3.3. Kriging interpolation of SWC
P. crassifolia is a shallow-rooted tree species with roots distributed mainly in the first 30 cm below the ground surface (Chen et al., 2016) and, where also accounts for 40 to 60% of the total active profile moisture storage in a temperate climate region (Western et al., 1999). The results of this study showed the spatial pattern of SWC in surface soil (especially depths shallower than 20 cm). Based on the simulation function for each plot, local Kriging interpolation was used for the spatial simulation of SWC (Fig. 5). The resulting spatial map of SWC was based on actual field values, and on the knowledge about the underlying spatial relationship obtained from sampling values.
SWC did not exhibit a consistent spatial pattern along the slope in the planted forest (plot #2) or in the natural forest area (plot #10), which suggest that the vertical axis spatial pattern of SWC in this water-limited ecosystem was not controlled by the position of slope. These results differed from those of (Brocca et al., 2010; Vereecken et al., 2014), who reported that SWC depended on water transport via subsurface lateral flow from the upper slope to the toe when water was in sufficient supply. SWC along the horizontal axis exhibited a significant decreasing trend from shrubland to grassland or forestland (Fig. 3 and Fig. 5), indicating that shrubland had a superior soil and water conservation capacity in this water-limited area.