State updating in Xin'anjiang model by Asynchronous Ensemble Kalman filtering with enhanced error models

For flood simulation in humid catchments, utilizing discharge observations to update the states of hydrological models may enhance performance. Asynchronous Ensemble Kalman Filter (AEnKF), an asynchronous variant of the Ensemble Kalman Filter (EnKF), holds substantial application potential in hydrological assimilation due to its ability to utilize more observations with almost no additional computational time. This study employs AEnKF to update the state variables of the Xin'anjiang model, necessitating the use of error models to perturb both model states and observations to generate ensemble spread. The Bias-corrected Gaussian Error Model (BGEM) is used to mitigate the systematic bias brought by perturbating soil moisture, and the Maximum a Posteriori Estimation Method (MAP) is employed for the estimation of hyperparameters of error models. Through synthetic and real-world data testing, it has been validated that the rectification of soil moisture perturbations using the BGEM significantly reduces the systematic bias induced by Gaussian perturbations. Moreover, the assimilation scheme introduced in this study, based on AEnKF with enhanced error models, outperforms the EnKF with those models. It substantially reduces the accumulation of past errors in the initial conditions at the start of the forecast, thereby aiding in elevating the accuracy of flood forecasting.