Le cycle de lʹeau et les activités au sein de lʹespace rural. Enjeux globaux, solutions locales et régionales

The analysis of the water cycle emphasizes the weight of surface evaporation compared to rain and the weight of water availability compared to evaporation. The ocean is a huge machine producing water vapour contrary to the continents, where evaporation is limited by plants that must shield themselves from drying and particularly by bare soils where the surface drying slows down the water losses. The availability of water for the biosphere depends on the difference between rain and evaporation. This difference, often called ‘net supply’, can be modified by anthropogenic actions on the vegetation; for example, the anthropogenic actions such as drying climate (reducing evapotranspiration) increases the net supply; on the opposite, the large development of more and more covering vegetation (arboreous system, bocages, savannah...) decreases the net supply (larger surface evapotranspiration all over the year). Moreover, the feedback between climate and continental interface can modify the rainfall pattern and the thermal equilibrium, leading ecosystems to extreme situations such as drying climate and desertification (ET lower and lower) or equilibrium situations corresponding to large development of vegetation (ET close to EP). The anthropogenic pressure and the human food requirements lead human beings to keep nearly the whole available water of the biosphere, particularly to practise irrigated agriculture. This practise is on average two times more efficient than the rainy agriculture (which is why it is attractive); moreover, its water efficiency decreases according to the environment and its aridity. Except for arboreous systems and agroforestry, agriculture is always a system that leads more or less towards aridity, but we have to bring it under control and to make it sustainable. A good management of the whole rain requires an efficient organization of the territory to optimise the water consumption in order to protect the environment and its biodiversity, to increase the agricultural production, to avoid pollutions and to reduce everywhere it is possible the natural degradation (deforestation–aridification–desertification), even to reverse it. It is the reason why, in situations nearly always unstable and very often difficult, water is inseparable from environment, and any approach on water management cannot be imagined and evaluated out of the context defined by the triptych ‘anthroposphere–biosphere–technosphere’. To cite this article: A. Perrier, A. Tuzet, C. R. Geoscience 337 (2005).