Abstract

In many environments, infiltrating water transits unsaturated bedrock en route to groundwater, surface water or the atmosphere via evapotranspiration. In recent decades, individual studies across a range of geologic and climatic conditions have demonstrated the importance of bedrock water storage (i.e. rock moisture, a term parallel to soil moisture) to ecosystem resilience and productivity. The circulation of water by deep roots in bedrock has direct consequences for spatiotemporal patterns of groundwater recharge, biogeochemical cycling, and geochemical weathering reactions. However, transpiration of rock moisture and associated impacts are largely inferred because we have limited tools for quantifying water fluxes, residence times, and storage volumes within the bedrock vadose zone. This lack of direct observation serves as a central impediment to directly linking behavior within the bedrock vadose zone to the larger context of a catchment's water budget, solute exports, or plant nutrient supply. Here, I review recent approaches for investigating hydrologic and associated biogeochemical processes within the fractured bedrock vadose zone drawing upon research associated with the Eel River Critical Zone Observatory in Northern California and the East River Watershed Function Scientific Focus Area near Crested Butte, Colorado. I focus on the challenge of partitioning water storage in the bedrock vadose zone between plant water uptake, groundwater drainage and storage, as well as identifying whether transpired water is sourced from unsaturated or saturated conditions. I compare inferences made via water budget analyses and stable water isotope and geochemical analyses to direct measurements of water composition and water content dynamics made within the bedrock vadose zone. I compare geophysical measurements of water storage such as nuclear magnetic resonance, neutron moderation, gamma ray logging, and electrical resistivity with in-situ sensing via lysimeters and time-domain reflectometry and transmission. Findings highlight both advances and challenges in making direct measurements that advance our ability to incorporate the fractured bedrock vadose zone into model frameworks for mass and energy exchange at the land surface.

Citation

Rempe, D.M. (2019): Progress and challenges in investigating the fate of water stored in the fractured bedrock vadose zone. American Geophysical Union 2019 Fall Meeting, San Francisco, CA, 9-13 December 2019 Abstract #H13A-03 (Invited).

This Paper/Book acknowledges NSF CZO grant support.