Responses of soil water dynamic processes and groundwater recharge to irrigation intensity and antecedent moisture in the vadose zone

In this study, a field experiment was conducted to investigate the soil water dynamics and water percolation through the deep vadose zone. A calibrated HYDRUS-1D model was used to simulate the process of soil water movement and the water budget. Based on the measured volumetric soil water contents, the model was well calibrated and validated. Then, we conducted scenario analyses to determine the combined effects of irrigation amount (IA), antecedent soil moisture (AM), crop evapotranspiration, and deep percolation (DP) in an irrigation event. Four IAs (5, 10, 15, and 20 cm) and three AM conditions (AM-1, AM-2, and AM-3) were controlled in the scenario analyses. The results indicate that according to the S-e's (effective saturation) values status and the observed or simulated depth, there could be different conclusions on the influence of DP. Under different IAs in dry (AM-1) and medium (AM-2) AM status, DP changed slightly; it was 0.39 and 2.47 cm in AM-1 and 0.40 and 2.48 cm in AM-2 for the summer maize and winter wheat crop, respectively; the AM had a crucial contribution to DP. While under the condition of wet AM (AM-3) or small observation depth, the water inputs could have a significant effect on DP. According to increasing irrigation intensity, the higher values of S-e (>0.6) in the whole profile were only displayed between 70 and 300 cm at AM-1, 70-500 cm at AM-2, and 70-below 600 cm at AM-3, which were gradually extended and moved down with increasing AM. Hence, the IA significantly affected the water percolation at a depth of 200 cm, whereas there was a weak influence at 600 cm except in AM-3. Furthermore, in the higher values of the S-e (>0.65) domain, the correlation between IA and DP was an exponential function and significantly under P < 0.05. In addition, DP began to occur when the soil water content was equal to or greater than 0.75 times that of the field water capacity or the S-e > 0.65. When the coarse silt layer became embedded in the silt clay soil profile, it lagged the process of water transport but did not affect permeability in the end.