A Physics-Driven Spectroscopic Approach for Rapid Estimation of the Soil Water Retention Curve

The knowledge about soil hydraulic properties (i.e., water retention and hydraulic conductivity characteristics) is of relevance for accurate determination of land surface fluxes. Yet, the laborious and time-consuming process of measuring the soil water retention curve (SWRC) with conventional laboratory methods poses a challenge. In addition, the measured soil water content and matric potential pairs obtained with standard methods are often fragmentary and consist of only a limited number of measurements across the desired soil water content range. Proximal and remote sensing methods are rapid and cost-efficient alternatives to quantify soil attributes across different scales. However, past studies that centered around proximal and remote sensing of soil hydraulic functions mainly rely on statistical relationships and a physically-based method is still lacking. In this presentation, we introduce an innovative physics-based laboratory method that allows the direct estimation of the complete SWRC across the entire range from saturated to dry conditions. The inputs to the model include measured data pairs of soil water content and reflectance within the shortwave infrared domain. The fundamental hypothesis behind the new method is that the soil reflectance spectra are a function of both soil water content and the pore scale distribution of capillary and adsorbed soil water. The performance of the proposed model was evaluated for 21 soils that vastly differ in physical and hydraulic properties. The RMSE and R-squared between retrieved and measured water contents at various matric potentials were found to be 0.03 m³ m⁻³ and 0.96, respectively, indicating the good performance of the proposed method. The results suggest that the new method is a rapid and efficient alternative to established laboratory measurement methods.