Accelerating Water Cycle in Mountain Catchments: The Role of Snow in Runoff Dynamics

Streamflow in central European mountain catchments is strongly influenced by snow. Rising air temperatures are causing a shift from snowfall to rainfall, a decrease in snow storage, and earlier snowmelt. Therefore, the question emerged of whether these changes could contribute to changes in catchment transit times and thus lead to acceleration of the water cycle. This study aims to quantify 1) whether the increasing number of partial snowmelt periods during winter resulting from increasing rainfall compared to snowfall affects the partitioning of the snowmelt runoff into soil and groundwater components, 2) how it affects selected hydrological signatures in late spring and early summer, and 3) examine the influence of catchment elevation on the above processes. To investigate changes in the runoff components, we used long-term simulations from 68 mountain catchments in Czechia covering the period from 1965 to 2019, using a conceptual, bucket-type catchment model. The model was evaluated against observed daily runoff and snow water equivalent (SWE). We analysed temporal trends in the fraction of fast (event) and slow (baseflow) runoff responses, calculated as monthly or seasonal fractions of the individual components to total runoff (Q_fast/Q_tot; Q_slow/Q_tot). The statistical significance of temporal trends was evaluated using the Mann-Kendall test. The elasticity index was calculated to describe how sensitive the fractions are to changes in SWE and snowmelt volume. Additionally, we investigated how the catchment characteristics, particularly elevation and geographic region, influenced these relationships to provide a more comprehensive understanding of water cycle dynamics across different mountains in Czechia. The preliminary results indicate that snow-poor years are characterized by a higher fraction of fast-response runoff during the winter months. In contrast, years with high maximum SWE lead to higher groundwater recharge which also contributed to higher low flows during late spring and early summer. Ultimately, the influence of SWE on the selected hydrological signatures becomes more pronounced with elevation.