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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, Auradé, 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 Auradé 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. Key Points: The long‐term critical zone (CZ) evolution controlling the regolith thickness strongly impacts the total water storage in watersheds, while the Quaternary geomorphological evolution influences the current hydrological partitioning and the separation of hydrologically active and inactive water storageBoth internal watershed characteristics and external forcings, such as current atmospheric forcing and recent land use need to be considered to infer stream persistence and to understand hydrological diversityThe observed hydrological diversity cannot be fully understood without considering a continuum of time scales in CZ evolution [ABSTRACT FROM AUTHOR]

Abstract: Chemical erosion rates have been determined on two upland granitic catchments under agricultural pressure in Brittany, France. Intensive agriculture has been carried out for at least 30 years in this region. The influence of geochemical processes related to agriculture on the chemistry of streamwaters is determined through a geochemical mass balance. The elemental export fluxes from these two agricultural catchments are then compared with other catchments around the world. The volume and concentrations of the precipitation are taken into account, as well as the inputs of organic and chemical fertilizers, groundwaters and streamwaters, to estimate the relative influence on export fluxes, and then evaluate the elemental fluxes released by weathering. The relatively high Si flux of about 1.8±0.9 kmol ha−1 yr−1 is directly attributed to the chemical weathering of soil and rock in the catchment system. However, the Si flux remains comparable to values found in both small and large-sized catchments under temperate and tropical conditions. On the other hand, extremely high fluxes of major cations (Ca, Na and Mg) are observed, ranging from 4.2±2.6 to 8.0±4.9 kmol ha−1 yr−1, which can be attributed to chemical weathering. These fluxes remain dramatically higher than those found in granitic catchments worldwide. Despite an integrated agriculture, the soil acidification induced by fertilizer application leads mainly to a release of major cations from the system, by processes of soil ion-exchange leaching as well as weathering of soil and rock. [Copyright &y& Elsevier]

Although nitrate export in agricultural catchments has been simulated using various types of models, the role of groundwater in nitrate dynamics has rarely been fully taken into account. We used groundwater dating methods (CFC analyses) to reconstruct the original nitrate concentrations in the groundwater recharge in Brittany (Western France) from 1950 to 2009. This revealed a sharp increase in nitrate concentrations from 1977 to 1990 followed by a slight decrease. The recharge concentration curve was then compared with past chronicles of groundwater concentration. Groundwater can be interpreted as resulting from the annual dilution of recharge water in an uncontaminated aquifer. Two aquifers were considered: the weathered aquifer and the deeper fractured aquifer. The nitrate concentrations observed in the upper part of the weathered aquifer implied an annual renewal rate of 27 to 33% of the reservoir volume while those in the lower part indicated an annual renewal rate of 2-3%. The concentrations in the deep fractured aquifer showed an annual renewal rate of 0.1%. The river concentration can be simulated by combining these various groundwater reservoirs with the recharge. Winter and summer waters contain i) recharge water, or water from the variably saturated zone with rapid transfer and high nitrate concentrations, and ii) a large contribution (from 35 to 80% in winter and summer, respectively) from the lower part of the aquifer (lower weathered aquifer and deep fractured aquifer). This induces not only a relatively rapid response of the catchment to variations in agricultural pressure, but also a potential inertia which has to be taken into account., (Copyright © 2012 Elsevier B.V. All rights reserved.)