A mechanistic model for gas transfer velocity in the Damodar river.

Gas transfer across river interfaces remains a poorly quantified process in the Earth's carbon cycle. Standard frameworks overlook how turbulence, biofilm impedance, and suspended sediment interact. We propose a mechanistic k model integrating (i) turbulence-driven transfer via the O'Connor & Dobbins formulation, (ii) biofilm suppression as a power-law function of thickness (δ), (iii) sediment suppression as an exponential function of suspended sediment concentration (Csed), and (iv) diurnal inhibition of k(Δkphoto). We apply this model to the Damodar River, India-a regulated, eutrophic tropical system. Field measurements (SF6 tracer injections, floating chambers, micro-profiling, ADCP surveys) reveal that biofilms suppress k by 18-37% in dry-season low-flow reaches (δ = 1.2-3.0 mm), while suspended sediments dominate monsoon fluxes, reducing k by 22-29% at estimated Csed of 300-850 mg L-1. The model captures observed spatial and seasonal patterns with RMSE = 1.82 cm h-1 and NSE = 0.64, outperforming conventional formulations (O'Connor and Dobbins, NSE = 0.42; Raymond et al., NSE = 0.45). Because sediment concentrations were estimated from published post-dam baselines, we report results as a sensitivity analysis across plausible Csed ranges. These findings highlight the need to incorporate biofilm-sediment interactions into gas flux models, particularly in anthropogenically altered rivers.