Estimation of soil water content at permanent wilting point using hygroscopic water content

Traditional methods for determining soil water content at permanent wilting point (theta(PWP)) are time consuming. The objectives of this study are to develop a model for predicting theta(PWP) based on easily measured hygroscopic water content and to compare the performance of the proposed model with three previous models: the Aina & Periaswamy model (AP model), the van den Berg et al. model (vdB model) and the Seybold & Harms model (SH model), which estimate theta(PWP) from particle size distribution and/or organic matter (OM) content. By assuming that the dry end of the soil water retention curve could be described with the linear Campbell & Shiozawa model, a simple linear model, theta(PWP) = 3 theta(RH50), was developed to predict theta(PWP) using theta(RH50). The theta(RH50) of the soils ranged from 0.002 to 0.157 g g(-1) and the theta(PWP) ranged from 0.014 to 0.279 g g(-1). The validation results on 48 soils showed that the root mean squared error (RMSE) and mean absolute error (MAE) of the proposed model were 0.007 g g(-1) and 0.005 g g(-1), respectively. Model comparison showed that the prediction accuracy of four models for theta(PWP) was in the order of proposed model > AP model > SH model > vdB model. Thus, theta(RH50) is a viable predictor for theta(PWP), which eliminates the need for soil particle size distribution and OM content. Highlights Water content at permanent wilting point (theta(PWP)) relates to hygroscopic water content. A model was developed to predict theta(PWP) using water content at relative humidity of 50%. The proposed model performed better than existing models.