High-Frequency Stable-Isotope Measurements of Evapotranspiration Partitioning in a Maize Field

Knowledge of the evaporation (E) and transpiration (T) components of evapotranspiration (ET) is important for ecohydrological modeling and agricultural productivity. The stable-isotope method offers the possibility to partition E and T due to the distinct differences in the isotopic signals of the sources. In this study, the concentration and isotopic ratios for oxygen-18 (18O) of water vapor in the ecosystem boundary layer of a growing maize field at the Hydrological Open Air Laboratory (HOAL) catchment in Austria were measured using a high-frequency field-sampling device. In conjunction with isotope samples from the soil and maize plants, these data were used to partition ET using the Keeling plot technique. Eddy covariance and sap flow measurements were used to provide a comparison to test the stable-isotope method. The fraction of transpiration (Ft) calculated with the stable-isotope method showed good agreement with the sap flow method. Overall daily average values of Ft were in a range from 43.0 to 88.5% with T accounting for an average value of 67.5% of the evapotranspiration over the nine days of the experimental period. Following a precipitation event of 9.7 mm, Ft increased from 63.4 to 88.5% over the next four days as the upper layer of the soil dried out while the plants accessed deeper soil water.