Simulating water and salt transport in subsurface pipe drainage systems with HYDRUS-2D

•Three subsurface pipe boundaries were applied in HYDRUS-2D during SPD.•The effects of hydraulic conductivity and width of resistance layer were analyzed.•Salt discharge in regions containing CLs were analyzed using HYDRUS-2D. Subsurface pipe drainage (SPD) has been widely applied in arid and semiarid regions to control soil salinization. To increase the accuracy of HYDRUS-2D simulations in SPD, three effective strategies, including the resistance layer (RL), actual infiltration surface (AIS), and equivalent radius (ER) treatments, were proposed to replace the standard seepage boundary (SSB) for subsurface pipes in HYDRUS-2D. First, the accuracy of the three strategies was calibrated using laboratory experiments with the multi-objective calibration method of the PEST program. Second, additional laboratory experiments were conducted to validate the three strategies. The RL, AIS, and ER strategies obtained a relatively high simulation accuracy in terms of the soil salt content (SSC), soil water content (SWC), and drainage and salt discharge processes that was significantly higher than that obtained with the standard seepage boundary (SSB) strategy. Moreover, compared with the AIS, ER, and SSB strategies, the RL strategy attained the highest simulation accuracy regarding the water drainage (root mean square error (RMSE) = 3.42 L; mean absolute error (MAE) = 2.91 L) and salt discharge (RMSE = 0.12 kg; MAE = 0.11 kg) in SPD. The relative position of the subsurface pipe has an important effect on the desalting rate of the clay layer (CL). Based on the SPD simulation by HYDRUS-2D with the RL strategy, we recommend placing the subsurface pipe below the clay layer to obtain the best water drainage and salt discharge effects in regions containing CLs. Furthermore, a positive logarithmic relationship between the drainage rate of the subsurface pipe and the CL saturated hydraulic conductivity was found (R2 = 0.99). Future research should focus on the upper and lower boundary conditions of HYDRUS-2D to further increase the SPD simulation accuracy.