Numerical simulation of water and salt discharge of combined drainage system in arid region based on multi-scale kinetic model
Aiming to clarify the appropriate parameters for"open ditch+subsurface pipe"combined drainage project,which meets the diverse needs of water conservation,salt control,mechanized cultivation and so on,a parameter optimization method based on the multiscale dynamics model for combined drainage system was developed in this study,and using the pilot area of the 27th Regiment of the Xinjiang Production and Construction Corps as an example.Based on the existing open ditch,a total of 990 scenarios were designed considering different subsurface pipe spacing(10-100 m),pipe depth(1.0-2.0 m)and winter irrigation quota(150-360 mm),the drainage and salt discharge efficiency of the combined drainage system under each scenario was simulated,and the quantitative relationship between the drainage and salt discharge amounts,groundwater depth and each variable was clarified,at last the appropriate parameters of the subsurface pipe layout and the corresponding winter irrigation quota were selected.The results show that salt discharge amounts from combined drainage system and subsurface pipe decrease linearly with the increasing pipe spacing,the discharge amounts from open ditch increase linearly with increasing pipe spacing.The preferred drainage path in the system is the deeper drainage project,subsurface pipe will dominate the drainage when the pipe's depth is greater than 1.8 m,subsurface pipe's drainage amounts account more than 99.5%and 83.0%in the reproductive period and winter irrigation period respectively,for better combined drainage,the pipe depth should be close to the depth of open ditch.Open ditch can only control the groundwater surface to its depth in a small area adjacent to open ditch's path of placement,subsurface pipe is necessary to better control the groundwater depth in the field.Smaller pipe spacing can better control the groundwater table,but the pipe spacing less than 20 m is too dense and unnecessary.The salt discharge amounts of the combined drainage system increase linearly with the increase of winter irrigation quota.When the pipe depth is greater than the depth of open ditch and the pipe spacing is small,open ditch under small irrigation quotas can't serve the purpose of drainage and salt removal,for example,under the pipe spacing of 20 m and the pipe depth of 1.8 m,the salt discharge of open ditch accounts for less than 1%of the total salt discharge of the system until the winter irrigation quota is greater than 240 mm.Based on the aforementioned results,taking the water-saving,salinity control,and moisture retention needs of arid regions into account,the appropriate winter irrigation quota for the experimental plot under combined drainage is 300 mm,with suitable pipe spacing and pipe depth being 60 m and 2.0 m,respectively.
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