Water retention models for soils mixed with waste residues: application of the modified van-Genuchten and Brooks-Corey models

Applying additives (e.g., biochar and rapeseed-oil residue) to soils is a common agronomic practice used to improve water-retaining capacity. An investigation of water availability and an applicability evaluation of traditional soil water retention curve (SWRC) are therefore necessary for the soil mixed with waste residues. We aimed to investigate the impact of additives on water retention and further to develop models for their prediction. Loam and sandy loam were mixed with different amounts of rapeseed-oil residue and biochar, respectively. And results show that the biochar and rapeseed-oil residue retain more water, with a minimum and maximum increase in soil moisture by approximately 26.2% and 92.7%, and 10.2% and 19.4%, respectively, relative to pure soil. Furthermore, based on the soil capillary theory, modified van–Genuchten (M-VG) and Brooks–Corey (M-BC) models were constructed and compared, which indicate that both the modified physics-based models (M-SWRC) have higher accuracy than the traditional SWRC models in soil moisture prediction; furthermore, the M-VG model outperforms the M-BC model, due to larger R² and smaller MAPE and RMSE. For the field soils mixed with additives, the soil suction density function has potentials for SWRC model modification based on the soil capillary theory.