Soil moisture prediction with the ensemble Kalman filter: Handling uncertainty of soil hydraulic parameters

•Soil hydraulic parameters are often uncertain and unidentifiable.•Different ways of handling parameter uncertainty in the EnKF are compared regarding soil moisture predictions.•Performing joint updates of parameters and states is the best method to account for parameter uncertainty. For predicting flow in the unsaturated zone, an adequate choice of the model parameters, especially the soil hydraulic parameters, is essential. It is difficult to determine these parameters, as the parameter estimation problem easily becomes ill-posed, e.g. due to pseudo-correlations among two or more of the unknown parameters. In the field, this problem is strongly related to the available observations which, in monitoring networks, are not optimized to be used for parameter estimation. In this paper, we investigate the potential of data assimilation using the ensemble Kalman filter (EnKF) with unsaturated zone models under conditions where model parameters are highly uncertain and not identifiable. Different ways of dealing with the parameter uncertainty, such as parameter updates and bias correction, are discussed and compared. It is shown that jointly updating all uncertain parameters and states is the best method to account for the error induced by parameter uncertainty.