Coupled temporal trends in river discharge and water quality across South Korea

Contemporary freshwater resource management seeks to simultaneously secure sufficient quantity and adequate quality for domestic, agricultural, livestock, and industrial uses, as well as to mitigate water-related risks. While river discharge and water level magnitudes have been extensively gauged worldwide, monitoring river water quality remains sparse in both spatial coverage and temporal resolution, largely due to substantial manpower and financial investment required. These limitations are further compounded by the frequent misalignment between hydrological watershed boundaries and administrative units, which complicates the direct integration of water quality measurements with corresponding river discharge records. Consequently, previous studies have typically been confined to a limited number of subbasins when investigating temporal trends and concentration-discharge (C-Q) relationships. Overcoming these limitations requires a coupled temporal perspective, in which the co-evolution of discharge and water quality can be systematically examined to understand watershed responses to changing climate conditions and to track the effectiveness of water management actions. In this study, we leverage 11 years (2014–2024) of hourly and daily observations from an automated national water quality monitoring network in South Korea, encompassing temperature, pH, total nitrogen, total phosphorus, and total organic carbon. This unique dataset enables coupled temporal analyses of river water quantity and quality trends across river networks spanning the country’s four major basins. Mann-Kendall test shows increasing trends in discharge at most gauging stations, contrasting with the largely insignificant trends reported during the 20th century. However, four homogeneity tests (Pettitt, Buishand, von Neumann, and standard normal homogeneity test) indicate that these increases are attributable to distinct change points rather than gradual monotonic trends. Although homogeneity tests are traditionally applied to identify artificial discontinuities caused by station relocations or changes in data-quality control procedures, the found change points here appear to reflect hydrological responses to climatic forcing. By jointly applying trend and homogeneity analyses to both discharge and water quality variables, we examine how abrupt hydrological shifts propagate through physiochemical, nutrient and organic carbon dynamics at the national scale. Understanding whether water quality responses to discharge regime changes are concurrent, lagged, or threshold-driven provides pivotal insight for total maximum daily load management and for assessing eutrophication risk under a changing climate.

Acknowledgements

This work was supported by the Creative-Pioneering Researchers Program through Seoul National University and by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIT) (No. RS-2025-00523350).