Abstract
The isotopic composition of rainfall (δ2H and δ18O) is an important tracer in studies of the ecohydrology, plant physiology, climate and biogeochemistry of past and present ecosystems. The overall continental and global patterns in precipitation isotopic composition are fairly well described by condensation temperature and Rayleigh fractionation during rainout. However, these processes do not fully explain the isotopic variability in the tropics, where intra‐storm and meso‐scale dynamics may dominate. Here we explore the use of atmospheric back‐trajectory modeling and associated meteorological variables to explain the large variability observed in the isotopic composition of individual rain events at the study site in central Kenya. Individual rain event samples collected at the study site (n = 41) range from −51‰ to 31‰ for δ2H and the corresponding monthly values (rain volume‐weighted) range from −15‰ to 15‰. Using the Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model, we map back‐trajectories for all individual rain hours occurring at a research station in central Kenya from March 2010 through February 2012 (n = 544). A multiple linear regression analysis demonstrates that a large amount of variation in the isotopic composition of rainfall can be explained by two variables readily obtained from the HYSPLIT model: (1) solar radiation along the trajectory for 48 hours prior to the event, and (2) distance covered over land. We compare the measurements and regression model results to the isotopic composition expected from simple Rayleigh distillation along each trajectory. The empirical relationship described here has applications across temporal scales. For example, it could be used to help predict short‐term changes in the isotopic composition of plant‐available water in the absence of event‐scale sampling. One can also reconstruct monthly, seasonal and annual weighted mean precipitation isotope signatures for a single location based only on hourly rainfall data and HYSPLIT model results. At the study site in East Africa, the annual weighted mean δ2H from measured and modeled values are −7.6‰ and −7.4‰, respectively, compared to −18‰ predicted for the study site by the Online Isotopes in Precipitation Calculator.
Highlights
The stable isotope composition of water (d2H and d18O) has become an important tool for investigating ecosystem processes (Dawson et al 2002)
Means weighted by the rain volume associated with each sample have a range of À15% to 10% for d2H (Fig. 1). These monthly means allow for comparison with data from the Online Isotopes in Precipitation Calculator (OIPC) (Bowen and Revenaugh 2003, Bowen et al 2005, Bowen 2012), which is based on data from the ongoing GNIP sampling program (Dansgaard 1964, Rozanski et al 1993)
We have demonstrated that highly variable isotopic composition of rainfall can be explained by incorporating the spatial and meteorological history of the associated air masses
Summary
The stable isotope composition of water (d2H and d18O) has become an important tool for investigating ecosystem processes (Dawson et al 2002). Post-condensation fractionation processes have been increasingly accepted as important drivers of isotopic composition (Friedman et al 1962, Dansgaard 1964, Stewart 1975, Gat 2005, Risi et al 2010a, Noone 2012) and have been included in isotopically-enabled General Circulation Models (GCMs) (Joussaume et al 1984, Jouzel et al 1987, Noone and Simmonds 2002, Lee et al 2007, Risi et al 2010c) These processes include equilibration of raindrops with the water vapor at the cloud base and while they are falling as well as evaporation of raindrops and re-entrainment of this evaporated moisture into clouds
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