Spatial-temporal variation characteristics of water- salt movement in coastal saline soil improved by flooding and subsurface drainage

Abstract: Flooding the field and leaching salts from the soil through a subsurface drainage is a typical method for reclaiming saline soil. However, water-salt movement is non-uniform in the drained region under flooding, which affects leaching efficiency. In the paper, the spatial-temporal variations of water-salt movement in coastal saline soil under flooding with subsurface drainage were simulated with Vedernikov infiltration equation and van der Molen leaching equation. The subsurface drainage systems for a drainpipe depth (1.2 m) and various spacing (3, 6 and 9 m) were laid in 3 simulated field trial plots respectively, and leaching water volumes were the same in the trial plots. Furthermore, it was assumed that the soil was homogeneous and isotropic, and the flooding head was negligible in the simulation study. The results showed that, the stable infiltration rate in the regions with 3, 6 and 9 m spacing varied between 3.14-4.26, 1.19-3.68 and 0.58-3.55 cm/d respectively, and soil water moved more quickly in areas near the drainpipes than in the midway between drainpipes, and the greater the drainpipe spacing, the more obvious the spatial variation of the infiltration rate in the drained region. In several zones between drainpipes, the spatial variation of infiltration rate was obviously different too, and was apparently greater in the zone near the drainpipe. The spatial distribution of leaching water volume was extremely uneven in the drained region, and the more water was allocated for the area closer to the drainpipes. Comparison between 3 subsurface drainage systems showed that, the greater the spacing of the drainpipes, the more the flooding water that was allocated above the lines of the drainpipes, and the amount of water allocated above the lines of the drainpipes with 9 m spacing was 1.39 and 2.00 times that above the lines of the drainpipes with 6 and 3 m spacing respectively. The apparent spatial difference of infiltration rate resulted in spatial uneven distribution of flooding water in the drained region, and accordingly the changes of soil salinity were greatly non-uniform, and the salts moved more quickly in areas near the drainpipes than in the midway areas between drainpipes, too. After flooding for 20 days, only in the 0-0.6 m area away from the drainpipes with 9 m spacing, salinity content of the 0-60 cm soil layer decreased to 3.00 g/kg. After flooding for 40 days continuously, only in the 0-1.6 m area close to the drainpipe, the salinity content of the 0-60 cm soil layer decreased to 3.00 g/kg. In order to make the salinity content of 0-60 cm soil layer dropped to 3.00 g/kg completely in the whole area between the drainpipes, it was necessary to flood the surface for 100 days, which would cause a large proportion of the irrigation water volume to flow away from the vicinity of the drainpipe, thereby wasting water resource and reducing leaching efficiency. The research provides a simple and feasible method for simulating water-salt movement in drained region under flooding for reclaiming saline soil, which can guide the design of irrigation and drainage to achieve effective improvement of saline soil.