Long term (1921–2011) hydrological regime of Alpine catchments in Northern Italy

Abstract

Long term (1921–2011) yearly and seasonal hydrological regime of 23 Alpine rivers in Northern Italy (ca. 102–104km2) was investigated here. First, for regulated catchment, the potential effect of flow storage was investigated using an index of potential flow regulation, and pre and post reservoirs’ installation flow analysis. For catchments displaying little regulation effect, non stationarity was studied using linear regression, including variable (segmented) slope analysis, and Mann Kendall test, traditional and progressive. The link of the observed trends against descriptive physiographic variables was then investigated, to highlight geographic and topographic patterns of changes of the hydrological cycle. Dependence upon global thermal and North Atlantic Oscillation NAO anomalies were analysed to highlight potential impact of large scale climate drivers against regional hydrological regimes. Also, the correlation between stream flows and climatic drivers of precipitation and temperatures in nearby stations was investigated, to highlight climate trends potentially driving hydrological changes, and potential changes in the nexus between climate and hydrology given by reservoirs’ operation.The results display for several Alpine rivers here likelihood of significant changes of hydrological fluxes, notably during Winter, Spring and Summer. Winter discharges is decreasing on average, but decrease is seen below 1800m a.s.l. or so, while increase is found above, and the more Northern the larger the increase. Discharges during Spring mostly decrease in time, and more so for increasing outlet altitude, while Summer specific discharges always decrease, and more notably with increasing altitude of the contributing catchment. NAO and global thermal anomalies are correlated against the rate of variation of hydrological fluxes, with the intensity of correlation linked to altitude, longitude, and basin’s size. Specifically targeted studies are necessary to investigate the underlying mechanisms of modified hydrological cycle within different catchments. Besides the presence of expectedly little changes as given by flow operation in regulated streams, the observed trends may be explained by modified hydrological cycle within the Alps of Italy, as given by (i) trading of rainfall for snowfall during Winter, resulting into larger flows, and affecting more highest catchments and Northern areas, (ii) lack of snow cover at thaw, and shrinking of ice covered areas, decreasing melt water deliver during Spring, and Summer, more evident at the highest altitudes, and (iii) increase of evapotranspiration driven by temperature, leading to increased soil moisture uptake and decreased stream fluxes at the intermediate altitudes. The proposed study seems of interest as a benchmark for the assessment of water resources in the Alps, and for conjecture upon future water availability.