The temporal stability of soil moisture spatial pattern and its influencing factors in rocky environments

Rock outcrops (ROCs) are widespread across the Earth's surface. However, little empirical research has addressed the dynamic characteristics of soil water stored in rocky environments, especially for soil moisture spatial pattern (SMSP) and its temporal stability. This study aimed to explore the spatiotemporal characteristics of SMSP in a karst topographical setting and to reveal the relevant factors most influencing it. In three adjacent sampling plots located in Shilin County, Yunnan Province, China corresponding to (1) a forestland, (2) a shrubland, (3) a grassland, their soil moisture was measured monthly at depths of 10, 20, 30, 40, 60, and 100 cm from May 2014 to April 2017, while key environmental parameters-rock coverage, vegetation aboveground biomass, root distribution, and precipitation-were also quantified in each plot. Our results showed that where ROCs emerged, soil moisture displayed higher spatial heterogeneity than reported from other terrestrial ecosystems, but this area also maintained high temporal stability in the SMSP, as indicated by the low standard deviation of relative difference sigma(delta(i)) and highly significant Spearman rank correlations (r(s)). Linear regression analysis demonstrated the feasibility of using a single representative location to estimate limited-scale soil water conditions in karst rocky environments (R-2-values >= 0.88). More importantly, the spatiotemporal dynamics of SMSP were apparently driven by the joint effects of soil depth, vegetation, and precipitation. The SMSP was markedly homogeneous and stable in the deep soil layer, as well as during the rainy season; however, with the onset of the dry season, the vegetation tended to induce a temporary change in the root-zone SMSP. Our study suggests that the multifactor-influenced soil moisture and high temporal stability of SMSP offer both a challenge and an opportunity for better understanding the hydrological models and dynamics of rocky environments.