Effects of Subsurface Drainage on Soil Salinity and Groundwater Table in Drip Irrigated Cotton Fields in Oasis Regions of Tarim Basin

As one global issue, soil salinization has caused soil degradation, thus affecting the sustainable development of irrigated agriculture. A two-year study was conducted in 2018 and 2019 to identify the effects of subsurface drainage spacing on soil salinity and groundwater level, the latter of which is in a high-water table in drip irrigation cotton fields in the Tarim Basin oasis in southern Xinjiang, China. Three subsurface drainage treatments, with a drain spacing of 10 m (W10), 20 m (W20), and 30 m (W30), respectively, and a drainage-absent treatment (CK), are tested. With CK, soil salinity in the 0–60 cm layer was accumulated within a year. In contrast, the subsurface drainage reduced the soil salinity at a leaching rate of 10–25%. When decreasing the drain spacing, it was found that the soil desalination rate increased significantly (p < 0.05) with good repeatability. Experimental results showed that the fitting equation of the soil salinity leaching curve could accurately describe the soil salinity leaching pattern of drip irrigation, and thus could be further used to inversely determine the theoretical drip irrigation leaching quota for those soils with different salinity degrees. As such, subsurface drainage could effectively control the groundwater table. Compared with CK, subsurface drainage deepened the groundwater table and mitigated the fluctuation of the groundwater level. These effects were strengthened by reducing the drain spacing. Correspondingly, the influence of the fluctuation of the groundwater table was reduced.