Response of water and salt movement to subsurface pipe drainage system in saline-alkali coastal areas of Hebei Province

To explore the application of subsurface pipe drainage technology in saline-alkali coastal areas of Hebei Province, key parameters of the drainage method (pipe depth and spacing) were tested in a farm field experiment in Nandagang, Hebei Province. Five treatments considered in the study included open-ditch drainage treatment (CK1, CK2) and 4 subsurface pipe drainage treatments (T1, T2, T3 and T4) with respectively 1.0 m/20 m, 1.2 m/30 m, 1.4 m/40 m and 1.6 m/50 m pipe depth/spacing. Groundwater depth, soil salinity, subsurface pipe optimal parameters and movement mechanisms of water and salt (e.g., salt ion component changes) were analyzed in the one-year experiment. The conclusions of the experiment were as follows: (1) Subsurface pipe drainage technology effectively controlled groundwater level, enhanced salt leaching by precipitation, lowered groundwater level and thereby prevented salt accumulation in surface soils. Subsurface pipe drainage technology was therefore suitable for application in saline-alkali coastal areas of Hebei Province with shallow groundwater levels. (2) In comparison to open-ditch drainage treatment, subsurface pipe drainage had better capacity to prevent water-logging disasters and reduced by 70% water-logging hazards during the rainy season. (3) Subsurface drainage technology significantly reduced salt content in arable soil layers (0~20 cm) by an average of 1.1‰ or even up to 1.8‰ during salt sensitive periods under cotton cultivation. (4) Considering the average decline and restoration rates of groundwater level and the economic cost of laying underground pipes, T2 treatment (with buried pipe depth of 1.2 m and spacing of 30 m) was the most optimal scheme for saline-alkali coastal areas of Hebei Province. (5) The migration rates of salt ions along with soil water movement were different in different soil types. Chloride ion migration rate was the fastest, followed by that of calcium ion.