From decadal runoff depletion to daily streamflow cycles: The role of riparian evapotranspiration in a forested catchment

Abstract

This study investigates the role of evapotranspiration in the hydrological balance of the small forested Vydrica catchment (Slovakia) by linking long-term climatic trends with short-term streamflow dynamics. A 60-year mass balance analysis (1961–2020) was integrated with high-frequency streamflow observations across nine severe summer drought events (1992–2025). The long-term analysis revealed a paradoxical trend: despite a slight increase in precipitation, annual runoff significantly decreased. This decline is driven by rising air temperatures and increased evaporative demand, causing altered seasonal runoff distribution and more severe summer low flows. The short-term event analysis demonstrated that diurnal streamflow fluctuations are strongly controlled by available baseflow. During moderate droughts, active riparian vegetation generates distinct diurnal streamflow cycles with a typical six-hour lag behind maximum air temperature. However, during extreme low-flow periods, the diurnal amplitude flattens and evapotranspiration losses drop by an order of magnitude, indicating that the groundwater table has fallen below the effective root zone. The findings prove that climate-driven long-term warming continuously depletes baseflow reserves, progressively restricting the daily transpiration capacity and resilience of riparian forests.