Combination of geophysical, hydrological and geochemical investigations to better understand and model past, present and future watershed functioning. – HYDROCRIZSTO

The project will be handled within four work-packages (WPs) with very strong interactions between the first three.
- WP1: underground imaging and groundwater survey: Subsurface geophysics. This WP consists in combining different geophysical methods in order to build a spatial geometric image of the different depth and superficial lithological structures of the catchment. Groundwater storage will be estimated by gravimetry and RMS;
- WP2: Surface/subsurface hydrology and water resources. Geophysical and geochemical approaches with biospheric and hydrologic modeling will be gathered to improve our knowledge of the hydrological functioning at the watershed scale (water storage, water pathways, water balances….);
- WP3: Water/rocks/vegetation interactions. Geochemical/isotopic tracers, mineralogical and ecological data, and laboratory experiments will be used to better identify and characterize the water/rock interactions and the biogeochemical signature of soil solution, springs and stream waters.
- WP4: Impact of climate change on water resources and soil mineral fertility. Calibrated models obtained through WP2 and WP3 will be used to estimate the evolution of water resources and soil mineral composition until 2100.
The link between the 3 first WPs can be summary as:
- WP1 will provide statistical information about the underground structure and water volume to WP2 and WP3
- WP2 will provide parameter distribution and water volumes to WP1 to assess petrophysical relationships and geophysical surveys. WP2 will also provide water pathways and travel time to WP3.
- WP3 will provide constraints from the water/rock processes and therefore evaluate the reliability of the results given by WP2.
The three first WPs will work inside an iterative loop between the WPs until a good match between modeling and observations is reached.

WP1 : geophysical studies at different scale have been realized (CSAMT, siesmic, RMN, ERT, gravimetry).
WP2 : Modeling of the water flux at the watershed scale and modeling of solutes and water at the plot scale in the first meter on soil.
WP3 : Experiments have been realized with natural soils from two different plots located in beech and spruce stands. Soil samples from 6 horizons (until 80 cm depth) are disposed on batch series (triplicate) with two different acids (hydrochloric acid and oxalic acid). The solutions are analyzed at different time from 24h to 150 days. DRX, SEM, chemical and isotopic (Sr) characterization are determined on soil and solution. Strontium isotope data aimed to decipher mineral weathering, atmospheric deposit and biological activity.
The results show highly variable chemical behavior versus element, type of soil, horizons and type of acid. In addition we observed unexpected results for calcium. Indeed the total quantity of Ca extracted after 150 days correspond to a very low proportion of the calcium exchangeable obtained with cobaltihexamine, underlining a complex bioavailability of this nutrient in soil.
WP4
The B-WITCH model has been used to simulate the chemical composition of soil solution on two sites (beech and spruce sites) of Strengbach catchment over 1987-2013 period. The calibration of model over the last twenty five years could permit us to estimate the chemical compostion evolution of surface water up to 2100. The runs have not been completed yet.

WP1 : The continuation of temporal record of gravimetry at the catchment scale during more than one hydrological year will allow to map the water recharge variation, in function of the pluviometry.
The RMS results allow to determined a map of the spatial distribution of water volume
WP2 : Streamlines of the subsurface contributing water to the surface draining network have been established and will be calculated for contrasted hydrological period.
WP3 : The isotopic ratios of the experimental solution, echangeable fraction, clays, bulk soils allow to identified the source of nutrient but also the mecanisms responsible of their availbility.
WP4 : The calibration of model over the last twenty five years could permit us to estimate the chemical compostion evolution of surface water up to 2100. The runs have not been completed yet.

1. Beaulieu, E., Lucas, Y., Viville, D., Chabaux, F., Ackerer, P., Goddéris, Y., Pierret, M-C. 2016. Hydrological and vegetation response to climate change in a forested mountainous catchment. Model. Earth. Syst. Environ. 2 :191.
2. Belfort B., Toloni I., Ackerer P., Cotel S., Viville D. Lehamnn F., 2018. Vadose Zone Modeling in a Small Forested Catchment: Impact of Water Pressure Head Sampling Frequency on 1D-Model Calibration. Geosciences, 8(2), 72; doi.org/10.3390/geosciences8020072
3. Younes A., Zaouali J., Fahs M., Slama F., Grunberger O., Mara T. Bayesian soil parameter estimation: results of percolation-drainage vs infiltration laboratory experiments, submitted to Journal of Hydrology (2018).
4. Pierret M.C., Cotel S., Ackerer P., Beaulieu E., Benarioumlil S., Boucher M., Boutin R., Chabaux F., Dambrine E., Delay F., Fourtet C., Friedmann P., Fritz B., Gangloff S., Girard J.F., Legtchenko A., Viville D., Weill S. and Probst A., in revision, The Strengbach Catchment: a multidisciplinary environmental sentry for 30 years. Vadose Zone Journal
5. Weill, S., Delay, F., Pan, Y., & Ackerer, P. (2017). A low-dimensional subsurface model for saturated and unsaturated flow processes: ability to address heterogeneity. Computational Geosciences, 21(2), 301-314.

Understanding how the environment reacts to anthropogenic or natural disturbances at short- and long-term time scales is one of the major societal and scientific challenges in the field of natural resource management and conservation. Among the wide field of the environmental challenges, this project aims to evaluate water and soil resources, related to climatic changes (rainfall regime, temperature increase) and anthropogenic actions (forest management) in medium altitude forested watersheds. For that, the project proposes a detailed understanding of the transport processes of water and its related chemical fluxes to elaborate physically based models, which should be applicable and adaptable in other climatic, ecologic and geologic environments. These models will be able to simulate and thus predict future evolution of such a natural ecosystem in response to disturbances like climate change or logging.
Thus, the aim of this project are (i) to develop a methodology based on tight coupling of several geophysical, hydrological and geochemical approaches to estimate water and solute fluxes and their associate models at a watershed scale and (ii) to evaluate the methodology on the Strengbach watershed (80 ha granitic catchment in NE of France- 90% vegetation cover- Vosges Massif). Since 1986, the climatic, hydrological and geochemical parameters of this watershed have been recorded, which represents one of oldest monitored sites on granitic basement in the world.
The project will be handled within four work-packages (WPs) with very strong interactions between the first three.
- WP1: underground imaging and groundwater survey: Subsurface geophysics. This WP consists in combining different geophysical methods in order to build a spatial geometric image of the different depth and superficial lithological structures of the catchment. Groundwater storage will be estimated by gravimetry and RMS;
- WP2: Surface/subsurface hydrology and water resources. Geophysical and geochemical approaches with biospheric and hydrologic modeling will be gathered to improve our knowledge of the hydrological functioning at the watershed scale (water storage, water pathways, water balances….);
- WP3: Water/rocks/vegetation interactions. Geochemical/isotopic tracers, mineralogical and ecological data, and laboratory experiments will be used to better identify and characterize the water/rock interactions and the biogeochemical signature of soil solution, springs and stream waters.
- WP4: Impact of climate change on water resources and soil mineral fertility. Calibrated models obtained through WP2 and WP3 will be used to estimate the evolution of water resources and soil mineral composition until 2100.
The link between the 3 first WPs can be summary as:
- WP1 will provide statistical information about the underground structure and water volume to WP2 and WP3
- WP2 will provide parameter distribution and water volumes to WP1 to assess petrophysical relationships and geophysical surveys. WP2 will also provide water pathways and travel time to WP3.
- WP3 will provide constraints from the water/rock processes and therefore evaluate the reliability of the results given by WP2.
The three first WPs will work inside an iterative loop between the WPs until a good match between modeling and observations is reached. Due to an EQUIPEX (CRITEX) most of the equipment required for this project is already installed and operational. The consortium is based on 7 multidisciplinary institutes, 1 foreign researcher, 1 private office, an university department for science popularization and the ONF (French forest national agency). Most of the teams involved in this project have already worked together on the Strengbach watershed but never at this level of interdisciplinarity.
This project will provide a better understanding of the long-term variation of water and mineral nutrient availabilities and of forest health in middle altitude mountain areas.