作者：Huang, L., Fan, X., Huang, L., Gao, S. & Song, C.

River mouths serve as critical interfaces linking terrestrial and marine systems, where water color integrates signals of watershed processes, human activities, and hydrodynamic conditions. However, the complex interplay of river discharge and optical constituents necessitates effective remote sensing monitoring. The Forel-Ule Index (FUI), a 21-scale colorimetric classification, offers effective robust indicator of water quality. This study quantifies the spatial patterns, decadal trends, and seasonal variability of water color across 3,209 global river mouths using Landsat-8 imagery from 2013 to 2024. The results show that global river mouths exhibited a mean FUI of 17.76 ± 2.65, with substantial spatial heterogeneity. High FUI values were predominantly observed in, densely populated low-latitude regions (e.g., Southeast Asia, western Central America), while lower values were observed in Arctic coastal regions and the mid-latitude areas of the Northern Hemisphere. Small river mouths generally showed higher FUI than large ones due to limited dilution capacity, with regional exceptions in the Southeast Asia, Amazon, West Africa and West North America. Over the past decade, global FUI displayed a slight overall increase, with rising trends prevailing in the Arctic and low-latitude Southern Hemisphere, contrasting with declines in mid-latitudes. Seasonally, FUI peaked in summer and reached a minimum in winter. Notably, nearly 30% of river mouths exhibited negative trends across seasons, particularly in summer and winter, whereas persistent increases were mainly concentrated in eastern Australia. This study provides a global assessment of river-mouth water color dynamics, revealing the distinct scale effects and regional disparities in river mouth water quality. It is expected to offer critical scientific evidence for understanding land–ocean interactions under the dual drivers of climate change and anthropogenic perturbations.

河口是连接陆地与海洋系统的关键界面，水色在其中整合了流域过程、人类活动与水动力条件的综合信号。然而，河流径流与光学组分的复杂相互作用，使得遥感监测成为必需。福雷尔—乌尔指数（FUI）是一种21级比色分类体系，可作为有效且稳健的水质指示指标。研究利用2013—2024年Landsat-8影像，量化了全球3209个河口的水色空间格局、年代际趋势与季节变化。结果显示：全球河口平均FUI为17.76 ± 2.65，空间异质性显著。高FUI值主要分布于人口稠密的低纬度地区（如东南亚、中美洲西部），而低值见于北极沿岸及北半球中纬度地区。受稀释能力所限，小河口的FUI普遍高于大河口，东南亚、亚马逊、西非与北美西部则存在区域性例外。过去十年，全球FUI总体呈轻微上升趋势，北极与南半球低纬度以升高为主，中纬度则以下降为主，二者形成鲜明对比。季节尺度上，FUI于夏季达峰、冬季最低。值得注意的是，近30%的河口在各季节均呈下降趋势（尤以夏季与冬季为甚），而持续性上升主要集中于澳大利亚东部。本研究提供了河口水色动态的全球评估，揭示了河口水质中显著的尺度效应与区域差异，有望为理解气候变化与人为扰动双重驱动下的海陆相互作用提供关键科学依据。

（来源：Journal of Hydrology 2026  DOI: 10.1016/j.jhydrol.2026.135628）