Hydrological variability and changes, today and tomorrow
Climate changes today as it always changed along the long Earth history, but anthropic actions are now playing their part in addition to astronomical and geophysical causes which have long been the only driving forces of its evolution. Mathematical modelling of Climate is certainly one of the greatest challenge ever faced by the scientific community, due to the multiplicity and complexity of involved phenomena, characterized by their interactions and their development along a large range of time and space scales. To diagnose changes, it is first necessary to better know, by as many studies of instrumental and historical time series as possible, the very nature of the natural variability of the different climate facets, which surely cannot be reduced to stationarity or to a linear trend. Modeling exercices are obviously necessary to size the field of possibilities and to help decision making, but the importance and difficulty of the task should incline to humility. Numerous uncertainties linked to climatological models, which are difficult to quantify, still limit their forecasting capabilities and their results should be handled cautiously especially as far as today and to tomorrow hydrology are concerned.