Climate and anthropogenic activities play an important role in streamflow regime variations, but analyses of hydrologic changes often do not differentiate between these factors. Trends in precipitation (Pm), temperatures (T), potential evapotranspiration (ET0), and outflow (O) in the Reno River Mountain Basin (RRMB) during the period 1926–2006 were analyzed with the aim of separating the effects on the outflow of climate change and anthropogenic factors. Human influences studied include water withdrawal for domestic and irrigation use, reservoir management, and land use changes. The abandonment of mountain areas, which mainly occurred in the 1950s, resulted in a reduction in cultivated areas from almost 40 to 5% and an increase in forests, pasture, and meadows from 44 to 78%. The minimum temperatures present a stronger positive trend respect to the maximum temperatures. Comparing the hydrological catchment budget, Pm, ET0, and O decreased by almost 7, 10, and 43%, respectively. Increasing actual evapotranspiration (ET) was the key hydrological variable to explain the outflow decrease in RRMB. The expansion of forests and pastures and the less human pressure that should lead to better conditions of water supply for the plants permitted ET to equal ET0. The overall results of the research showed the effects of external anthropogenic factors acting on the outflows were more relevant than climate change.
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