Abstract
Extreme streamflow nonstationarity has probably attracted more attention than mean streamflow nonstationarity in the assessment of the impacts of climate change on the water cycle. Nonetheless, a significant decrease in mean streamflow could lead to conditions of scarcity of freshwater in the long-term period, seriously compromising the sustainability of the demand for civil, agricultural, and industrial uses. Regional analyses are useful to better characterize an area’s nonstationarity, since a clear trend at a global scale has not been detected yet. In this article, long-term and high-quality series of streamflow discharges observed in five rivers in the Central Italian Alps, including two multicentury series and two new precipitation and streamflow series not analyzed before, are investigated to statistically characterize individual trends of mean annual runoff volumes. Nonparametric pooled statistics are also introduced to assess the regional trend. Additional climatic and nonclimatic factors, namely, precipitation trends and land cover transformations, have also been considered as potential change drivers. Unlike precipitation, runoff volumes show a marked and statistically significant decrease of −1.45 mm/year, which appears to be homogeneous in the region. The land cover transformation analysis presented here revealed extensive woodland expansions of 510 km2 in 2018 out of the 2650 km2 area measured in 1954, representing 38% of the area investigated in this study: this anthropic driver of enhanced hydrologic losses can be recognized as an additional likely cause for the regional runoff volume decrease.
Highlights
The assessment of the impact of climate change on streamflow discharges continues to be one of the most crucial issues in hydrologic research and water engineering applications [1]
510 km2 in 2018 out of the 2650 km2 area measured in 1954, representing 38% of the area investigated in this study: this anthropic driver of enhanced hydrologic losses can be recognized as an additional likely cause for the regional runoff volume decrease
Applying the results reported in [38], the 20% increase in woodland we identified in the investigated area for the 1954–2018 period would result in a 20 mm decrease in runoff volume, on average, a value that can explain a major part of the 36 mm of runoff volume losses that cannot be explained by claiming the enhanced potential evaporation due to temperature increase only, assessed with a −36 mm/century rate for the Adda watershed in [14]
Summary
The assessment of the impact of climate change on streamflow discharges continues to be one of the most crucial issues in hydrologic research and water engineering applications [1]. Changing trends could have dramatic consequences on a number of issues, such as the sustainable exploitation of freshwater resources or the mitigation of flood risk. This problem has been the subject of study for several years, definitive and general outcomes have not been achieved yet. The existence of a global trend in streamflow discharges has recently been questioned by the scientific community. According to the most recent comprehensive analyses [4,5], there is little certainty of a correlation between an increasing trend in global river discharge and global warming in the 20th century. Studies investigated streamflow discharge trends in near-natural rivers in more than 400 small watersheds located in 15 countries across Europe from 1962 to 2004 [6,7]
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