In situ monitoring of rain infiltration and evapotranspiration in the critical zone using self-potentials

Characterizing and monitoring water flow in the critical zone is of uttermost importance to understand the water cycle. Water link several process within critical zone from aquifer recharge and solute transfer to eco-hydrology, many eco-systemic services and biogeochemical reactions. However, the in situ quantification of water flow is technically challenging using traditional hydrological methods and numerous gaps of knowledge remain. The self-potential (SP) method is a passive geophysical method that relies on the measurement of naturally occurring electrical field. One of the contributions to the SP signal is the streaming potential, which is of particular interest in hydrogeophysics as it is directly related to both the water flow and porous medium properties. Unlike tensiometers and other point sensors, which use the measurement of state (e.g., matric pressure) at different locations to infer the intervening processes, the SP method measures signals generated by dynamic processes (e.g. water movement). However, the amplitude of the SP signal depends on multiple soil properties which are dependent to soil type, moisture content, and water chemistry (composition and pH). During the last decades, many models have been proposed to relate the SP signal to the water flow. In this contribution, we will present a soil-specific petrophysical model to describe the electrokinetic coupling generated from different water fluxes in the critical zone: rain water infiltration and water uptake from tree-roots. We tested a fully coupled hydrogeophysical approach on a large SP dataset collected in a two-dimensional array at the base of a Douglas-fir tree (Psuedotsuga menziesii) in the H.J. Andrews Experimental Forest in central Oregon, USA. We collected SP measurements over five months to provide insight on the propagation of transpiration signals into the subsurface with depth and under variable soil moisture. The coupled model, which included a root-water uptake term linked to measured sap flux, reproduced both the long-term and diel variations in SP measurements, thus confirming that SP has potential to provide spatially and temporally dense measurements of transpiration-induced changes in water flow. Similar set-ups are being installed on several test-sites of the French Critical Zone observatory network, OZCAR: Larzac, LSBB, Strengbach. This will allow us to test the approach under different climatic conditions, different soil types and in different ecohydrological systems.