Modeling spatial and temporal variations in soil moisture in China

On the basis of station observations, an atmospheric field (ObsFC) was constructed for the Community Land Model version 3.5 (CLM3.5). The model (CLM3.5 driven with ObsFC, hereafter referred as to CLM3.5/ObsFC) was used to simulate soil moisture (SM) from 1951 to 2008 in China. The resulting SM was compared with in situ observations, remote-sensing data and estimations made by various land models, indicating that CLM3.5/ObsFC is capable of reproducing the temporospatial characteristics and long-term variation trends of SM over China. Using an in situ observation-based forcing field improves the simulation of SM. Analysis of SM simulated using CLM3.5/ObsFC shows that the overall spatial pattern of SM was characterized by a gradually decreasing and alternating distribution of arid-humid zones from the southeast to northwest. Regionally averaged SM was the driest over southern Xinjiang Province and western Inner Mongolia, while the most humid regions were located over the Northeast Plain, Jianghuai region and the Yangtze River basin. The long-term variation trends of SM were generally characterized by increases in arid and humid regions and decreases in semiarid regions. Moreover, the variation was relatively intense from the mid-1970s to the mid-1990s in the arid region. The time series was more stable in the humid region except for a period near 1970 and after the year 2003. A downward trend was most prominent in the semiarid region from the 1990s to the end of the time series. For 1951-2008, in the arid, semiarid and humid regions, the SM volume percentage changed by 2.35, -1.26 and 0.08, respectively. The variation trends and intensity remarkably differed among the different regions, with the most notable changes being over the arid and semiarid regions north of 35 degrees N.