| The technology of film mulched drip irrigation ?FMDI? has been applied widely for planting cotton in Xinjiang, China, where scant precipitation and high evaporation have result in serious salinization.Salts in the soil have be leached downwards from the shallow root zone ?0-40 cm? and accumulated in the lower root zone ?40?60 cm? on account of the little quota and high irrigation frequency of FMDI.Drainage with Subsurface pipes ?SSPs?, according to the advantages of land saving and high drainage efficiency, has been widely used in the world for amelioration of saline-alkaline soils. SSPs are typically placed in the saturated zone where they serve to maintain the water table as well as to drain salts from leaching. However, whether and how the SSPs can maintain the high efficiency of water drainage and salt discharge is a big question. Therefore, it is very important to cognize clearly the rules of water movement and salt transport under the drainage condition when the SSPs are laid at the unsaturated soil layer for Xinjiang and other arid and semi-arid regions.The paper main study the water flow and salt transport processes in soils representative of the cotton field under FMDI in order to evaluate the salt discharge efficiency of SSPs in the unsaturated root zone. Series experiments ?including the leaching from soil surface and drainage with SSPs, supply water and drainage from SSPs? were conducted in order to quantify drainage processes, calibrate the hydraulic parameters and verify the established models. HYDRUS-2D/3D was employed to simulate the dynamics of soil water flow and salt transport during drainage with SSPs. The simulated results were used to evaluate the salt discharge efficiency and discuss the feasibility of the improved methods?including leaching from soil surface, seep-proof beneath SSPs under the leaching from soil surface condition, water supply and drainage from single SSP, water supply and drainage from interval SSPs?.The main results are outlined as follow:?1? A field leaching experiment, supplied water from soil surface and drainage with SSPs, was conducted when the SSPs was buried in unsaturated soil at 500 cm space interval and 60 cm below the soil surface, and the leaching period lasted for 16 days under 3?10 cm pond water head in the soil surface. After the leaching experiment, most ?74.3%? of the salt in 0-60 cm soil layer was leached downwards. However, because of lateral percolation around the experimental plot's boundary and limited by the factors such as low groundwater table and small surface area of the pipe for collecting flow ?Scf? from nearby domain, only about 0.3% of the salt in the 0?60 cm soil layer was discharged via single SSP. It meant that the common drainage method under soil surface leaching condition have low salt discharge efficiency and should be improved.?2? Results of the field leaching experiment were used to calibrate hydraulic parameters and verify simulations of soil water flow and salt transport during the drainage processes with SSPs.Simulated and observed soil water content distributions were in good agreement with root mean squared error ?RMSE? and coefficient of determination ?R2? of 0.025 cm3 cm-3 and 0.88, respectively.Correspondingly for soil salinity distributions, the values of RMSE and R2 were 2.74 g kg'1 and 0.93,respectively. Therefore, the calibrated numerical models and soil hydraulic parameters could be used to analyze the dynamics of soil water and salt under the drainage condition with SSPs located in unsaturated soil, and evaluation of alternative methods for salt discharge.?3? First improved method—Leaching from soil surface and seepage control beneath SSPs. Under the soil surface leaching condition, seepage-proof material ?SPM? with suitable width would be set beneath SSPs to increase the Scf and salt discharge efficiency. While, when the SPM was too wide, the increase degree of salt discharge efficiency may be limited but both the difficulty and cost of construction would increase vastly. In general, SPM between 20 and 100 cm could increase the proportion of salt discharge amount to the initial total salt mass in the 0-60 cm soil layer ?SDR? and water use efficiency for salt discharge ?WUESD?. Compared with the treatment without SPM, the SDR and WUESD were enhanced from 9.8% and 1.86 kg m-3 to a maximum of 17.2% and 3.13 kg m-3,respectively. In addition, the SDR and WUESD were further enhanced by decreasing the space interval between pipes from 500 cm, typically found in commercial fields, to 200 cm or 100cm.?4? Second improved method—Supply water and drainage from single SSP. In this mode, SSP could be used to both supply water and discharge leachate. Compared with leaching from soil surface,the water consumption would decrease obviously when the water was supplied from SSP to wash the soil around SSP. However, drainage immediately when irrigation with SSP ceased, the water drainage amount and salt concentration was limited fairly even the negative pressure condition was maintained in the SSP. Moreover, it was difficult to maintain an effective negative pressure condition in the SSPs using vacuum pump and remove salt from soil effectively in large area. Simulated results indicated the mode had limited practical value as very little salt could be discharged through a SSP, and the maximum?WUESD? was only 2.19 kg m-3 even when ?SPM? was laid beneath SSP and the space interval between pipes was narrowed to 100 cm.?5? Third improved method—Water supply and drainage from interval SSPs. In this mode, water supplied from one SSP and drainage from both sides of the adjacent was superior to water supply and drainage from single SSP. The drainage rate and water drainage amount increased with the rise of water supply pressure head. Along the extension of irrigation time, WUESD increased first and then decreased after 4?6 d date from irrigation beginning. If the cost of construction was disregard and only consider the ecological benefit, when the space of adjacent supply and drainage SSP was 50 cm, the water supply pressure head was 100 cm and the water supply duration was 10.9 d, the maximum WUESD was 6.86 kg m-3. If a SPM with width of 100 cm was laid beneath the SSP, the WUESD would increase 71%(reach to 11.73 kg m-3). |
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