Evapotranspiration and its partitioning for three agro-ecosystems: using an isotope-coupled two-source model.

The contribution of transpiration (T) to evapotranspiration (ET) is a requirement for understanding changes in carbon assimilation and water cycling in a changing environment, and has received intensive attention from the scientific community in recent years. However, few existing studies address the dynamics that drive temporal and seasonal variability of T/ET or whether the different methods agree under natural conditions, across diverse agro-ecosystems. In this study, we applied an isotope coupled two-source model, was used to partition ET for three agro-ecosystems (rice, wheat and maize). The model was coupled with a plant physiology approach to calculate a canopy conductance and an isotopic non-steady state scheme to estimate transpiration isotopic flux. Good agreements between the observed and predicted ET flux and the isotope composition of leaf water indicate that our model is a promising tool for simulating the isotopic fractionation processes associated with evaporation and transpiration for different crop ecosystems. The estimated T/ET ranged from 0 to 1, with an almost continuous increase in the early growing season with leaf area index (LAI) less than 2.5 and then convergence towards a stable value beyond LAI of 2.5. The seasonal change in T/ET could be described quite well as a function of LAI for the three ecosystems, implying that LAI is first-order factor affecting ET partitioning. The two-source model results show that the growing-season T/ET was 0.50, 0.84 and 0.64, while the isotopic approach show that T/ET was 0.74, 0.93 and 0.81 during the measurement period for rice, wheat and maize, respectively. The two-source model results were supported by soil lysimeter and eddy covariance measurements made during the same time period for wheat (0.87). Uncertainties and errors for different T/ET techniques were discusses.   Our analysis suggests that further improvement to the Craig-Gordon model predictions and to evapotranspiration isotope measurement is necessary to achieve accurate flux partitioning at the ecosystem scale.