Impact of Climatic and Land Use Changes on River Flows in the Southern Alps.

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, lasting
over 150 years, of annual runoff for the five major Italian rivers in the Central Alps
are presented and compared with precipitation, temperature and land use changes.
A homogeneous decreasing trend of annual runoff is observed, and the significance
of such a trend at the local and regional scale is tested with Mann-Kendall, Sen-
Theil and Sen-Adichie statistical tests. It is shown that for some rivers, the increased
agricultural water demand and land use changes are a likely major source of nonstationarity,
possibly more relevant than meteorological ones. A natural feedback
which is being observed also at the global scale is discussed on the basis of land
use in the Adige river basin by comparing cadastral maps of the mid-nineteenth
century with recent aerial photographs in four sample areas. Results are consistent
with the reduced speed of deforestation observed at the global scale and the natural
afforestation observed in Europe occurring over the last decades. This process can
play a major role in regulating the hydrological cycle and mitigating flood and
drought extremes, but also enhancing evapotranspiration losses and thus reducing
runoff volumes.