Use of a land surface model to evaluate the observed soil moisture of grassland at the Tongyu reference site

A land surface model driven by the continuous three-year observed meteorological data witha time interval of 30 minutes at the Tongyu station, a reference site of the Coordinated EnhancedObserving Period (CEOP), was used to evaluate the observation bias of soil moisture (SM) data andanalyze the variation of SM at different time scales. The saline-alkaline soil of the grassland atthe Tongyu site makes the measured SM too high, especially in boreal summer of 2003--05. The simulatedannual mean SM has the lowest value in 2004 and its three-year variation corresponds to the changeof precipitation, whereas the observation shows the increasing trend from 2003 to 2005. Compared tothe variation range between -60% and 40% for the anomaly percentage of the simulated daily meanSM during May--October of 2004, the measured data show the higher values more than 40%. The magnitudeof the variation trend of the observed daily mean SM in 2003 and 2005 is generally consistent with thesimulation. The largest deficiency for the soil moisture observation of the grassland is the overestimatedvalue in the drought year with less precipitation. The simulated monthly mean SM has the lowest valuein March due to the large contribution of evaporation relative to precipitation and this phenomenoncan not be reproduced in the observation.