Distribution and variation of water and salt in soil profile under controlling subsurface drainage

Abstract: Soil salinization and drought are important factors restricting the sustainable development of agriculture in irrigation areas. Unreasonable irrigation and salinity leaching can enhance groundwater level and soil salinity accumulation. The free subsurface drainage can drain excess soil moisture from the field and speed up the leaching rate of soil salinity, subsequently leading to the lowering of the groundwater level and soil salt accumulation. However, the free subsurface drainage may cause excessive drainage and the drought stress of crops during the irrigation intermission. By raising the height of drainage outlet of subsurface pipe, controlled drainage methods can reduce field drainage volume and improve water use efficiency. Therefore, controlled drainage would change the movement of soil water and salt, which will affect soil salinity and regulate the soil water condition. To assess the distribution and variation of soil water and salt affected by controlled drainage and drain spacing, three drainage treatments were designed in this study including the free drainage (FD), controlled drainage (CD) and reducing subsurface pipe spacing of the CD (CD1/2). For the FD treatment, the spacing of subsurface pipe was 50 m and the depth of subsurface pipe was 1.6 m. The CD treatment raised drainage outlet height of FD by 0.4 m. On the basis of CD treatment, the treatment of CD1/2 decreased the spacing of subsurface drain to 25 m. The static and dynamic spatial distribution of soil water content and salinity were analyzed and the variation characteristics of soil water and salinity before and after the irrigation during the typical irrigation period were evaluated. The experiment was conducted during the growth period of maize at the Yichang test station in the Hetao Irrigation District of China. The results showed that compared with the free drainage, the controlled drainage increased the soil relative water content by 8.27%, enhanced the soil water increase rate during the irrigation stage, and reduced the soil water decrease rate during the intermittent stage. The controlled drainage also improved the distribution uniformity of salt in soil profile, and it decreased the horizontal and vertical variation coefficient by 45.88% and 32.55%. At the same time, the controlled drainage reduced the soil salinity by 36.73% in soil profile, enhanced the desalination area by 29.17% during the irrigation stage and reduced the salt accumulation area by 14.29% during the intermittent stage. After reducing the drain spacing of control drainage, the soil water increase rate was reduced during the irrigation stage, and the soil water decrease rate was increased during the intermittent stage, meanwhile the horizontal distribution uniformity of soil salinity before irrigation was improved while the horizontal distribution uniformity of soil salinity after irrigation was decreased. The controlled drainage enhanced desalination rate, salinity distribution uniformity, and soil water retention. Thus, it has the potential to alleviate soil secondary salinization and improve agricultural water use efficiency.