Evaluation of upward flow of groundwater to freezing soils and rational per-freezing water table depth in agricultural areas

In arid agricultural areas with shallow water table depth, water exchange between vadose zone and groundwater is intensive during the freezing-thawing period, and thus has a significant impact on crop growth. This study is aimed to develop a one-dimensional model that couples heat and variably saturated water in seasonally freezing-thawing agricultural areas. To alleviate numerical oscillations occurring during the phase change at zero temperature, a three-level iteration scheme was developed for solving the water and heat coupling model, which improves numerical stability for variably saturated water and heat modeling. The model accuracy was evaluated by comparing the simulation results with those from SHAW under various soil texture and bottom conditions. The square of correlation coefficient (R-2) of freezing and thawing depth and soil temperature profiles are above 0.95 and the values of root mean square error (RMSE) of total soil water content are lower than 0.05 cm(3)/cm(3), which suggest the model accuracy. The model was further applied to simulate the hydrothermal conditions in the Yonglian irrigation field of Hetao Irrigation District in Inner Mongolia, China. Model calibration and validation were conducted, and the validated model was used to predict hydrothermal changes under scenarios with different soil surface temperatures and pre-freezing water table depths. The results show that the soil freezing process has great impact on the fluctuation of water table depth when the pre-freezing water table depth is shallower than 180 cm. The maximum freezing depth has a negative relation with the pre-freezing water table depth when it is shallower than 180 cm. An empirical formula was established to estimate the upward flow of groundwater to freezing soils. The soil moisture and thermal conditions were analyzed in the germination and seedling stages after the freezing-thawing period, and the analysis leads to a recommended pre-freezing water table depth of 100 cm, which provides suitable soil moisture and thermal conditions for the crop growth.