Effects of climate change on runoff variability in mid-latitude montane basins

The study analyzed long-term changes of runoff variability of headwater montane basins in Central Europe as a response to the effects of climate change and modifications to the environment.

The aim was to compare the patterns of variability of the indicators of hydrologic alteration, derived from long-term time series of daily discharge observations in montane basins with the recent premises of climate change effects on surface runoff dynamics in the Central Europe region. In particular, there were tested the following assumptions: (i) recent climate warming will result in the shifts of the runoff seasonality and distribution and in (ii) higher variability of runoff, displayed by a higher frequency of floods and droughts, while (iii) the indicators of runoff balance will remain without significant changes.

These hypotheses were tested in a set of 8 unregulated montane catchments, spreading over the border mountain ranges of the Czech Republic - the Šumava Mountains (Bohemian Forest), Krušné hory (Ore Mountains), Jizera Mountains, Krkonoše (Giant Mountains), Orlické Mountains and Beskydy Mountains. All basins are of comparable size (30-90 km2), and without significant hydrological regulations. Their west-east geographical distribution allows for tracking the potential effects of the gradient of climate continentality in the Central European region. The uninterrupted time series of daily discharge observations from 1953 to 2018 were used for the analyses at the gauging stations. 

We focused on indicators that reflect the aspects of the runoff regime, that are likely to be affected by the assumed effects of the changing climate. Variety of time series analysis and statistical techniques was applied, including the set of 33 Indicators of hydrologic alteration (IHA), 34 Indicators of Environmental flow components, frequency and distribution of the peak an low flows, statistical testing of significance of changes using Mann-Kendall test, breakpoint analysis, analysis of deficit and surplus volumes and homogeneity testing using Buishand, Petitt and SNHT tests.

The study has identified the significant shifts in the hydrological response of montane basins that are apparent in seasonality, balance, and variability of discharge. The analyses proved (i) changes in runoff response reflecting the timing of the observed changes in air temperatures, (ii) the shift of spring snowmelts towards earlier spring and a corresponding decline of may flows, occurring in all of the investigated regions, (iii) diverging trends of high flows across the basins, (iv) changing dynamics of rainfall-runoff response (v) better sensitivity of indicators, reflecting low magnitude events and (vi) decline of low flow indicators across the basins.