Development and application of a new package for MODFLOW-LGR-MT3D for simulating regional groundwater and salt dynamics with subsurface drainage systems

Subsurface drainage systems have been widely used to help manage soil salinization worldwide. However, designing a subsurface drainage system requires predictions of regional groundwater and salt dynamics. While these dynamics can be obtained using MODFLOW’s drain package (DRN), this package requires that the drain conductance be specified, which is difficult to determine for subsurface drainage systems with different layout parameters, including the pipe spacing, depth and radius. In this study, a new subsurface drainage (SDR) package based on the Hooghoudt equation was developed within the MODFLOW-LGR-MT3D framework to simulate the effects of subsurface drainage pipes on groundwater flow and salt dynamics. The accuracy and applicability of SDR were tested by a synthetic case, a published indoor experiment and two field experiments. Comparison with indoor experimental data shows that the RRMSE of cumulative drainage volume was 5.08% and the RRMSE of groundwater table depth was 3.69%. The simulation results of outdoor experiment show that the RRMSE of drainage volumes of a single pipe ranged from 3.14% to 4.69%, and the relative errors of cumulative drainage volumes were below 6% and the relative errors of drainage discharge were 3.1–16.3%. The results demonstrate that SDR can accurately simulate leaching water and solute transport under transient flow conditions. The model was then applied to the Yonglian irrigation area of Inner Mongolia, China, for subsurface drainage system design, and 3 different layout schemes of subsurface drainage systems were presented. The total salt discharge increased by 38–54% after the application of the subsurface drainage system, 36–45% of which was discharged by subsurface drainage pipes. This study provides a pilot example for regional subsurface drainage system design. • The new Subsurface Drainage (SDR) package is based on the Hooghoudt equation. • The SDR package can accurately capture the drainage flux under complex transient conditions. • The SDR package can be used for designing layout parameters of subsurface drainage systems in regional scales. • Subsurface drainage systems remove 36–45% of total salt discharge. • Subsurface drainage systems decrease salt accumulation from phreatic evaporation.