Technical note: Evaporating water is different from bulk soil water in δ2H and δ18O
Abstract. Soil evaporation is a key process in the water cycle and can be conveniently quantified with δ2H and δ18O in bulk surface soil water (BW). However, recent research shows that larger soil pore water evaporates first and differs from small pore water in δ2H and δ18O, which disqualifies quantification of evaporation from BW δ2H and δ18O. We hypothesize that BW has different isotopic compositions than evaporating water (EW). Therefore, our objectives are to test the hypothesis, and to
... othesis, and to evaluate if the difference alters the calculated evaporative water loss. We measured isotopic composition in soil water in two continuous evaporation periods in a summer maize field. Period Ⅰ had a duration of 32 days following a precipitation event and Period Ⅱ lasted 24 days following an irrigation event with a 2H-enriched water. BW was obtained by cryogenically extracting water from samples of 0–5 cm soil taken every three days; EW was derived from condensation water collected every two days on plastic film placed on soil surface. Results showed that when newly added water was heavier than pre-event BW, δ2H of BW in Period Ⅱ decreased with the increase of evaporation time, indicating evaporation of heavy water; when newly added water was lighter than pre-event BW, δ2H and δ18O of BW in Period Ⅰ and δ18O of BW in Period Ⅱ increased with increasing evaporation time, suggesting evaporation of light water. Moreover, relative to BW, EW had significantly smaller δ2H and δ18O in Period Ⅰ and significantly smaller δ18O in Period Ⅱ (p 0.05). Our results have important implication for quantifying evaporation process with water stable isotopes.