Isotopic equilibrium between precipitation and water vapor: evidence from continental rains in central Kenya

An accurate understanding of the relationships between the isotopic composition of liquid water and that of water vapor in the environment can help describe hydrologic processes across many scales. One such relationship is the isotopic equilibrium between falling raindrops and the surrounding vapor. The degree of equilibration is used to model the isotopic composition of precipitation in isotope-enable general circulation models and land-atmosphere exchange models. Although this equilibrium has been a topic of isotope hydrology research for more than four decades, few studies have included vapor measurements to validate modeling efforts. Recent advances in laser technology have allowed for in situ vapor measurements at high temporal resolution (e.g., >1 Hz). Here we present concomitant rain and vapor measurements for a series of 17 rain events during the 'Continental' rainy season (June through August) at Mpala Research Center in central Kenya. Rain samples (n=218) were collected at intervals of 2 to 35 minutes (median of 3 minutes) depending on the rain rate (0.4 to 10.5 mm/hr). The volume-weighted mean rain values for δ¹⁸O, δ²H and D-excess (δ²H - 8* δ¹⁸O) were 0.1 ‰, 10.7 ‰, and 10.1 ‰. These values are more enriched than the annual weighted means reported for the area (-2.2 ‰, -7.6 ‰, and 11.0 ‰, respectively). Vapor was measured continuously at ~2Hz (DLT-100, Los Gatos Research), with an inverted funnel intake 4m above the ground surface. The mean vapor isotopic composition during the rain events was -10.0 +/- 1.2 ‰ (1 σ) for δ¹⁸O and -73.9 +/- 7.0 ‰ for δ²H. The difference between the rain sample isotopic composition and that of liquid in isotopic equilibrium with the corresponding vapor at the ambient temperature was 0.8 +/- 2.2 ‰ for δ¹⁸O and 6.2 +/- 7.0 ‰ for δ²H. This disequilibrium was found to correlate with the natural log of rain rate (R² of 0.26 for δ¹⁸O and 0.46 for δ²H), with lower rain rates having larger disequilibrium. There was also a temporal pattern in the disequilibrium for δ¹⁸O, with the first five rain events having significantly larger (p < 0.01) disequilibrium (4.4 ‰) than the subsequent rain events (0.6 ‰). The temporal pattern suggests that, in addition to the relationship with rain rate, there is some relationship between rain-vapor equilibrium and larger-scale controls such as vapor source region, precipitation recycling and air mass trajectory.