The Influence of Flow and Bed Slope on Gas Transfer in Steep Streams and Their Implications for Evasion of CO2

The evasion of greenhouse gases (including CO2, CH4, and N2O) from streams and rivers to the atmosphere is an important process in global biogeochemical cycles, but our understanding of gas transfer in steep (>10%) streams, and under varying flows, is limited. We investigated gas transfer using combined tracer injections of SF6 and salt. We used a novel experimental design in which we compared four very steep (18.4-29.4%) and four moderately steep (3.7-7.6%) streams and conducted tests in each stream under low flow conditions and during a high-discharge event. Most dissolved gas evaded over short distances (similar to 100 and similar to 200-400m, respectively), so accurate estimates of evasion fluxes will require sampling of dissolved gases at these scales to account for local sources. We calculated CO2 gas transfer coefficients (K-CO2) and found statistically significant differences between larger K-CO2 values for steeper (mean 0.465min(-1)) streams compared to those with shallower slopes (mean 0.109min(-1)). Variations in flow had an even greater influence. K-CO2 was substantially larger under high (mean 0.497min(-1)) compared to low flow conditions (mean 0.077min(-1)). We developed a statistical model to predict K-CO2 using values of streambed slopexdischarge which accounted for 94% of the variation. We show that two models using slope and velocity developed by Raymond et al. (2012) for streams and rivers with shallower slopes also provide reasonable estimates of our CO2 gas transfer velocities (k(CO2);md(-1)). We developed a robust field protocol which could be applied in future studies.