Progress in developments and applications of the HYDRUS model and associated coupling model packages

HYDRUS model is widely used in agricultural irrigation and pollution transport because it can comprehensively consider the movement of water, heat, and pollutants in the soil-plant-atmosphere system. However, there is no summary of water and solute transport simulations under the influence of plant roots and the simulation of Low Impact Development (LID) facilities. This paper systematically summarized the simulation of water, salt, nitrogen, heavy metals and organic substances under the influence of plant roots and the simulation of LID facilities, and analyzed the simulation accuracy. The results show that HYDRUS can well simulate the movement of water, salts, and pollutants under the influence of plant roots, but the simulation accuracy is affected by many factors such as plant roots, soil and its physical, and chemical and biological characteristics. Additionally, since the relevant model parameters can be determined by historical documents, experimental measurements, and model prediction and inversion, the simulation results are also different. Future research should focus on the following aspects: (1) Considering the effects of different planting years, different root types, and different root characteristics such as root length, root diameter, root volume, and root density in the simulation of plant root effect by HYDRUS. (2) Accumulating more transport parameters for new pollutants, including diffusion, adsorption, and degradation, to improve the simulation of pollutant transport. (3) Enhancing the description and simulation of the heterogeneity of unsaturated zone media. (4) Strengthening the acquisition and determination of parameters through the integration of Machine Learning and Artificial Intelligence. (5) Further developing and applying HYDRUS coupling models to enable comprehensive simulations of the entire process of surface water, soil water, and saturated groundwater during seepage.