Determination of soil water retention curves from thermal conductivity curves, texture, bulk density, and field capacity

The soil water retention curve (SWRC) is frequently expressed using the van Genuchten (VG) model, which has four parameters: saturated water content (theta s), residual water content (theta r), alpha, and m (1-1/n). Soil thermal conductivity (lambda), which is linked to the hydraulic properties of unsaturated soil, has been a proxy variable used to estimate SWRC. In this study, we present a new approach to estimate the VG model parameters. Parameters theta s, alpha and m are calculated from the information of soil texture, bulk density (rho b), and a measured water content at field capacity (theta fc, at -33 kPa or -10 kPa), and theta r is estimated from the thermal conductivity versus water content curve, lambda(theta), based on similarities between SWRCs and lambda(theta) curves. The new approach was evaluated with laboratory and field measurements on 23 soils of various textures, rho b values, and theta values. Results showed that for repacked core samples, intact core samples, and in situ field soils, the new approach estimated SWRCs with average root mean square errors (RMSEs) of 0.042, 0.030, and 0.049 m3 m-3, respectively. The new approach offers a quick and effective way to estimate SWRCs accurately with measured lambda(theta) curves, texture, bulk density, and theta at field capacity.