Abstract:The technology of film mulched drip irrigation（FMDI is applied widely to cotton cultivation in Xinjiang, China. Resulted from its characteristics of high ratio and small amount of irrigation, a problem of shallow roots and weak adversity resistance is often found in practice and thus would limit further development of FMDI. Since the irrigation water is supplied deeper in the root zone, the subsurface drip irrigation (SDI) is easily to be considered as an alternative to solve the problem. To avoid probable damage from mechanical farming and prolong the application duration, the irrigation tape in a traditional SDI system is usually buried in deeper root zone (e.g. beneath 35cm from the soil surface), which might be extremely inconvenient for seedling irrigation and management maintenance of the irrigation system. In fact, with the technological progress and cost reduction for manufacturing the irrigation tape, the practical use of disposable drip irrigation tape is becoming more and more popular. The objective was to explore the reasonable depth of disposable drip irrigation tape in SDI by using numerical simulation method. The HYDRUS-2D/3D software was used to simulate the dynamics of soil water and salt, and a field experiment was conducted in Manasi County of Xinjiang autonomous region to validate the numerical model and selected hydraulic parameters. Simulated and observed soil water content distributions were in good agreement with the maximum mean absolute error (Me) and root mean square error (Rm) of 0.034cm3/cm3 and 0.040cm3/cm3, the minimum correlation coefficient (R) of 0.8 and Nash-Sutcliffe efficiency coefficient (Ns) of 0.34~0.62, respectively, between them. Correspondingly for soil salinity distributions, the values of maximum Me and Rm were 3.31g/kg and 4.24g/kg, the minimum R was 0.6 and Ns were -0.06~0.38, respectively, which was also in acceptable range. Then the transport processes of soil water and salt under SDI with different burying depths H (5cm, 15cm and 30cm, respectively) for irrigation tape were simulated by using the validated numerical model and hydraulic parameters. The results showed that salt was gradually driven away from the tape by irrigation water, with soil salinity decreased around the tape but increased near the wetting front. While the increase of H resulted in decrease of evaporation loss, the shallow soil layers near surface would be more and more difficult to be wetted by irrigation water. Synthesizing the factors such as recycling convenience of disposable tape, water requirement for cotton seedling, desalination demand of root zone and water use efficiency for desalination, local disposable tape of SDI was recommended to set at about 15cm, a moderate depth from the soil surface.
