Biochar is a very important soil amendment in improving soil water retention. A multi-scale mathematical model was developed for investigating heat and mass transfer between unsaturated sandy soils and biochar. As both the sandy soil and the biochar were porous media, a water equilibrium model was set up based on existing experimental results. One-dimensional numerical simulation in unsteady state was conducted for obtaining the water content distribution in the biochar and the sandy soil. The numerical results agreed well with the experimental data, verifying this mathematical model can be used to evaluate the effects of biochar on sandy soil water retention. The effects of the environmental conditions, such as the relative humidity and the mass transfer coefficient (Hm), on the sandy soil water retention with biochar added, were further evaluated by the mathematical model. The numerical results showed that the application of biochar would be more effective in areas where soil evaporation was very strong. (C) 2015 Elsevier Ltd. All rights reserved.