Determination of kinetic isotopic fractionation of water during bare soil evaporation

Abstract

A process-based understanding of the water cycle in the atmosphere is important for improving meteorological and hydrological forecasting models. Usually only net fluxes of evapotranspiration – ET are measured, while land-surface models compute their raw components evaporation –E and transpiration –T. Isotopologues can be used as tracers to partition ET, but this requires knowledge of isotopic kinetic effects which impact the stable isotopic composition of water pools (e.g., soil, plant, surface waters) during phase change and vapor transport by soil evaporation and plant transpiration. Craig and Gordon (1965) introduced the kinetic fractionation in their model. It’s defined as the ratio of the transport resistances in air of the isotopologue to the most abundant isotopologue. Previous studies conducted laboratory experiments for free evaporating water (Merlivat, 1978. Cappa et al. 2003) or bare soil evaporation (Braud et al. 2009) with only a low temporal resolution. The goal of this study is to provide estimates of this factor at higher temporal resolution. We performed a soil evaporation laboratory experiment to determine the kinetic fractionation factor by applying the Craig and Gordon model. A 0.7 m high column (0.48 m i.d.) was filled with silt loam (20.1 % sand, 14.9 % loam, 65 % silt) and saturated with water of known isotopic composition. Soil volumetric water content, temperature and the isotopic composition of the soil water vapor were measured at six different depths. At each depth microporous polypropylene tubing allowed the sampling of soil water vapor and the measurement of its isotopic composition in a non-destructive manner with high precision and accuracy as detailed in Rothfuss et al. (2013). In addition, atmospheric water vapor was sampled at seven different heights up to one meter above the surface. Finally, air relative humidity and temperature were monitored at one meter height. Results showed that soil and atmospheric isotopic composition profiles could be monitored at high temporal and vertical resolutions during the course of the experiment. The kinetic fractionation factor could be calculated by using an inverse Graig and Gordon model and the Keeling plot method at high temporal resolution over a long period. We observed an increasing isotopic composition in the evaporating water vapor due to more enriched surface water. This leads to a higher transport resistances and an increasing kinetic fractionation factor