A study of karst hydrosystem recharge at the parcel scale, using modeling and correlation analysis - Low noise underground laboratory of Rustrel site

La caractérisation des flux d’eaux qui rechargent réellement les hydrosystèmes souterrains reste un frein à la compréhension du fonctionnement hydrogéologique des milieux souterrains. Lors d’événements pluvieux, quelle part de l’eau est évapo-transpirée ? Quelle part est temporairement stockée dans le sol ? Ces incertitudes sont particulièrement fortes dans le cas de la recharge des milieux hétérogènes tel que le karst. En général, les calculs de recharge des hydrosystèmes karstiques se basent sur une représentation simplifiée de l’évapotranspiration qui considère seulement le climat et pas le fonctionnement de la végétation. Dans cette étude, un modèle de végétation permettant de simuler les transferts d’eaux entre le sol et l’atmosphère en contexte forestier (le modèle CASTANEA), a été appliqué à une parcelle de Chêne vert. L’infiltration efficace (un indicateur de la recharge) estimé avec CASTANEA a été comparée à celle estimée par des approches classiques ainsi qu’à des séries long terme de flux d’eaux souterraines (9 années). Les résultats de cette analyse révèlent que l’infiltration efficace modélisée à partir d’un modèle de végétation comme CASTANEA est plus satisfaisante que les approches classiques ne tenant pas compte du fonctionnement de la végétation. Ce travail ouvre des perspectives intéressantes pour mieux tenir compte du fonctionnement de la végétation et de l’usage du sol sur la recharge des hydrosystèmes karstiques.Assessing the recharge of underground hydrosystems remains an obstacle to understand their hydrologeological functioning. During a rain event, which part of the rain is evapotranspired ? And how much is temporarily stored within the soil ? These questions are particularly relevant in heterogeneous media such as karst hydrosystems. Currently, the models used to compute recharge of karst hydrosystems, rely on simplistic formulations of evapotranspiration that do not account for vegetation functioning. In this study, we used the vegetation process based model CASTANEA, which is designed to compute water transfer between soil, plant and atmosphere. We computed effective infiltration (an index of recharge) with CASTANEA and with other classical approach (based on precipitation minus ETP), and for a welldocumented holm oak site in Provence. Our results provide evidences that effective infiltration computed with CASTANEA yield more satisfactory correlation with measured outflow than simulations based on the classical approach. Our results provide a promising way to improve the simulation of karst hydrosystem recharge

Monitoring water flow in the critical zone using self-potentials: toward the quantification of rain infiltration and evapotranspiration

International audienceCharacterizing and monitoring water flow in the critical zone is of uttermost importanceto understand the water cycle. Water link several process within critical zone from aquiferrecharge 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, manymodels 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 up-take term linked to measured sap flux, reproduced both the long-term and diel variations inSP 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 OZCAR test-sites (Larzac, LSBB, Strengbach). This will allow us to test the approach under different climatic conditions, different soil types and in different ecohydrological systems

Monitoring water flow in the critical zone using self-potentials: toward the quantification of rain infiltration and evapotranspiration

International audienceCharacterizing and monitoring water flow in the critical zone is of uttermost importanceto understand the water cycle. Water link several process within critical zone from aquiferrecharge 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, manymodels 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 up-take term linked to measured sap flux, reproduced both the long-term and diel variations inSP 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 OZCAR test-sites (Larzac, LSBB, Strengbach). This will allow us to test the approach under different climatic conditions, different soil types and in different ecohydrological systems

Mesurer le potentiel spontané électrique en milieu forestier pour observer les échanges hydriques dans le continuum subsurface-végétation-atmosphère

International audienceThe electrical self-potential measurements are influenced by the variation of the water flowing in the soil and in tree trunks. We study here how such data acquired in a forest environment are sensitive to the different processes of water transfers occurring in the continuum subsurface-vegetation-atmosphere. In that goal, we designed and installed an experiment of electrical self-potential measurements on several tree trunks and roots and in the soil in a forest plot. Those measurements are completed with observations of the environmental variables.Les mesures de potentiel spontané électrique sont influencées par la variation des flux hydriques circulant dans le sol et au sein des arbres. Nous étudions ici comment ces données acquises en milieu forestier sont sensibles aux différents processus de transferts hydriques qui ont lieu dans le continuum subsurface-végétation-atmosphère. Pour cela, nous avons conçu et installé une expérience de mesures de potentiel spontané sur le tronc et au pied de plusieurs arbres d'une même parcelle forestière et complété ces mesures par des observations des variables environnementales

The geophysical toolbox applied to forest ecosystems – A review

International audienceStudying the forest subsurface is a challenge because of its heterogeneous nature and difficult access.Traditional approaches used by ecologists to characterize the subsurface have a low spatial representativity. This review article illustrates how geophysical techniques can and have been used to get new insights into forest ecology. Near-surface geophysics offers a wide range of methods to characterize the spatial and temporal variability of subsurface properties in a non-destructive and integrative way, each with its own advantages and disadvantages. These techniques can be used alone or combined to take advantage of their complementarity. Our review led us to define three topics how near-surface geophysics can support forest ecology studies: 1) detection of root systems, 2) monitoring of water quantity and dynamics, and 3) characterisation of spatial heterogeneity in subsurface properties at the stand level. The number of forest ecology studies using near-surface geophysics is increasing and this multidisciplinary approach opens new opportunities and perspectives for improving quantitative assessment of biophysical properties and exploring forest response to the environment and adaptation to climate change

Self-potential signals related to tree transpiration in a Mediterranean climate

International audienceTranspiration is a crucial process in the water cycle and its quantification is essential for understanding terrestrial ecosystem dynamics. Solely relying on sap flow measurements may not fully assess tree transpiration due to its complexity. Self-potential (SP), a passive geophysical method, may provide constraints on transpiration rates even if many questions remain about tree electrophysiological effects. In this study, we continuously measured tree SP and sap velocity on three tree species for one year in a Mediterranean climate. Using wavelet coherence analysis and variational mode decomposition, we explored the empirical relationship between tree SP and transpiration. Our analysis revealed strong coherence between SP and sap velocity at diurnal time scales, with coherence weakening and phase shifts increasing on days with higher water supply.We estimated electrokinetic coupling coefficients using a linear regression model between SP and sap velocity variations at the diurnal scale, resulting in values typically found in porous geological media. During a dry growing season, the electrokinetic effect emerges as the primary contribution to tree SP, indicating its potential utility in assessing transpiration rates. Our results emphasize the need for improved electrode configurations and physiochemical modeling to elucidate tree SP in relation to transpiration

First evidence of correlation between Evapotranspiration and Gravity at a daily time scale from two vertically spaced superconducting gravimeters

International audienceKey Points:•For the first time, two vertically spaced gravimeters allow to interpret small gravity hydrologically induced signal (<5 nm/s²).•Superconducting gravimetric signal are correlated with evapotranspiration at daily time step.•Gravimetry enables an integrative estimate of evapotranspiration particularly relevant for hydrology.Abstract: Estimating evapotranspiration (ET) is a primary challenge in modern hydrology. Hydrogravimetry is an integrative approach providing highly precise continuous measurement of gravity acceleration. However, large-scale effects (e.g., tides, polar motion, atmospheric loading) limit the fine time-scale interpretation of the gravity data and processing leads to residual signal noise. To circumvent this limitation, we exploited the difference between two superconducting gravimeters (SGs) vertically spaced by 512 m. The gravity difference allows to remove common large-scale effects. Daily variation of the gravity difference is significantly correlated with daily evapotranspiration as estimated using the water balance model SimpKcET (p-value = 4.10(-10)). However, this approach is effective only during rain-free periods. In the future, comparison with direct ET measurements (e.g., eddy-covariance, scintillometer) may confirm and strengthen our interpretation. Improved hydrogravimetric data processing could extend the proposed approach to other experimental sites equipped with a single SG. Plain Language Summary: Land evaporation and vegetation transpiration are crucial parameters in ecohydrology because evapotranspiration constitutes more than two-thirds of precipitated water at the continental scale. However, this invisible flux is difficult to characterize, especially at kilometric scale, and its quantification is challenging for the hydrologist community. Continuous gravity monitoring using a superconducting gravimeter is a direct estimation of the mass change of lands with high precision. At a mountain site in southern France, we highlight a significant association between evapotranspiration calculated by a numerical model and the mass loss of the mountain. This approach provides a novel way to monitor evapotranspiration that will reinforce traditionally used methods

Intra-specific variability in deep water extraction between trees growing on a Mediterranean karst

International audiencePlant transpiration is a major component of water fluxes in the critical zone, which needs to be better characterized to improve our ability to understand and model the hydrological cycle. In water-limited ecosystems such as those encountered on karst environments, climate-induced changes in transpiration are expected to be strongly influenced by the ability of the vegetation cover to resist or adapt to drought. However, because of the high heterogeneity of karst environments, the amount of water available for trees can change within a stand, which may lead to significant differences in drought vulnerability resistance between trees of the same species. So far it is not known if soil and subsoil environment influence the magnitude of deep water extraction, at the intra-specific scale. Here, we investigate the variability in deep water extraction for six individual Quercus ilex trees growing on a karst substrate in a Mediterranean forest. We combined three approaches: (i) electrical resistivity tomography to determine the variability of soil/subsoil characteristics, (ii) isotope tracing to determine the origin of water transpired by plants, and (iii) predawn and midday leaf water potential (Ψ) to assess the trees’ water stress and transpiration regulation. Along the summer season, deep water extraction increased with drought intensity. Deep water use varies between individuals and according to drought intensity. At moderate water stress levels, we found no significant relationship between the origin of xylem water and soil/subsoil characteristics or individual stress level. However, at the peak of the drought (average predawn Ψ < −2 MPa), individuals that had the least total available water in soil/subsoil (0–2 m) relied more on deep water and were also subject to less water stress. These results suggest that trees with less favorable soil/subsoil conditions (i.e. low water retention capacity) in the near surface (0–2 m) adapt their root systems to exploit deep water reserves more intensively so as to enhance their drought tolerance, while trees with more favorable surface conditions exhibit greater water stress and may be more vulnerable to extreme droughts because of a lower root development in deeper horizons