Vegetation dynamics and climate seasonality jointly control the interannual catchment water balance in the Loess Plateau under the Budyko framework

Within the Budyko framework, the controlling parameter (omega in the Fu equation) is widely considered to represent landscape conditions in terms of vegetation coverage (M); however, some qualitative studies have concluded that climate seasonality (S) should be incorporated in omega. Here, we discuss the relationship between omega, M, and S, and further develop an empirical equation so that the contributions from M to actual annual evapotranspiration (ET) can be determined more accurately. Taking 13 catchments in the Loess Plateau as examples, omega was found to be well correlated with M and S. The developed empirical formula for omega calculations at the annual scale performed well for estimating ET by the cross-validation approach. By combining the Budyko framework with the semi-empirical formula, the contributions of changes in omega to ET variations were further decomposed as those of M and S. Results showed that the contributions of S to ET changes ranged from 0.1 to 74.8% (absolute values). Therefore, the impacts of climate seasonality on ET cannot be ignored, otherwise the contribution of M to ET changes will be estimated with a large error. The developed empirical formula between omega, M, and S provides an effective method to separate the contributions of M and S to ET changes.