Résumé
We develop a holistic approach of watershed hydrology, focusing on the interfaces between critical zone components and relying on interdisciplinarity. We mainly explore the first Unsolved Problem in Hydrology (UPH): Is the hydrological cycle regionally accelerating/decelerating under climate and environmental change, and are there tipping points? and 5 other UPH (3,4,7,12,19). As feedbacks exist between the critical zone components, forming a complex system, it can lead to the emergence of tipping points and to shifts between hydrological regimes.A hydrological regime shift is indeed observed since 1950 in the Sahel (semi-arid region, Africa). Stream flows continuously increased in this region despite a severe regional drought in the 70s-90s. This suggests that a tipping point could have been surpassed, because of changes in climate and/or human practices. To explore this hypothesis, we develop a new modeling approach, relying on long term observations (1950-present) of hydrology, geomorphology and land used/land cover. We use System Dynamics modeling to capture feedbacks between water, soil structure, vegetation and flow connectivity between hillslope, channel and aquifer.The modeling results accurately represent the observed decade-scale evolution of the hydrological regime at several watershed scale (1-10000 km²). This shows that tipping points exist in semi-arid land hydrology and allows to explore which processes are at play. We identified domains within watersheds where regime shifts occurred at small scale, propagated across scales and led to large-scale shifts. Ultimately, we aim at identifying areas where the risk of an irreversible hydrological regime shift is high under various climatic and socio-economical possible futures.This study illustrates that using system dynamics to represent the critical zone is complementary to classical hydrologic modeling based on hydrodynamics. While the system-based approach represent correctly the decadal trends of the hydrological regime, the hydrodynamics approach did so for floods and drought at minute/year time scales.