Assimilation of passive and active microwave soil moisture retrievals

Near‐surface soil moisture observations from the active microwave ASCAT and the passive microwave AMSR‐E satellite instruments are assimilated, both separately and together, into the NASA Catchment land surface model over 3.5 years using an ensemble Kalman filter. The impact of each assimilation is evaluated using in situ soil moisture observations from 85 sites in the US and Australia, in terms of the anomaly time series correlation‐coefficient, R. The skill gained by assimilating either ASCAT or AMSR‐E was very similar, even when separated by land cover type. Over all sites, the mean root‐zone R was significantly increased from 0.45 for an open‐loop, to 0.55, 0.54, and 0.56 by the assimilation of ASCAT, AMSR‐E, and both, respectively. Each assimilation also had a positive impact over each land cover type sampled. For maximum accuracy and coverage it is recommended that active and passive microwave observations be assimilated together.