Spatial pattern of soil moisture and its temporal stability within profiles on a loessial slope in northwestern China

•Vertical and horizontal distributions of soil moisture depended on vegetation types.•In order of importance, vegetation type, soil depth and the wetness index had effect on temporal stability of soil moisture.•Relationships between the standard deviation of relative differences and the wetness index varied nonlinearly with soil depth.

SummaryTemporal stability of spatial distributions of soil moisture are usually observed after repeated surveys of soil moisture across an area. To understand how temporal stability of soil moisture varied with soil depth under the combined influences of vegetation and local topography, we collected soil moisture data at intervals of 10 cm within 1-m profiles on a loessial slope in China in four plots (61 m × 5 m) under different types of vegetation (Korshinsk peashrub, KOP; purple alfalfa, ALF; natural fallow, NAF; millet, MIL). Measurements of soil water content were made by neutron probes on 15 occasions between 2010 and 2012. Soil moisture distributions in both the vertical and horizontal dimensions were investigated to describe its spatial pattern and to lay the groundwork for better understanding its temporal stability characteristics. The results indicated that: (1) soil moisture presented different vertical but similar horizontal trends in the four plots, with significant correlations of soil moisture occurring primarily among adjacent soil layers irrespective of vegetation types, mostly in soil profiles under KOP and ALF and less frequently in soil profiles under NAF and MIL; (2) based on Spearman rank correlation coefficients, with increasing depth temporal stability generally increased under KOP and MIL, but first increased and then decreased under ALF, and increased after the first three measurements under NAF; (3) based on the relative difference technique, points with extreme moisture tended to remain representative at more depths than did points with average moisture and their time stability increased with increasing soil depth; and (4) the correlation between MRD (mean relative differences) and the wetness index weakened with soil depth. The relationship between SDRD (the standard deviation of MRD) and the wetness index varied nonlinearly with soil depth. Vegetation type, soil depth and the wetness index, in descending order of influence, had significant effects on the temporal stability of soil moisture. Among selected soil properties, saturated hydraulic conductivity, bulk density and soil organic carbon all significantly affected the SDRDs. These observations are expected to add valuable information to the theory of temporal stability and for the practices of soil moisture management.