Groundwater modelling needs a good way to combine model and observation data. The ensemble Kalman filter (EnKF) and ensemble smoother (ES) are methods used for this. The EnKF updates the model with data one by one, while the ES updates the model with all data at once, saving time. A new method called the iterative ES continuously updates the model with measurements. In a study comparing these methods, the iterative ES performed better than the standard ES and was similar to the EnKF, but with less computational cost. This shows that the iterative ES is a promising method for improving groundwater modelling.