Exploring the Critical Zone Heterogeneity and the Hydrological Diversity Using an Integrated Ecohydrological Model in Three Contrasted Long-Term Observatories

An integrated ecohydrological modeling approach was deployed in three long-term critical zone (CZ) observatories of the French CZ network (CZ Observatories-Application and Research) to better understand how the CZ heterogeneity modulates the water cycle within territories. Ecohydrological simulations with the physically based model EcH2O-iso constrained by a wide range of observations crossing several disciplines (meteorology, hydrology, geomorphology, geophysics, soil sciences, and satellite imagery) are able to capture stream water discharges, evapotranspiration fluxes, and piezometric levels in the Naizin, Aurade, and Strengbach watersheds. In Naizin, an agricultural watershed in northwestern France with a schist bedrock underlying deep weathered materials (5-15 m) along gentle slopes, modeling results reveal a deep aquifer with a large total water storage (1,080-1,150 mm), an important fraction of inactive water storage (94%), and relatively long stream water transit times (0.5-2.5 years). In the Aurade watershed, representative of agricultural landscapes of the southwestern France developed on molasse, a relatively shallow regolith (1-8 m) is observed along hilly slopes. Simulations indicate a shallow aquifer with moderate total water storage (590-630 mm), an important fraction of inactive water storage (91%), and shorter stream water transit times (0.1-1.3 years). In the Strengbach watershed, typical of mid-mountain forested landscapes developed on granite, CZ evolution implies a shallow regolith (1-5 m) along steep slopes. Modeling results infer a shallow aquifer with the smallest total water storage (475-575 mm), the shortest stream water transit times (0.1-0.7 years), but also the highest fraction of active water storage (18%). Plain Language Summary Understanding how water is stored and released in landscapes is essential for predicting water availability in a changing world. It is of course driven by the local climate, but also by the landscape settings, from the vegetation to the belowground structure inherited from the geological history. In three intensively studied observatories across France, we used numerous field measurements and a numerical model of water-vegetation-subsurface interactions to better depict how differences between landscapes shape the water cycle. We found that the geological history determines how much water is stored in watersheds, through the thickness of water-holding rocks and soils. How this storage contributes to surface processes (including river flow) is modulated by more recent changes, from valley morphology and tectonics to agricultural practices and precipitation patterns. This study highlights that considering the continuity between long and short-term landscape processes is key to a detailed understanding of the water cycle.

Automated summary

An integrated ecohydrological modeling approach was used to investigate how the heterogeneity of the critical zone modulates the water cycle within territories. The study revealed that the geological history determines the water storage in watersheds, and this storage is modulated by recent changes in landscape settings.