4.4 Equilibrium between vapour and precipitation
The difference in isotope ratios of δv and precipitation in the atmosphere, ΔRatm, generally differed from zero (i.e. the equilibrium) at both sites, with higher deviation from the equilibrium between mid-August and mid-September (Fig. 8). Responses of ΔRatm after precipitation events showed heterogenous patterns, reflecting varying differences from the equilibrium throughout the monitoring period. In August, grassland showed only positive ΔRatm , depicting more enriched δv compared to precipitation. On an event basis, grassland and tree sites differed slightly inΔRatm and the timeseries ofΔRatm for δ18O and δ2H showed similar patterns.
Boxplots showed higher ΔRatm values at the grassland site (though note that grassland included more data, Fig. 9). T-tests did not show significant differences between grassland and tree site for ΔRatm δ2H (p=0.055) but showed significance for ΔRatmδ18O (p=0.012), again showing a difference between the two vegetation types in their variation from equilibrium. MedianΔRatm values of δ2H were 2.95 ‰ and 0.51 ‰ for δ18O at the grassland, and 0.51 ‰ for δ2H and 0.36 ‰ for δ18O at the tree site. Maximum and minimum ΔRatm values at the grassland ranged between -30 and 46 ‰ and between ­­­‑3.7 and 7.7 ‰ for δ2H and δ18O, respectively. At the tree site they varied between -31 and 26 ‰ and -4 and 2.7 ‰ for δ2H and δ18O, respectively. Thus, the difference from equilibrium of δv with precipitation was greater at the grassland than at the tree stand. The higher and more abundant positive values at grassland reflect a higher isotopic enrichment of δv.