Spatial patterns of soil moisture at transect scale in the Loess Plateau of China

Understanding the spatial variability and impact factors of soil moisture is important when modeling the antecedent soil moisture status of a catchment.Soil moisture is a limiting factor of ecosystem development in semi-arid and arid areas.Soil moisture spatial patterns and impact factors are particularly important for ecosystem restoration in the semi-arid areas of the Loess Plateau.This study identifies soil moisture spatial patterns and impact factors(terrain,land use types and wet season) in Yangjuangou catchment in north of the Loess Plateau from the measurement of soil moisture in the top 100 cm soil layer along five different transects on a loess hill before and after the wet season.Statistical analysis revealed that the soil moisture spatial variation in deeper soil(40cm) was more significant than that in upper soil(20cm),despite the influence of wet season.On a typical slope which had natural grassland at upper position and planted R.pseudoacacia forest land at lower position,the average soil moisture in planted forest land was 3.8% lower than that in grassland in the 20cm to100cm soil depth,while 5% in the 100cm depth.Comparatively,in the scope of same kind of land use type,the difference of soil moisture between different slope positions was less than 2%,which was obvious smaller than the difference between the land use types.These laws kept consistent before and after the wet season.Moreover,the soil moisture content in different land use types was ranked as natural grassland planted R.pseudoacacia forest land(8 years) planted R.pseudoacacia forest land(20 years) planted Hippophae reamnoides forest land,no matter before or after the wet season.Although the topography of planted forest land has advantageous conditions(shading slope,lower position,smaller gradient or terraces) for retaining soil moisture,the soil in planted land was still found to be have a much lower soil moisture content than the soil in natural grasslands,especially in deeper soil.Thus it is considered that the influence of terrain on soil moisture content is masked by the influence of surface vegetation.Therefore,when modeling hydrological processes in the Loess Plateau,it is recommended that the pattern of vegetation types within the simulated area should be paid special attention.Hydrologic models of the Loess Plateau based only on terrain properties should be used cautiously.The soil moisture content at all sample sites increased after the wet season,however,the spatial patterns of soil moisture was not significantly changed.Sites which had high soil moisture content relative to other sites before the wet season retained their higher soil moisture content after the wet season.This was also the case for sites with low soil moisture content.Significant correlation was found between the soil moisture before and after the wet season at each site.Soil moisture spatial patterns displayed temporal stability despite the influence of the wet season.This suggests that soil moisture spatial patterns remain consistent at certain time scales.The knowledge gained from this study is an important addition to soil moisture spatial pattern research.