A multi‐century meteo‐hydrological analysis for the Adda river basin (Central Alps). Part II: Daily runoff (1845–2016) at different scales

Abstract

A high-quality daily runoff time series of the Lake Como inflow and outflow, the longest for Italian Alps, was reconstructed for the 1845-2016 period in the Adda river basin. It was compared with contemporary monthly precipitation and temperature observations and estimated potential evapotranspiration losses. Trend analyses were conducted for daily flow maxima and 7-day duration minima of inflows into the lake showing a non-significant decrease and a significant increase, respectively. Although the annual precipitation time series exhibits a non-significant decrease, annual runoff volumes decrease with a rate of −136 mmÁcentury −1 , with a significance level of 5%. Possible causes of variability of rainfall and runoff as North Atlantic Oscillation (NAO), Atlantic Multidecadal Oscillation and Western Mediterranean Oscillation indexes and sunspot activity were also explored. Wavelet spectra analyses of monthly precipitation and runoff show some changes in the energy both at small and large scales and are effective in pointing out phenomena as droughts and the effects of dams' regulation. Conversely, wavelet coherence spectra indicate a weak correlation of NAO and sunspots with precipitation. In addition, the analysis of temperature and potential evapotranspiration tendencies suggests that the decrease of runoff has to be ascribed mostly to anthropogenic factors, including water abstraction for irrigation and increased evapotranspiration losses due to natural afforestation and, only in part, to climatic variability.

A multi‐century meteo‐hydrological analysis for the Adda river basin (Central Alps). Part I: Gridded monthly precipitation (1800–2016) records

“…The decrease of the runoff coefficient series seems to highlight long‐term changes in evapotranspiration probably mainly induced by increasing temperature, as will be discussed more in detail in the companion paper by Ranzi et al . (this issue), focused on the 172‐long time series of daily runoff. Another contribution to the variation of the evapotranspiration over the study basin could derive from changes in land use and coverage occurring over the study period, with the expansion of forests enhanced by the gradual decline of human communities and pastures over the mountainous areas (Guidi et al ., 2014; Ranzi et al ., 2017).…”

“…The region is characterized by a very heterogeneous orography, which is predominated by the mountain environment and includes some of the main glaciers of the Central Alps contributing, to limited extent, to the annual hydrological cycle of the study basin. A more detailed description of the study basin and of the water uptake regulation is provided in the companion paper (Ranzi et al ., this issue).…”

“…This area is covered by a relevant number of long meteorological observations and it has one of the longest daily runoff records in Italy which has been recently recovered over the period 1845–2016. This record is presented and analysed (Ranzi et al ., this issue) in the second part of the multi‐century meteo‐hydrological analysis for the Adda river basin of which this paper is the first part. The upper part of Adda river basin represents therefore a very interesting region to better investigate the response of Alpine water resources to climate variability and change.…”

Hydroclimatic Variability and Land Cover Transformations in the Central Italian Alps

“…To achieve this aim, upstream hydropower reservoirs contribute by releasing relevant volumes stored out of the irrigation season. In [14], long-term changes in monthly runoff regimes in the Adda river are shown, indicating a decrease in seasonality due to the reservoirs used for hydropower generation plants, requiring the storage of relevant water volumes during summer and autumn and the release in the cold months of January-April and November-December. However, on an annual basis, the natural runoff is not affected by reservoir management because of the water volume continuity.…”

“…In researching this paper, extended and high-quality time series of streamflow for five main rivers were collected: the rivers Adige, Mincio, Chiese, Oglio, and Adda, originating from the southern side of the Central Alps, from east to west. In particular, the time series of the Adda river [13,14] and the Adige river [15] span a period that is longer than a century and thus provide a nonordinary set of data that are highly valuable. The nonstationarity in precipitations and hydrologic losses can be deemed to be potential drivers of changes in runoff volumes on an annual time scale.…”