Frequency domain reflectometry and Hydrus-1D for parameterizing the van Genuchten equation in a sandy entisol

This study hypothesized that estimating van Genuchten equation parameters using Hydrus-1D, with matric potential data (from tensiometry) and moisture data (from gravimetry and FDR probe), yields soil-water retention curves (SWRC) more consistent with field conditions compared to laboratory methods. The objective was to obtain and compare van Genuchten parameters from Hydrus-1D and laboratory methods. For this, an instantaneous profile experiment was conducted in Fortaleza/CE, Brazil, using four plots (1.5 × 2.0 × 0.5 m) in a loamy sand Entisol. Each plot had an FDR probe access tube (Diviner 2000) and tensiometers (Hg manometer) at depths of 0.2 and 0.4 m. For gravimetric determinations, soil samples were collected at 0.20 and 0.40 m depths concomitantly with tensiometer and FDR readings, and the mass-based water content was converted into volumetric water content using bulk density. When the variation in soil moisture over time reached a threshold drainage rate of ∂θ/∂t = 0.01 cm³ cm⁻³ d⁻¹, disturbed and undisturbed samples were collected for lab analysis of bulk density, particle density, porosity, and SWRC, fitted to the van Genuchten model (m = 1 − [1/n]). Hydrus-1D estimated four van Genuchten parameters, followed by ANOVA, MANOVA, PCA, and cluster analysis in SAS. Model performance was assessed via efficiency coefficient and RMSE. Results showed that inverse modeling with Hydrus-1D using gravimetry and FDR data yields SWRC better representing field conditions, with FDR probes proving practical and effective for SWRC estimation.