A water availability and low-flow analysis of the Tagliamento River discharge in Italy under changing climate conditions

This study estimated the effects of projected variations in precipitation and temperature on snowfall-snowmelt processes and subsequent river discharge variations in the Tagliamento River in Italy. A lumped-parameter, non-linear, rainfall-runoff model with 10 general circulation model (GCM) scenarios was used to capture river response variations attributed to climate-driven changes in 3 future time periods in comparison to the present climate. Spatial and temporal changes in snow cover were assessed using 15 high-quality Landsat images collected during the 2001–2003 time period, which were further used to define different elevation bands to incorporate the elevation effects on snowfall-snowmelt processes. The 7Q10 low-flow probability distribution approximated by the Log-Pearson type III distribution function was used to examine river discharge variations with respect to climate extremes in the future. On average, the results obtained for 10 scenarios indicate a consistent warming rate for all time periods, which may increase the maximum and minimum temperatures by 2.3 °C (0.6–3.7 °C) and 2.7 °C (1.0–4.0 °C), respectively, by the end of the 21st century compared to the present climate. Consequently, the exponential rate of frost day decrease for 1 °C winter warming in lower-elevation areas is approximately three-fold (262%) higher than that in higher-elevation areas, revealing that snowfall in lower-elevation areas will be more vulnerable under a changing climate. In spite of the relatively minor changes in annual precipitation (−17.4 ~ 1.7% compared to the average of the baseline (1991–2010) period), snowfall will likely decrease by 48–67% during the 2080–2099 time period. The accumulated effects of a decrease in winter precipitation and an increase in evapotranspiration demand on winter river discharge will likely be compensated for by early snowmelt runoff due to increases in winter temperatures. Nevertheless, the river discharge in other seasons will decrease significantly, with a 59% decrease in the predicted river discharge in October over 100 yr. The low-flow analysis indicated that while the magnitude of the minimum river discharge will increase (e.g. a 25% increase in the 7Q10 estimations for the winter season in the 2080–2099 time period), the number of annual average low-flow events will also increase (e.g. 16 and 15 more days during the spring and summer seasons, respectively, in the 2080–2099 time period compared to the average during the baseline period), leading to a future with a highly variable river discharge. Moreover, a consistent shift in river discharge timing would eventually cause snowmelt-generated river discharge to occur approximately 12 days earlier during the 2080–2099 time period compared to the baseline climate. These results are expected to raise the concern of policy makers, leading to the development of new water management strategies in the Tagliamento River basin to cope with changing climate conditions.