Electrical resistivity imaging to link evapotranspiration and groundwater fluxes in the critical zone

Abstract:

We investigate the spatial and temporal controls on water partitioning in the critical zone using a research framework that incorporates pedological, geological, and ecological variables. Previous analyses have found that ET and groundwater are coupled under certain temporal and spatial regimes within many forested watersheds. Two conceptual models developed to describe the connection between groundwater and ET are: (1) riparian interception, and (2) hydraulic pumping. In riparian interception, vegetation captures water moving laterally; whereas in hydraulic pumping, ET drives soil matric potential gradients towards the surface resulting in passive uptake of groundwater. Quantification of these processes has been difficult due to the challenge of separating hillslope and riparian controls. To identify controls influencing this connection, we implement a combination of established tools, including electrical resistivity (ER), sap flow sensors, water-level gauges, soil moisture sensors, sediment thermal probes, and weather stations. ER results show high spatial variability in ground electrical conductivity and at both diel and seasonal timescales. An analysis of ER data showed that the greatest diel fluctuations in ground electrical conductivity occur in areas closet to large, or numerous clustered trees. These data allow us to test two hypotheses: (1) Hillslopes with both riparian and upslope vegetation will transition from riparian interception to hydraulic pumping as low baseflow (flow from groundwater to stream) conditions are approached. Hillslopes lacking riparian vegetation will be controlled by hydraulic pumping during baseflow recession. (2) Throughout hydraulic pumping, the degree of diel fluctuation in baseflow will be correlated with soil moisture changes. As soil moisture decreases, transpiration and baseflow will become more decoupled. Preliminary analysis of data from the deployed instruments, combined with co-located water and soil samples for isotopic analyses, has yet to unveil distinctly different hillslope and riparian controls on ET-groundwater interactions. Yet, we have identified interesting temporal connections amongst tree uptake, ground electrical conductivity, atmospheric temperature, watertable elevations, and stream discharge.