Partitioning and controlling factors of evapotranspiration: 1. Hydrological modeling constrained with isotope-based water balance decoupling

Exploring the components of evapotranspiration (ET) is essential but challenging because of the difficulties in ET partitioning. This study aims to improve the accuracy in partitioning ET under farmland and apple orchards by the hydrological model HYDRUS-1D constrained with isotopes, and further identify the effects of land use change on soil water balance (SWB). In particular, the long-term average SWB was first decoupled by combining multiple isotopes (delta 2H, delta 18O, and delta 3H), and then used to constrain HYDRUS-1D to improve model performance. With the isotope -based partitioned SWB, some hydrological model parameters difficult to be obtained experimentally were inversely optimized. Consequently, the performance of HYDRUS-1D in ET partitioning was greatly improved. Specifically, with the isotope method as a reference, the HYDRUS-1D with optimized parameters had errors of 20%+/- 2.1, 4%+/- 2.0, and 6%+/- 0.8 for evaporation (E), transpiration (T), and ratio of transpiration to evapotranspiration (T/ET), which were much smaller than those from the model with default parameter (171% +/- 34.5, 57%+/- 5.8, and 58%+/- 5.8). The apple orchards increased T by decreasing other SWB components. The framework of optimizing hydrological model by isotope -based SWB decoupling is promising in improving model accuracy, and the partitioned SWB components can be used to promote our understanding of ecohydrological processes.