夹砂层土壤Green-Ampt入渗模型的改进与验证

对于土层夹砂结构,湿润锋穿过砂层上界面时,入渗率变为稳渗率。为确定各因素下夹砂层土壤的稳渗率,在Green-Ampt入渗模型基础上,引入导水度系数(小于1)来量化上层土壤的导水程度,建立了改进的夹砂层土壤Green-Ampt入渗模型。采用HYDRUS-1D软件,模拟了不同土壤质地、初始含水率、压力水头、砂层埋深和砂层厚度条件下的稳渗过程,根据模拟结果分析了夹砂层土壤的入渗规律及其影响因素,稳渗率主要受土壤质地、压力水头和砂层埋深的影响。在相同压力水头、初始含水率和砂层厚度下模拟获得不同砂层埋深的稳渗率,并采用改进的Green-Ampt入渗模型拟合,求得导水度系数和进水吸力值。分析发现导水度系数变化较小,为简化计算,取其平均值0.95。在此基础上,提出了由土壤物理特性参数进气值倒数估算进水吸力的计算公式。利用秦王川地区的夹砂层土壤积水入渗试验及已有文献资料验证所建模型的有效性,结果表明所建模型待定参数少,计算误差基本在5%以内,且试验设计较简单,可为农田水分管理及工程防渗技术提供理论依据。

Improvement and verification of Green-Ampt model for sand-layered soil

Abstract: In the northwest of China, soil structure with sand interlayer is a common soil configuration. For soil structure with sand interlayer, infiltration rate will turn to steady infiltration rate when wetting front passes through the upper interface of sand layer. Normally, in the traditional Green-Ampt infiltration model, the water entry suction is difficult to determine. Furthermore, due to the hydraulic conductivity of wetting pattern is assumed to the saturated hydraulic conductivity of upper soil in the model, the error exists when calculating the steady infiltration rate of soil with sand interlayer. In order to compute the steady infiltration rate of the soil with sand interlayer accurately and conveniently, hydraulic conductivity coefficient, which was less than 1, was introduced to quantify the extent of water conductivity for upper layer soil on the basis of Green-Ampt infiltration model. More importantly, under the influence of various factors, the improved Green-Ampt infiltration model for soil with sand interlayer was set up, which included the undetermined parameters to the hydraulic conductivity coefficient and water entry suction. With the HYDRUS-1D software, the procedure of steady infiltration under various factors including soil texture, soil initial water content, pressure head, sand depth and sand thickness was simulated. At the same time, the law of infiltration and its influencing factors were analyzed according to the simulation results. The results showed that steady infiltration rate was mostly influenced by soil texture, pressure head and sand depth, but the steady infiltration rate of soil with sand interlayer was not affected by the soil initial water content and the thickness of sand interlayer. In view of the above, the steady infiltration rate could be acquired through simulation by changing sand depth while remaining pressure head, initial water content and sand thickness constant. Then the values of hydraulic conductivity coefficient and water entry suction were determined by fitting with the improved Green-Ampt infiltration model. The determination coefficient of fitting curve was larger than 0.99. In addition, the value of hydraulic conductivity coefficient changed in the range of 0.91 to 0.99. The average value of 0.95 was adopted in order to simplify the calculation. After that, the determined hydraulic conductivity coefficient was put into the improved Green-Ampt infiltration model. The improved Green-Ampt infiltration model was used further to match, with the purpose to determine the water entry suction. It was found that water entry suction was inversely proportional to the reciprocal of air entry value. Therefore, according the reciprocal of air entry value, which was one of the soil physical characteristic parameters, the calculation formula of water entry suction by estimation was put forward. In order to verify the reliability and universality of the improved model in this research, the water infiltration experiments of the soil with sand interlayer in Qinwangchuan area were conducted. The model was verified both by the experimental results and the available literature data. The improved model was proved to be better than the conventional calculation method for steady infiltration rate because the undetermined parameters were less, the solution for parameters was simpler, and the practicability was better. Simultaneously, the error was basically within 5%. In this paper, the developed model relied on the saturated hydraulic conductivity of upper soil and the reciprocal of air entry value of van Genuchten-Mualem model of sand layer soil. And the two parameters were the basic parameters of soil hydraulic properties, which could be obtained from the simple laboratory tests. Therefore, this model can provide valuble information for agricultural water management and engineering seepage controlling technique.