ABSTRACT

The water cycle describes the movement of water on, above, and below Earth's surface and establishes where water exists. Fluxes quantify the rate of water movement among reservoirs such as groundwater, surface water, and atmospheric water vapor. Evapotranspiration is the primary mechanism supporting the surface-to-atmosphere water flux; it is the combined effect of evaporation from surface-water bodies and transpiration by plants drawing water from the soil and evaporating it from leaf surfaces. The National Research Council has identified understanding the interconnections between evapotranspiration and groundwater fluxes to be one of the most important challenges facing hydrologists today. This project addresses a critical knowledge gap in how subsurface water storage mediates the connection between evapotranspiration and groundwater dynamics such as water-table elevation and flow rates. Elucidating the connections between evapotranspiration and groundwater recharge that may limit irrigation agriculture and aquifer pumping is directly relevant to societal needs for food and water. The study focuses on a long-term research site in Oregon. Based on global climate models of the Pacific Northwest, stresses placed on groundwater by prolonged evapotranspiration are likely to become increasingly important to water availability for downstream communities. Results from this work will be incorporated into undergraduate curriculums. Underrepresented undergraduates will be engaged and mentored throughout the project.

Two conceptual models have been developed to explain evapotranspiration-baseflow interactions - riparian interception and hydraulic pumping - but their implications for critical zone models have yet to be explored. This project will test these conceptual models through isotopic measurements and subsurface imaging. The project will (1) use the temporal and spatial change in soil and tree xylem water isotopes to examine subsurface connections between transpiration, groundwater and streamflow; (2) image changes in moisture content in the subsurface through the weathered saprolite and into the unweathered critical zone; and (3) assess the importance of subsurface properties and antecedent moisture on the transfer of the evapotranspiration signal to the stream. The research will provide novel contributions by (1) identifying the mechanisms by which subsurface hydrological responses are coupled to tree physiological processes at the hillslope scale and (2) integrating real-time observations, isotopic analysis, and geophysical approaches to identify how evapotranspiration-groundwater interactions vary with space, time, and antecedent moisture. These results can transform the understanding of the interactions among surface water, groundwater, and soil moisture and the role of vegetation dynamics controlling the multi-scale hydrological functioning of terrestrial ecosystems.

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