Estimating the spatial relationships between soil hydraulic properties and soil physical properties in the critical zone (0–100 m) on the Loess Plateau, China: A state-space modeling approach

Abstract Soil hydraulic properties (SHP) such as the soil water retention curve and soil saturated hydraulic conductivity (Ks) of the deep profile in the Earth's critical zone (CZ) are important factors for investigating the water cycle process in the CZ. However, details are lacking about the SHP for the deep profile as well as their relationships with other soil properties. In the present study, SHP were obtained for a 100-m profile by soil core drilling, where the objectives were to understand the spatial distributions of SHP and to quantify the relationships between SHP and soil properties based on state-space model analysis and linear regression analysis. The results showed that SHP were not significantly related to the silt content and there was no cross-correlation between SHP and the soil organic carbon content. The soil physical properties (bulk density, sand content, and clay content) could account for most of the total variation in SHP. Compared with linear regression analysis, state-space modeling described the spatial relationship between SHP and soil physical properties much better. This study provides information about the SHP in deep profiles, thereby provide important parameters for investigating the water cycling process in the CZ and for developing pedotransfer functions.