Study on Water and Salt Transport under Different Subsurface Pipe Arrangement Conditions in Severe Saline-Alkali Land in Hetao Irrigation District with DRAINMOD Model

As an effective method to improve saline-alkali land, the drainage from subsurface pipes has been extensively studied in typical arid and semi-arid agricultural areas (Hetao Irrigation District). However, there are few studies on the improvement of subsurface pipe layout and the long-term soil salinization control in the process of leaching and soil amendment with subsurface pipes in this area. This study investigated the water and salt migration in the process of amending the heavy saline soil. Field experiments growing sunflowers and numerical model calculation were combined in this research. It was found in the field experiment that the salt concentration in the surface pipe drainage was positively correlated with the salt content in the soil and the depth of the pipe, while it was negatively correlated with the amount of irrigation water and the spacing of crops. Thus, the soil desalting rate (N) and salt control rate (SCR) were positively correlated with the depth of the pipe, and they were negatively correlated with the spacing. The leaching effect of irrigation would decrease when the soil salt content decreased. On the basis of field experiments, the DRAINMOD model and drainmod equation were used to calculate the water and salt migration in 38 different field plots during 2019 and 2020. When N was the same, the soil salinity in several plots with large burial depth could be controlled below the salt tolerance threshold of sunflowers during the growth period in the second year. The quantitative relationship between N and SCR, soil salt content before leaching, water amount of leaching, pipe spacing and buried depth was already established. These results can help develop strategies for desalination and salt control in the soil in the arid and semi-arid areas with the optimal layout of subsurface pipes.