5.3 Testing the assumption of equilibrium between vapour and precipitation

The equilibrium assumption between the isotopic signatures of δv and precipitation was not always held during our field campaign. Previous studies showed that the equilibrium assumption is more robust at subseasonal, longer time scales than for individual rain events. Lee et al. found that during rain events, vapour in the surface layer developed in general a state of equilibrium with the falling raindrops. In our study, this assumption was not robust at the subseasonal scale and did not confirm an establishment of an equilibrium.
ΔRatm (i.e. difference in isotope ratios of water vapour and precipitation in atmosphere) was greater and showed more positive values in summer reflecting that vapour was more enriched than precipitation during summer. Potential reasons for these results are (as discussed by ): raindrops formed at high elevation , precipitation came from convective events with big raindrops or high tree transpiration rates from deeper sources prevented vapour from equilibrium with precipitation. Additionally, high transpiration rates lead to isotopic enrichment of δv and could generate higher deviation from δv with precipitation.
Testing the equilibrium assumption is especially important for areas with a distinct microclimate like cities as previous studies showed that equilibrium estimates can be biased . Further, different regions of the World show diverging results for ΔRatm depending on climate, altitude and latitude. E.g. Mercer et al. showed that the equilibrium assumption does not hold in continental mountain environments. By going beyond the standard assumption of equilibrium in urban ecohydrology, we can improve simple mixing models, complex process-based, isotope-aided ecohydrological models like EcH2O-iso , estimations in keeling plot and the Craig and Gordon approach (cf. ).