Effects of Spacing and Depth of Subsurface Drain on Water and Salt Transport in the Field.

[Objective] Subsurface drains are commonly used in drainage system. This paper investigates the effect of their spacing and depth on water movement and salt transport in field soil under flood irrigation in Hetao Irrigation District.[Method]The study was based on data measured from a field, which were used to calibrate the HYDRAS-2D model. The validated model was then used to predict spatiotemporal changes in water flow and salt transport when the drain depth was 1.5m by varying the drain spacing from 5 to 100 m, and when the drain spacing was 25 m by varying the drain depth from 0.6 to 2.5 m.[Result] 1 The model is accurate for simulating both water flow and salt transport, with the coefficient of determination R² for soil profile moisture and salinity being 0.81 and 0.71, the RMSE error being 0.038 and 0.026, and the Nash coefficient NSE being 0.93 and 0.86, respectively. 2 The cumulative displacement and salt discharge increased with the decrease in drain spacing and the increase in drain depth. When the drain depth was 1.5 m, decreasing drain spacing from 100m to 5m increased total drainage by 4.96 times and increased the salt discharge by 5.06 times. When drain spacing was 25 m, increasing drain depth from 0.6 m to 2.5 m increased the total drainage by 64.24 times, and the total salt discharge by 60.08 times. 3 The water content in the soil profile decreased with the decrease in drain spacing and drain depth during the drainage period, and the decrease decreased as time elapsed. The salt content in the soil profile decreased with the decrease in drain spacing and depth in non-drainage period, and the decrease increased as time elapsed. 4 Horizontally, salt content in the soil increased with the increase in the distance from the drain in non-drainage period, and the increase decreased with an decrease in drain spacing and an increase in drain depth. When drain depth was 1.5 m, the difference in soil salinity between the drain spacing 5 m and 100 m increased from 26.67% in the region proximal to the drains to 39.88% in the middle of the drains. When the drain spacing was 25 m, the difference in soil salinity between the drain depth 0.6 m and 2.5 m increased from 30.56% in the region adjacent to the drain to 50.39% in the middle of the drains. Vertically, soil water content varied in a "sickle" shape with the increase in soil depth. The initial drainage of salt increased with the increase in drain depth, while in non-drainage period it decreased with the increase in drain depth.[Conclusion]Reducing drain spacing or increasing drain depth can improve the drainage and salt discharge, thereby reducing water storage in soil profile during drainage period and soil salt content during non-drainage period. Our results suggested that the optimal drain spacing was 15-50 m and the drain depth was 1.0-2.0 m for Hetao irrigation district. [ABSTRACT FROM AUTHOR]