Hydraulic Connectivity Between the Sava River and Lake Jarun During Drought: A High-Resolution Monitoring and Time Series Approach

Abstract

This study offers the first detailed insight into the hydraulic relationship between the Sava River and Lake Jarun, an urban lake formed from a former river meander in Zagreb, Croatia. The main objective was to quantify and understand the temporal variability of their hydraulic connectivity under drought conditions. Continuous, high-resolution (hourly) water level monitoring of Lake Jarun, established in April 2022, was combined with interpolated Sava River water levels derived from national gauging stations. Using a suite of statistical tools – Pearson correlation, linear regression, cross-correlation, and the sliding window correlation method – the study assessed the strength, direction, and timing of river–lake interactions. The results revealed a clear but dynamic subsurface connection, with the lake typically responding to river fluctuations after a delay of five to six days. During the exceptional drought in the summer of 2022, this response weakened significantly, and the time lag extended beyond 21 days, indicating reduced hydraulic connectivity. The sliding window approach further revealed temporal variability, with sharp declines in correlation during low-flow periods and rapid recovery during high-water events. These findings highlight the non-stationary nature of hydraulic connectivity in regulated floodplains and the sensitivity of such systems to hydrological extremes. In addition to providing the first high-resolution dataset for Lake Jarun, this study introduces a time-adaptive analytical framework that enhances the understanding of short-term river–lake interactions and supports future management of urban water systems under increasing climatic pressure.