Measuring Gas Transfer Velocity in a Steep Tropical Stream: Method Evaluation and Implications for Upscaling

Abstract

Greenhouse gas emission estimates from streams rely, in part, on accurate measurements or estimates of the gas transfer velocity, which describes the physical efficiency for gas exchange across the water-air interface. Numerous methods for measuring or modeling gas transfer velocity exist, yet few studies compare these different methods. Additionally, current models of gas transfer velocity in streams are predominantly derived from measurements in low-gradient, temperate, or boreal streams. Here, we measured gas transfer velocity using four different methods in a high-energy, tropical headwater stream under a range of flow conditions, and compared these measurements to indirect estimates from four empirical models. Our results show that, when compared to the use of a biologically inert gas tracer (propane), floating chambers produced lower gas transfer velocity values. Using carbon dioxide (CO₂) as a tracer gas was unreliable without considering other natural sources and sinks of CO₂ and yielded gas transfer velocities lower than when using propane. Existing empirical models tended to underestimate gas transfer velocity, compared to the inert tracer gas. When using empirical models to upscale the emission flux along an entire stream reach, implementing them at finer spatial resolutions yielded flux estimates closer to measured fluxes. We also highlight the extreme spatial variability of gas transfer velocity across small spatial scales, which contrasts with its relative stability across changing hydrological conditions. The discrepancies between methods highlight the need for further research in measuring and upscaling gas transfer velocity, particularly in very turbulent steep streams.