Characteristics of Dissolved Organic Matter from a Transboundary Himalayan Watershed: Relationships with Land Use, Elevation, and Hydrology

Himalayan rivers play pivotal roles in regional water and food supplies, global sediments, and carbon budgets. We studied a transboundary mountainous Kosi River in the Himalayas by utilizing ultraviolet-visible spectroscopy and excitation-emission matrix coupled with parallel factor analysis. We have found that agricultural land use promoted the production of abundant proteinlike fluorescence in the surface waters [∼1.1-1.8 Raman unit (RU) for two proteinlike components that accounts for ≥97% of relative abundance] as opposed to natural vegetation land cover (e.g., forests and grasslands), despite the low dissolved organic carbon (DOC, 0.50-1.02 mg L^-1), implying more reactive fluorescent dissolved organic matter (FDOM) that can potentially cause more CO₂ outgassing. Furthermore, we observed a decreasing trend of slope ratio (S_R) and proteinlike fluorescence with elevation as opposed to a humiclike component, potentially due to more concentrated croplands and consequent nutrient inputs to rivers at lower elevations. In addition, poststorm samples shifted to more humiclike FDOM and chromophoric DOM (CDOM) sides compared to baseflow water. These findings highlighted the importance of land uses and other environmental conditions, such as elevation and hydrology, on DOM characteristics and dynamics and water quality, which can potentially be amplified due to the steep gradients in mountainous watersheds. Measures should be taken for proper and sustainable land uses to mitigate river water eutrophication and pollution and to protect drinking water sources for millions of populations.