外包土工布暗管排盐条件下水盐运移规律

为揭示外包土工布暗管埋设在非饱和带时淋洗后水分和盐分的运移规律,该文设计了模拟暗管排水的室内试验,研究2种土壤初始状态下(非饱和状态和田持状态),排水初期暗管与地下水位的相对位置及其排水排盐情况,从开始淋洗至暗管停止排水全过程中地下水埋深及含盐量变化规律、暗管的排水排盐效果及土壤剖面的水盐动态运移规律。结果表明:在暗管周围包裹土工布的情况下,土壤初始状态无论是非饱和还是田持,当暗管开始排水时地下水均已完全淹没暗管,此时的排盐量最大,流量呈先增大后减小的变化趋势,且地下水位先升高后降低,地下水含盐量随着淋洗水量的增加由累积转变为脱盐。对比淋洗非饱和土壤(试验1)和淋洗田持土壤(试验2)的试验结果,试验2中暗管的排水、排盐效果优于试验1,在试验1中淋洗非饱和土壤时,土壤脱盐率在垂直方向上随土壤深度的增加逐渐降低,0~20 cm土层的脱盐率(>85%)最大,降至无盐水平,暗管周围土壤脱盐率相对较小(<60%),仍处于中度盐渍化水平;水平方向上,0~20 cm土层的脱盐率差异不大,20~40 cm土层中距暗管越远其脱盐率越小。试验2在试验1基础上进行,淋洗田持土壤时,0~20 cm土层盐分不再变化,30~40 cm土层的脱盐率增大(>60%)。此外,试验1中淋洗脱盐效果大于暗管排盐效果,暗管主要排出暗管以上土壤盐分;试验2中暗管排盐效果增强,暗管不仅排出暗管周围土壤盐分,而且排出暗管以下土层及地下水中盐分,随着淋洗水量的增加,土壤由脱盐型转变为排盐型。研究结果表明外包土工布暗管的应用效果受地下水与暗管相对位置的影响,合理提高淋洗水量可以增强暗管排水排盐效果及土壤脱盐效果,有效改善土壤盐渍化。研究结果可为西北内陆干旱地区不同地下水埋深条件下暗管排盐技术的推广和应用提供理论支撑和科学指导。

Characteristics of water and salt transport in subsurface pipes with geotextiles under salt dischargeconditions

Abstract: Soil salinization is an important environmental problem restricting agricultural development. Fresh water leaching combined with subsurface drainage is one of the most effective, fast and widely used methods for salt removal. However, some existing researches only considered the influence of subsurface pipes on drainage, salt discharge and water-salt distribution, there were few studies about the dynamic change of groundwater and the transport of soil water-salt under subsurface pipes drainage in Xinjiang. Moreover, many existing studies adopted sand gravels as filtering layer of subsurface pipes, there was obviously insufficient research on subsurface pipe technology using geotextile envelope as filter layer. This study designed an indoor drainage experiment of subsurface pipe with geotextile envelope, studied the relative position of subsurface pipe depth and groundwater depth at initial stage of drainage and the condition of water-salt discharge, the water-salt discharge effect of subsurface pipe and the dynamic transport law of water and salt in the soil profile during the whole process from the beginning of leaching to the end of drainage. The results showed that whether the initial state of soil is unsaturated or field-holding, the subsurface pipe would not begin to drain until the groundwater completely submerge subsurface pipe, at this time, the salt discharge was the largest, after that the drainage flow increased first and then decreased, the groundwater level also rose first and declined then, and the salt content of groundwater changed from accumulation to desalination with the increase of leaching water. Meanwhile, comparing the results of two leaching tests, the water-salt discharge effects of subsurface pipes in test 2 were better than those in test 1. When unsaturated soil was leached, the soil desalination rate gradually decreased with the increase of soil depth, the desalination rate of 0-20 cm soil (> 85%) was the largest, and decreased to the non-saline level. the soil desalination rate around subsurface pipe was relatively small (< 60%), and still at moderate saline level. In the horizontal direction, the soil desalinization rate the around subsurface pipe was relatively large, and the desalinization rate of 0-20 cm soil layer had little difference. The further away the 20-40 cm soil layer was from subsurface pipe, the smaller the soil desalinization rate was. Test 2 was carried out on the basis of test 1, the salinity change of 0-20 cm soil was small, and the desalinization rate of 20-40 cm soil layer increased (> 60%). In addition, the desalinization effect of test 1 was greater than that of salt drainage by subsurface pipe, which mainly discharged soil salt above subsurface pipe. For test 2, the salt discharge effect of the subsurface pipe was increased, not only removed salt from soil around subsurface pipe, but also removed salt from the soil and groundwater below subsurface pipe. With the increase of leaching water, the soil changed from desalting type to salt discharging type. Finally, this study showed that the application of subsurface pipe with geotextile envelope is affected by the relative position of groundwater depth and subsurface pipe depth, and affected by initial soil water content. Reasonable increase of leaching water volume can increase the water-salt discharge effect and soil desalination effect, effectively improve soil salinization. In addition, the specific applicable conditions of subsurface pipe with geotextile envelope, the influence of the relative position of subsurface pipe and groundwater on water-salt discharge effect, and the determination of reasonable leaching water amount under different initial soil water contents are all issues worthy of further research. This research results can provide theoretical support and scientific guidance for the popularization and application of subsurface pipe salt discharge technology under different groundwater depths in arid areas of northwest China.