| Hydrological processes at and above the surface, such as evaporation, transpiration, mixing, and transport, have a significant effect on regional and global climate. Water vapor evaporated from the surface carries an isotopic signature of the evaporation.;In this study, we have: (1) developed theoretical models, laboratory experiments, and field projects to determine the isotope ratios in evaporation; (2) clarified and reinforced the existence of the two-way exchange processes between the earth's surface and the atmosphere; and (3) suggested that forest transpiration uses deep soil water (winter precipitation) in late summer when soil moisture reaches its minimum level.;The objective of this research is to quantify the isotope ratios of the evaporating water vapor by measurements in the atmospheric boundary layer (ABL). By sampling at multiple levels in the ABL and defining a mixing line using water vapor mixing ratio and isotope ratios, we are able to separate, identify, and characterize the water vapor source at the earth's surface, which is one of the two water vapor end-member sources.;A two-component advective-diffusive model is developed as an improvement to the Craig-Gordon model. The model assumes the two-way exchange between the atmosphere and the liquid water reservoir and the equilibrium fractionation at the surface. We find that the dimensionless parameter N, which represents the balance between advection and diffusion, controls the isotope ratios in the evaporation.;We have conducted aircraft vapor sampling using dry-ice-cooled canisters in the ABL over New England. Isotope data confirm the mixing line hypothesis. We find that the isotope ratios of water vapor from surface evapotranspiration fractionate with respect to surface water.;Water vapor is also sampled above a coastal salt marsh. Less consistent gradients of d D and Q are observed near the surface. The deuterium flux ratios are obtained from the mixing line method. Their values show significant uncertainty due to the small gradients and variable surface conditions. Source characteristics are also difficult to obtain using the mixing lines for the same reasons. Equilibrium fractionation can not be verified by the data. The water vapor isotope ratio at the lower level is close to the equilibrium value when the humidity is high. The d D - S plot is used to identify the mixing and evaporation of surface water. Water coming into the marsh during the high tide period is from the mixing of sea water and lake water. Evaporation increases d D and S following an "evaporation line" that has different slope from a "mixing line". (Abstract shortened by UMI.)... |
