The multi-scale spatial variance of soil moisture in the semi-arid Loess Plateau of China

An understanding of soil moisture heterogeneity across spatial scales has been considered to be critical to eco-hydrological research and particularly important for vegetation restoration in semi-arid areas. This study aimed to investigate the spatial variance of deep soil moisture at multiple scales in the semi-arid Loess Plateau of China. The relative importance of the related factors and the dominant driver of soil desiccation were especially discussed. The impact factors in this study are related to slope positions, land use types and precipitation. To understand the relative importance of these factors to soil moisture, the experiment was conducted at three spatial scales (slope, small catchment and region) according to the impact scope of each factor. The soil moisture to a depth of 4 m had been obtained by in situ sampling in nine catchments along the precipitation gradients from south to north across the semi-arid area of the Loess Plateau. The terrain, soil and vegetation properties were investigated synchronously. The common characteristics of soil moisture spatial variance were extracted by comparing the soil moisture profile in the plots/catchments with different climate and terrain properties. At slope scale, the deep soil moisture (> 100 cm) was heterogeneous in grasslands, while relatively homogeneous in planted shrub/forest lands. At catchment scale, the influence of slope position on soil moisture was not significant compared with the influence of land use types. The deep soil moisture difference between slope positions was less than 4% in grasslands and less than 1.5% in planted shrub/forest lands, while reaching 9.7% between land use types. In the catchments, the soil moisture of different land use types commonly ranked as farmland > grassland > planted forest land, despite the climate difference in each catchment. At regional scale, precipitation was a significant factor influencing soil moisture spatial variance, but the influence was rather slow compared with the influence of afforestation. The land use type was the dominant factor of soil moisture spatial heterogeneity, rather than the slope positions and precipitation change. The afforestation was the major driver of soil desiccation in the semi-arid Loess Plateau of China. In consideration of the soil moisture conservation and the vegetation sustainable development, we suggested that abandoning slope farmland to natural grassland was a more effective measure for ecological recovery in the semi-arid Loess Plateau of China.