Land use drivers of riverine methane dynamics in a tropical river basin, India

Rivers are globally significant natural sources of atmospheric methane (CH₄). However, the effect of land use changes on riverine CH₄ dynamics, particularly in tropical zones, remain ambiguous, yet important to predict and anticipate the present and future contribution of rivers to the global CH₄ budget. The present study examines the magnitude and drivers of riverine CH₄ concentration and emission in the tropical Krishna River (KR) basin, India. The large spatial variability of CH₄ concentration (0.03 to 185.34 μmol L ^-1) and emissions (0.04 mmol m^-2 d^-1 to 1666.24 mmol m^-2 d^-1) in the KR basin was linked to the site-specific features of the catchments through which rivers are draining. Several fold higher CH₄ concentration and emission was observed for the urban river sites (64.63 ± 53.17 µmol L^-1 and 294.15 ± 371.52 mmol m² d^-1, respectively) than the agricultural (1.05 ± 2.22 µmol L^-1 and 3.45 ± 9.72 mmol m² d^-1, respectively) and forested (0.49 ± 0.23 µmol L^-1 and 1.26 ± 0.73 mmol m² d^-1, respectively) sites. The concentrations of dissolved oxygen, total phosphorus, and Chlorophyll-a were significant hydrochemical variables strongly coupled with the dissolved CH₄ concentrations. On the other hand, percentage of built-up area emerged as the most important landscape-level driver indicating that urbanization has an overriding effect on riverine CH₄ concentration in the agriculture dominated KR basin. Our study supports the growing notion that tropical urban rivers are hotspot of CH₄ emission. Furthermore, we show that the pattern of increasing in riverine CH₄ concentration with built-up area (%) is a general feature of Asian river basins. As the urban land cover and population following an exponential increase, Asian rivers might contribute substantially to the regional and global CH₄ budget.