A two-update ensemble Kalman filter for land hydrological data assimilation with an uncertain constraint

In the present study, a new constrained ensemble Kalman filter, which we refer to as weak constrained ensemble Kalman filter (WCEnKF), is introduced that satisfies the closure of the water balance equation. The proposed WCEnKF contains two update steps; it first incorporates observations from Gravity Recovery And Climate Experiment (GRACE) to improve model simulations of water storages, and second, it uses the additional climatic observations of precipitation, evaporation, and also ground-based water discharge to establish the water budget closure.222