Using HYDRUS 2D/3D to Evaluate Soil Water Movement in Drip-irrigated Young Populus tomentosa Plantations on Sandy Loam Soil

     Understanding the rules of soil water movement under drip irrigation can provide data support and theoretical basis for developing precise drip irrigation strategies. In this study, a two-years-old Populus tomentosa plantation under surface drip irrigation on sandy loam soil was selected to measure the dynamics of soil water potential (ψ_s), wetting front and soil water content (θ) during irrigation and water redistribution periods were investigated in field experiments. Then, the observed data in the field were used to evaluate the accuracy and feasibility of the HYDRUS-2D/3D model for simulating the short-term soil water movement. Besides, the validated model was used to simulate the dynamics of wetting front under different initial soil water content (θ_i). During irrigation, the variation of ψ_s, horizontal and vertical movement distances of the wetting front, and θ within the wetting volume with irrigation duration could be described by the logistic function (R² = 0.99), the logarithm function (R² = 0.99), the power function (R² = 0.82), and the polynomial function (R² = 0.99), respectively. At the end of irrigation, the horizontal and vertical movement distances of the wetting front reached 22.9 cm and 37.3 cm, respectively. The ψ_s and θ within the soil wetting volume were 61.6% and 30.9% higher than those at the start of the irrigation, respectively, but the ψ_s decreased to its initial level about 120 hours later after the stop of irrigation. The average deviations of the horizontal and vertical wetting radius between the simulated and measured values were 1.3 and 4.5 cm, respectively. The mean RMSE and RMAE of HYDRUS-2D/3D for simulating θ at the end of irrigation and during water redistribution were 0.021 cm³∙cm^-3 and 9.7%, respectively. The movement distances of wetting front in the experimental plantation under various soil drought degrees (soil water availabilities were 40%, 60%, 73% and 80%) were obtained through scenarios simulations using HYDRUS-2D/3D. And it was found that the wetting front could move further under higher θ_i, and the movement distance of the wetting front was always smaller in the horizontal direction than in the vertical direction under different θ_i conditions. Consequently, HYDRUS-2D/3D can be used to well simulate the short-term soil water movement in drip-irrigated young P. tomentosa plantations on sandy loam soil. In addition, the constructed figure (describes the variations of the horizontal and vertical soil wetting distances with the irrigation duration) can be used to determine the reasonable irrigation duration for the plantations of P. tomentosa and other tree species on sandy loam soil.