Long term statistics of river flow regime: climatic or anthropogenic changes?

River flow time series are far from being stationary and always experienced changes in the past, also dramatic in long time horizons. In recent years it seems that both climatic and anthropogenic factors are accelerating the variability of hydrological processes. It is not clear, however, whether climatic or anthropic factors represent the major forcing to the hydrological cycle. Long term statistics of river flow regime for some Italian rivers in the Central Alps are presented and compared with precipitation, temperature, land use and glacierised areas data. It is shown that for some rivers the increased agricultural water demand and land use changes are a major source of non stationarity, possibly more relevant than meteorological ones. Another natural feedback which is being observed at the global scale is discussed: the reduced speed of global deforestation and the natural afforestation in Europe occurring over the last two decades. This process can play a major role in regulating the hydrological cycle, mitigating flood and drought extremes, but also enhancing evapotranspiration losses and thus reducing runoff volumes.