Estimating the unimodal and bimodal van Genuchten model parameters using the simplified evaporation and dewpoint methods

The pressure plate method has been widely used for describing the soil water retention curve (SWRC). However, the simplified evaporation method (SEM) appears as an alternative because of its high resolution and automatization. On the other hand, the unimodal and bimodal van Genuchten models (VGM) have been used for describing the SWRC by computing the shape parameters. However, parametric pedotransfer functions (PTFs) have been developed based on the pressure plates method, focusing on the unimodal VGM. Therefore, the objectives of this study were: (1) to measure the SWRC with the simplified evaporation method (SEM) coupled to the dewpoint potentiometer (DP) in soils with different land use and soil texture, (2) to adjust and compare the VG unimodal and bimodal parameters, and (3) to estimate the VG unimodal and bimodal parameters through regression techniques including a linear approach and random forest. Thirty topsoils (disturbed and undisturbed cores) were sampled across a climate gradient and measured by duplicated. Preliminary results showed that the high resolution of the SWRC data obtained with the SEM + DP leads to a better fit when using the bimodal VGM.