Target-referenced interpretation of surface water quality monitoring data using entropy-weighted indices: a case study of the Tanjiang River Basin, China

Routine monitoring data provide an important basis for evaluating river water quality and supporting watershed management. In this study, a target-referenced assessment framework was applied to interpret surface water quality monitoring data in relation to section-specific regulatory objectives. By linking the entropy-weighted Water Quality Index (WQI) with target-based reference conditions, the Water Quality Compliance Index (WQCI) was used to provide a composite interpretation of the relative distance between observed water quality conditions and designated management targets. Monthly water quality data from 80 monitoring sections in the Tanjiang River Basin, China, during 2019–2024 were analysed. The conventional Water Quality Index (WQI) was first calculated using entropy-derived indicator weights, and target WQI values were then derived from the corresponding regulatory water quality classes. The WQCI was used to quantify the relative distance between observed water quality conditions and designated management objectives. The results show clear temporal and spatial differences in water quality compliance across the basin. Non-flood seasons generally exhibited better compliance conditions than flood seasons, while several monitoring sections showed persistently lower compliance stability. Indicator-level analysis suggests that DO and COD contributed strongly to spatial–temporal differentiation within the monitoring dataset. Compared with the conventional WQI, the WQCI provided additional information on whether similar overall water quality scores corresponded to different compliance pressures under different target classes. The WQCI framework used in this study offers a practical approach for linking routine monitoring results with regulatory objectives and identifying priority areas for river basin management.