Possible soil tension controls on the isotopic equilibrium fractionation factor for evaporation from soil

The stable isotopes of hydrogen and oxygen (δ²H and δ¹⁸O) are useful conservative tracers for tracking the movement of water in soil. But although the tracking of water infiltrating through the soil profile and its movement as run‐off and groundwater recharge are well developed, water movement through the soil can also include evaporative fractionation. Soil water fractionation factors have, until now, been largely empirical. Unlike open water evaporation where temperature, humidity, and vapour pressure gradient define fractionation, soil water evaporation includes fractionation by soil matrix effects. These effects are still poorly characterized. Here, we present preliminary results from a simple laboratory experiment with four soil admixtures with grain sizes that range from sand to silt and clay. Our results show that soil tension seems to control the isotope fractionation of resident soil water. The relationship between soil tension and equilibrium fractionation appears to be independent of soil texture and appears well supported by thermodynamic theory. Although these results are preliminary, they suggest that future work should go after soil tension effects as a possible explanatory factor of soil water and water vapour fractionation.