Effects of Subsurface Drainage Regulation on Water and Salt Dynamics in Saline–Alkali Soils Under Laboratory Conditions

Regulating agricultural drainage based on field water‐salt control requirements is essential for sustainable development. To explore the effects of regulation timing and outlet elevation on water and salt transport, a laboratory drainage experiment was conducted to study the effects of delayed drainage for 48 h and controlled drainage with outlet heights increased by 0.1 m and 0.2 m (CD 10 and CD 20 ), respectively, on the migration patterns of soil water and salt. Compared with free subsurface drainage, CD 10 and CD 20 reduced the cumulative drainage volume by 8% and 6%, the amount of salt discharge by 7% and 5%, and the desalination rate by 9% and 3%, respectively. CD 10 and CD 20 also distinctly increased the salt concentration of the drainage water, with the increase being most evident during the last drainage stage, with values of 46% and 67%, respectively. Moreover, they improved the uniformity of water and salt distributions, with the horizontal coefficient of variation (CV) of soil moisture decreasing by 32% and 49% and of salinity decreasing by 38% and 8%, respectively. Overall, delayed drainage had no significant effect on drainage salt removal or soil water‐salt distribution. Controlled drainage has considerable potential for improving soil moisture and enhancing the spatial uniformity of water and salt distributions.