Spatial variability of soil moisture vertical distribution and related affecting factors in hilly and gully watershed region of Loess Plateau

Taking the semivariogram in geostatistics as a main analytical tool,this paper studied the spatial variability of soil moisture vertical distribution,and its relations to the land use type and topography in hilly and gully watershed region of Loss Plateau.The semivariogram curve of soil moisture vertical distribution could be well fitted by spherical model,suggesting that there was a strong or moderate spatial autocorrelation.The variation range was 2-5 meters.Orchard mainly affected the soil moisture content at the depth of 1-2 meters.In May,the soil moisture content was maximal and showed a uniform distribution;while in August,the soil moisture content was minimal because of the high air temperature and strong leaf transpiration.In sloping farmland and terrace,soil moisture content was relatively high,and had less vertical change.Terrace had the minimal soil moisture content in September,one month later than sloping farmland.Due to the developed tree root system,woodland had greater effects on the variability of soil moisture vertical distribution,which was manifested in a trend of increase-decrease-increase-getting stable.Overall speaking,shrub land had lower soil moisture content,whose vertical variation mainly occurred at the depth of 0-2 meters,while grassland had higher soil moisture content,whose vertical variation mainly occurred at the depth of 1 meter or less.The spatial variability of soil moisture vertical distribution had no correlation with topographical factors slope and aspect.