Spatial dynamics of soil moisture in a complex terrain in the semi-arid Loess Plateau region, China

Soil moisture is an important hydrological variable in reforestation practices in a water-limited region of the Loess Plateau of northwestern China. The objective of this study was to quantify the spatial dynamics of soil moisture on a complex terrain. During 2004-2006, a total of 313 sample points in two kinds of grid (2 x 2 m and 20 x 20 m) were arranged for soil moisture measurements (two soil layers: 0-30 and 30-60 cm) with Time Domain Reflectometry. The geostatistical properties of soil moisture patterns, the variance and correlation structure of the soil moisture, and the effects of terrain factors on soil moisture were analyzed. The results suggested that our sampling grid captured the spatial variability of soil moisture distributions for this complex terrain. Principal Component Analysis and Cluster Analysis statistics showed that soil moisture decreased as slope gradient increased; that sunny aspects (112.5 degrees-292.5 degrees) had relatively lower soil moisture than did shady aspects (292.5 degrees-112.5 degrees); that soil moisture was lowest in the SWW direction and highest in the NWN direction; and that hillslope aspect was the main factor affecting soil moisture in the 0-to 30-cm soil layer, whereas the main factor for the 30-to 60-cm layer was slope gradient. It was found that the relative values of soil moisture for steep slopes (>36%) with shady aspect (292.5 degrees-112.5 degrees), gentle slopes (<36%) with sunny aspect (112.5 degrees-292.5 degrees), and steep slopes with sunny aspect were 99, 82, and 80, respectively-assuming a soil moisture value of 100 for gentle slopes with shady aspect. The results of this study are expected to be relevant to and useful for reforestation planning and design, parameterization of distributed hydrology models, and land productivity assessment in the study region.