悬帷段作用下暗管排水流量计算公式探讨

Discussion on the formula of subsurface drainage discharge considering hanging curtain section

暗管排水在降低地下水位过程中普遍存在悬帷段,悬帷段的存在对于暗管排水理论计算提出了挑战。该研究考虑悬帷段对排水流量的影响,提出了悬帷段作用下的暗管排水流量计算公式,基于HYDRUS模型得到不同暗管间距、暗管埋深、不透水层深度以及土壤质地条件下的排水流量模拟值及理论计算公式中的作用水头参数,对比分析了不同土壤质地以及悬挂水头影响下排水流量理论公式的适用性。结果表明提出的考虑悬帷段的暗管排水流量公式计算值与模拟值具有很好的吻合性,提出的6种组合公式计算值与模拟值相关系数均大于0.99,平均绝对误差均小于11%,其中Hooghoudt-Ploeg-B.И.阿拉文公式以及Hooghoudt-Ploeg-Kirkham公式误差更小,此2个理论公式也通过了已有文献的验证,而不考虑悬帷段条件时采用Hooghoudt公式计算得到的理论值则普遍小于模拟值,误差可能超过50%;从不同土壤质地来看,提出的考虑悬帷段的暗管排水流量计算公式在粉土和壤土的适用性最高,其次是砂土;此外,该研究初步证实了悬挂水头和单长暗管排水流量与渗透系数比值之间成线性相关关系。该研究成果对丰富和发展农田排水理论及技术有重要意义。

Hanging curtain drains are widely installed at a shallow depth during the processes of lowering groundwater tables in a subsurface drainage system.The presence of hanging curtains has posed a great challenge to the theoretical formula of subsurface drainage discharge.The heads above the pipe and at the middle location between the pipes were also considered as parameters.In this study,a modified formula was proposed for the subsurface drainage discharge considering hanging heads.Three formulas were also selected to explore the equivalent impervious depth,including the Kirkham equation,classic drainage formula in the Soviet Union and Hooghoudt formula.A reasonable assumption was then made for the six kinds of series formulas.Furthermore,an HYDRUS model was used to simulate the drainage discharge and heads above the pipe and at the middle location between the pipes using the theoretical formula under different drain spacing,drain depth,depth of impervious layer,and soil texture.Specifically,three types of drain spacing(6,20,and 40 m),three depths of impervious layer(2,5,10 m),three drain depths(0.8,1,and 1.2 m),and four soil textures(sand,silt,loam,clay)were set in the comprehensive tests.409 groups of relevant data were then obtained during simulation.The better theoretical formulas of subsurface drainage discharge were determined to compare the calculated and simulated values considering the hanging curtain section.The applicability of formulas was also verified in various soil textures at the different heights of hanging curtains.A Hooghoudt formula was selected to evaluate the simulation.Additionally,a correlation analysis was made on the hanging head,as well as the ratio of discharge per unit length and hydraulic conductivity.The results showed that the formula of equivalent impervious depth given by van der Ploeg was smaller than that by van der Molen and Wesseling.There was a larger difference between the two aforementioned formulas,as the increase in the ratio of impervious depth and drain spacing.The calculated value of the Hooghoudt formula was also significantly smaller than the simulated one without considering the hanging curtain.In the case of the hanging curtain,the calculated discharges using six kinds of series formulas were all matched well with the simulated values with the correlation coefficients larger than 0.99 and mean absolute errors smaller than 11%.Meanwhile,a series of formulas were established using the Hooghoudt formula with the equivalent impervious depth by van der Molen and Wesseling,Kirkham orВ.И.Аравин,andС.Н.Нумеровequation.It was found that better performance of modified formulas was achieved to well match with the larger correlation coefficients and the smaller mean absolute errors than other cases.In soil texture,the theoretical formula considering hanging curtains in silt and loam performed the highest applicable levels,followed by that in the sand.There was a smaller change of the head at the middle location between pipes,while a larger drain spacing during the decreasing process of hanging head.Once the hanging head was not available,the formulas can be estimated by the discharge per unit length of the pipe and hydraulic conductivity.There was also a better linear correlation between the hanging head and the ratio of discharge per unit length of the pipe and hydraulic conductivity with the correlation coefficient of 0.96.The finding has a great significance to enriching and developing the theory and technology of agricultural drainage.