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.