Tracking the Evaporation Front with Stable Isotopes of Water in a Drying Sand Column

We performed a long-term evaporation experiment to improve our ability to derive water flow and transport parameters from the isotopic information temporally archived in natural soils. Specifically, we treated the surface of an initially water-saturated sand column with a continuous dry gas stream. We monitored the column’s weight loss and in situ the concentration and isotopic composition of water vapor at different depths as well as in the outgoing gas stream. After several days, stable isotope and vapor concentration readings stabilized. We identified this as the stage I evaporation phase, which is characterized by a stagnant evaporation front near the surface and a constant evaporation stream that is maintained by capillary rise of water from lower depths. Stage II evaporation, which is characterized by gradual downward movement of the evaporation front due to breakdown of the capillary rise, started after 120 days. We identified it by the gradual decrease of both vapor concentration in the exported gas stream and the column’s weight loss rate. At the depth of the in situ probes, we identified the passing-through of the evaporation front at the different observation depths by respective increases of isotopic compositions before and a decrease of vapor concentration afterwards. Our setup was perfectly suitable to observe stage I and stage II evaporation from the combination of vapor concentration and isotope readings. The employed in situ probes proved to be critical for the success.