A calculation method of unsaturated soil water content based on thermodynamic equilibrium

Abstract. Accurate determination of soil water content is crucial for studying the local ecological water cycle, agriculture, forests and grasslands management, slope stability, environmental processes, and ecological problems. Current measurement methods of soil water content have challenges with the complicated operation, high costs, significant system errors, and soil type-specific accuracy. In this study, we proposed an alternative method to measure soil water content using the ratios of mass and density of water to vapor based on system science and thermodynamics. We derived the mass ratio and density ratio of water to vapor as a function of temperature from published data and validated the accuracy of the proposed method using observed temperature, relative humidity, and volumetric water content in two soil textures of medium and fine sand. We further demonstrated that the mass ratio function is independent of the nature of the water-containing media using theoretical and statistical analysis. The absolute error of soil water content between the calculated and measured ones is less than 1 % and is positively correlated with temperature. This study significantly improves the measurement accuracy of soil water content, eliminates the effect of water-containing medium on soil water content measuring, and has excellent potential for application in the field soil and even rock water content measurement.