Spatial variability of shallow soil moisture and its stable isotope values on a karst hillslope

Soil moisture (theta) and its stable isotope values are two of the most commonly used parameters for studying hydrological processes in soils. Despite their unique ability to aid in distinguishing between soil water evaporation and plant transpiration, the spatial variability of soil water isotope values is not fully understood. In the current study, 10 m x 10 m grids were established within a 90 m x 120 m plot on a highly heterogeneous karst hillslope. Two sampling campaigns were conducted during the early growing season, on April 15, and during the mid-growing season, on August 18, 2011. Stratified soil samples were collected from the shallow soil layer (0-30 cm) to measure theta and its stable isotope values, which were represented by soil water delta D values (delta D-theta). Related soil properties, land cover and topography were also measured and treated as influencing factors. On both sampling dates, theta decreased with depth, while delta D-theta stayed constant for all soil layers and were similar to the most recent rainfall values. Additionally, the variance of delta D-theta was smaller than that of theta, especially on August 18 when the most recent rainfalls had similar delta D values. The high contrast between theta and delta D-theta caused the impacts of other factors on delta D-theta to be masked by the impact of the recent rainfall. Soil moisture presented a moderate to strong spatial dependence, which was consistent with the spatial variability of the influencing factors that were significantly correlated with theta. Soil water delta D value exhibited weak spatial dependence and random spatial patterns that were different from the other influencing factors. Moreover, significant correlations between these same influencing factors and delta D-theta disappeared after a partial analysis with theta as a controlled variable, which means delta D-theta was indirectly affected by these influencing factors through theta. This suggests that the spatial variability of delta D-theta was being controlled at fine scales. Our results highlight the importance of analyzing the spatial variability of delta D-theta, its influencing factors and B in shallow soil layers separately. (C) 2015 Elsevier B.V. All rights reserved.