Extended Data Fig. 9: Comparison of modelled versus directly measured diffusive methane (CH4) emissions.
(a) Modelled diffusive CH4 fluxes in GRADES reaches where an empirical measurement is available in GRiMeDB. Measured fluxes span seven orders of magnitude while modelled fluxes are more constrained (2 orders of magnitude). One potential source of the discrepancy is the difference in gas transfer velocities (k) between the measured and predicted values, given that the vast majority of flux observations in GRiMeDB are measurements from a single day in a relatively short reach, while the modelled fluxes use monthly modelled discharge averages along a long river reach (4–6 km). When selecting pairs of observations with comparable k values, indicating that the hydrological conditions between the modelled and observed value are similar (predicted k is between 0.5 and 1.5 times the measured k; black points in (b)), the relationship between modelled and measured fluxes is evident. Dashed line in both panels is the 1:1 line, and note the change in the axis in panel (b).
