A Mechanistic Model for Gas Transfer Velocity in Damodar River

Abstract Riverine gas exchange is a critical yet poorly constrained component of global carbon cycling, with traditional models often failing to account for the complex interplay of turbulence, biofilm barriers, and sediment dynamics. Here, we present a mechanistic gas transfer velocity (\(\:k\)) model that explicitly integrates: 1. turbulence-driven transfer via shear velocity (\(\:{u}*\)), 2. biofilm resistance (\(\:{R}bio\)​) as a function of thickness (\(\:δ\:\)) and porosity (\(\:φ\:\)), 3. sediment clogging (\(\:{R}sed\)) scaled to suspended load (\(\:{C}sed\)), 4. diurnal photosynthetic adjustments (\(\:{\:k}ₚₕₒₜₒ\)). We apply this model to the Damodar River, India—a regulated, eutrophic system where biofilm proliferation and monsoon-driven sediment pulses create an ideal testbed for evaluating interfacial gas exchange processes. Field measurements (tracer injections, micro-profiling, and Acoustic Doppler Current Profiler, ADCP surveys) reveal that biofilms suppress k by 18–37% in dry-season low-flow reaches (\(\:δ\:\)=1.2–2.5 mm), while suspended sediments dominate monsoon fluxes, reducing k by 22–29% at \(\:{C}sed\)>300 mg/L. These findings underscore the necessity of incorporating biofilm-sediment interactions into gas flux models, particularly in anthropogenically altered rivers. Our framework provides a transferable tool for refining carbon budgets and informs management strategies targeting nutrient and sediment loads in river systems.