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

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. ©, 2015, Chinese Society of Agricultural Engineering. All right reserved.