Spatial patterns of bias during steady-state evaporation from the assumption of equilibrium between vapor and precipitation
Abstract
Stable water isotope ratios during evaporative processes are typically calculated using the Craig-Gordon model, which estimates the isotope ratios of the evaporating vapor using the isotopic compositions of and the humidity gradients between the evaporating liquid and the surrounding atmosphere. Globally, over timescales longer than the residence time of atmospheric vapor the average isotope ratio of evaporating water should closely match that of precipitation. This assumption has frequently been applied at smaller scales in steady-state evaporation models, such as in studies of the isotope ratios of lakes, leaf water, and soil water. However, until recently observations of isotope ratios in near-surface vapor were difficult to obtain at high spatial or temporal resolution, and as a result, this assumption has rarely been validated. Large seasonal differences in atmospheric vapor transport may result in vapor that is out of isotopic equilibrium with local annual precipitation, and potentially bias estimates of isotope ratios from evaporating water pools. To investigate the spatial and seasonal structure of this isotopic disequilibrium, we use an ensemble of GCM simulations that include water isotopes (nine different GCMs) to compare model simulated surface vapor isotope ratios with vapor isotope ratios in equilibrium with annual precipitation. We find that regions with strong atmospheric subsidence tend to have vapor isotope ratios that are lower than expected (by up to 30‰ in δ2H), but that the remainder of continental areas, particularly in high latitudes, tend to have vapor isotope ratios that are higher than expected relative to vapor in equilibrium with annual precipitation (by 5-50‰ in δ2H). We develop a general theory to model the bias introduced by assuming equilibrium with annual precipitation isotope ratios, and show that this bias depends both on the degree of disequilibrium between atmospheric vapor and local precipitation isotope ratios and the humidity gradient between the evaporation site and the atmosphere. Strong seasonal differences of up to 60‰ in δ2H in this isotopic disequilibrium are apparent in the GCM ensemble, indicating that implications for predicting isotope ratios of evaporating water pools depend on season, and that these relationships likely vary in past climates.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFMPP14B..01F
- Keywords:
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- 3344 Paleoclimatology;
- ATMOSPHERIC PROCESSESDE: 1041 Stable isotope geochemistry;
- GEOCHEMISTRYDE: 1655 Water cycles;
- GLOBAL CHANGEDE: 1833 Hydroclimatology;
- HYDROLOGY