Temporal stability of soil water content on slope during the rainy season in gully regulation watershed

To increase reserve farmland resources, ensure food security, and to improve the ecological environment, large-scale gully regulation projects in Yan'an area of the Loess Plateau have been undertaken in China since 2013. Understanding the spatial and temporal distribution characteristics of slope soil moisture during the rainy season is, therefore, important for vegetation selection used in restoration measures in this area. The temporal stability of soil water content in the upper 100 cm soil depth in a gully regulation watershed was examined using six measurements taken during the rainy season (May–November 2016). Temporal stability analysis of the soil water content was undertaken using Spearman’s rank correlation coefficient and the relative difference method. Results showed that soil water content in the 0–50 cm and 0–100 cm soil depths demonstrated moderate temporal and spatial variability, and soil water content variability gradually decreased with increasing soil depth. Soil water content in both depths had strong temporal stability, having a positive correlation with soil depth. Furthermore, temporal stability may decrease with increasing soil water content (p < 0.05). The number and position of representative locations were not constant, which varies with the soil depth and the estimation method. Five methods (mean relative difference, minimum relative difference standard equilibrium, temporal stability index, mean absolute deviation and root mean square error) can be used to determine the representative location of temporal stability. However, the prediction accuracy of the soil water content at representative locations obtained using the different methods for mean slope water content differed. Comparatively, the mean absolute deviation method had a higher accuracy, followed by the minimum relative difference standard equilibrium method. Results indicate that the prediction accuracy of five methods increased with soil depth.