[Analysis the Source and Spatiotemporal Variation of Dissolved Organic Matter in the Chaohe River Watershed].

Dissolved organic matter （DOM） is widespread in aquatic environments and plays a crucial role in various biogeochemistry processes. Urbanization and anthropogenic activities have significantly altered the source and characteristics of DOM in rivers. Therefore, analyzing the spatial and temporal variation of DOM on a watershed scale to trace its source for effective water quality management is important. In this study, the composition characteristics of DOM in the Chaohe River watershed, which is the source of drinking water in northern China, were analyzed using the EEM-PARAFAC method. Furthermore, the source of DOM was further identified by analyzing the relationships among optical parameters. The results revealed the four components in DOM： component 1 resembled fulvic acid, while component 2 and component 4 exhibited characteristics similar to those of humic compounds. Component 3 displayed tryptophan-like acidity, with the highest intensity observed during both high and low flow periods. Temporal and spatial variation in fluorescence parameters further indicated a distinct source of DOM across the three water periods. Notably, non-point source pollution was prominent during the low water period, whereas soil-related characteristics dominated during the flood season. Spatial analysis revealed that under natural vegetation cover, terrestrial detritus significantly influenced DOM originating from the source area. However, urbanization-induced anthropogenic pollution had become more pronounced in the middle reaches. Additionally, the downstream area with extensive aquatic vegetation coverage experienced notable impacts from phytoplankton proliferation. Correlation analysis demonstrated varying degrees of association between water environmental parameters, nitrogen and phosphorus content, optical parameters, and DOM across three hydrological periods. These findings revealed that water environment dynamics and nutrient sources govern the temporal and spatial distribution patterns of DOM.