulting in passive uptake of groundwater. Independent verification and quantification of these processes in the subsurface has been difficult due to the challenge of separating hillslope and riparian controls. This challenge arises from the inability to address these control(s) at the proper scale. Previous studies have been limited to point (too small) and catchment (too large) scales and thus have not been able to isolate the hillslope-scale controls. We aim to test two working hypotheses: (1) Hillslopes with both riparian and upslope vegetation will transition from being controlled by riparian interception in the early season to hydraulic pumping in the late season as low-flow, baseflow conditions are approached. Hillslopes lacking riparian vegetation will be controlled by hydraulic pumping during baseflow recession. (2) During hydraulic pumping, the degree of diel fluctuation in baseflow will be correlated with soil and rock moisture content in the critical zone. As soil and rock moisture decreases, transpiration and baseflow will become more decoupled.