Evaluation of Water–Salt Regulation and Economic Performance of Sand-Column–Assisted Subsurface Pipe Drainage in Saline–Alkali Soils Containing Low-Permeability Layers

Soil salinization is a major constraint on the sustainable development of global agriculture. As an effective technique for reclaiming saline soils, subsurface pipe drainage often exhibits limited drainage and salt removal efficiency under complex soil conditions characterized by low-permeability layers. To enhance drainage and salt removal efficiency, this study proposes a sand-column–assisted subsurface pipe drainage system, in which sand columns are installed directly above subsurface pipes and between adjacent pipes to establish stable vertical percolation pathways. Based on two years of field experiments and HYDRUS-3D numerical simulations, the regulatory effects of sand-column–assisted subsurface pipe drainage on the spatiotemporal variation of profile salinity, drainage and salt removal capacity, and groundwater dynamics were systematically evaluated, and comparisons were conducted with conventional subsurface pipe drainage under fixed-quota and fixed-time irrigation schemes focusing on drainage capacity, desalination efficiency, and economic benefits. The results indicated that over the two-year experimental period, soil salinity in the 0-40 cm plough layer of plots with sand-column-assisted subsurface pipe drainage declined from 15.48 g/kg to 8.53 g/kg, achieving a desalination rate of 44.85%, and no salt accumulation was observed in deeper soil layers. Under both fixed-quota and fixed-time irrigation conditions, sand-column–assisted subsurface pipe drainage exhibited superior groundwater control performance compared with conventional subsurface pipe drainage. Under the same irrigation amount, sand-column–assisted subsurface pipe drainage was more suitable for rapidly reducing surface soil salinity in the plough layer, whereas conventional subsurface pipe drainage showed more pronounced advantages in total salt removal and sustained salt discharge capacity. The average salt removal rate of sand-column–assisted subsurface pipe drainage was 49.23% higher than that of conventional subsurface pipe drainage at 1.5 d, whereas the cumulative salt removal of conventional subsurface pipe drainage was on average 16.19% higher than that of sand-column–assisted subsurface pipe drainage at 15 d. Under identical irrigation durations, the cumulative salt discharge of sand-column–assisted subsurface pipe drainage was generally higher than that of conventional subsurface pipe drainage. At a pipe spacing of 10 m, the per-hectare infiltrated water volumes for conventional subsurface pipe drainage and sand-column–assisted subsurface pipe drainage were 1399.95 m³ and 2128.05 m³, respectively, with salt removal by sand-column–assisted subsurface pipe drainage being 9.40% higher than that under conventional subsurface pipe drainage. Comprehensive economic evaluations under the two operating scenarios indicated that under fixed-quota irrigation leaching conditions, sand-column–assisted subsurface pipe drainage system with sand columns installed only above subsurface pipes showed overall economic advantages in terms of EIRR, ENPV, EBCR, and payback period. Under fixed-time irrigation leaching conditions, conventional subsurface pipe drainage exhibited superior overall economic benefits compared with sand-column–assisted subsurface pipe drainage with sand column uniformly installed both above the pipes and between adjacent pipes, whereas sand-column–assisted subsurface pipe drainage with sand columns installed only above the pipes exhibited better economic performance than conventional subsurface pipe drainage.