Simplified calculation method and its validation of infiltration capacity for film hole irrigation

In order to simplify the numerical calculation of cumulative infiltration capacity under film hole irrigation, the film hole cumulative infiltration was decomposed into vertical infiltration capacity and lateral infiltration capacity, and lateral infiltration capacity of unit film hole perimeter was introduced to calculate film hole lateral infiltration capacity. The soil vertical one-dimensional infiltration characteristics and film hole infiltration characteristics were simulated by using HYDRUS-1D/3D software. According to the simulation results, the influence of film hole diameter on lateral infiltration capacity of unit film hole perimeter was analyzed. It was found that the film hole diameter had little impact on lateral infiltration capacity of unit film hole perimeter, and its impact on lateral infiltration capacity of unit film hole perimeter could be ignored. At present, the influence factors of one dimensional infiltration and film hole diameter on lateral infiltration capacity of unit film hole perimeter was the soil texture, initial moisture content and irrigation water depth. Based on this, the vertical one-dimensional infiltration depth and lateral infiltration capacity of unit film hole perimeter were quantitatively analyzed by using Philip infiltration model, and the influence of soil texture, initial moisture content and irrigation water depth on lateral infiltration coefficient and vertical one-dimensional infiltration coefficient were studied. The results showed that the soil texture had large impact on both the relative sorptivity rate(the ratio of sorptivity rate of lateral infiltration capacity of unit film hole perimeter to sorptivity rate of vertical one dimensional infiltration) and the relative steady infiltration rate(the ratio of steady infiltration rate of lateral infiltration capacity of unit film hole perimeter to steady infiltration rate of vertical one dimensional infiltration), the relative sorptivity rate decreased slightly with the increase of initial moisture content, the decrease amplitude of the relative sorptivity rate was 5.49%-24.72%,and irrigation water depth had less influence on relative steady infiltration rate, the maximum amplitude of the relative steady infiltration rate change was 21.80%. With respect to the relative steady infiltration rate, the change of relative sorptivity rate had less impact on cumulative infiltration capacity under film hole irrigation, to simplify the calculation, the relative sorptivity rate was considered as a fixed value for the same soil texture. The relative steady infiltration rate increased with the increase of irrigation water depth, the increase amplitude of the relative steady infiltration rate was 2.55%-42.76%, and the initial moisture content had little influence on relative steady infiltration rate, and the maximum amplitude of the relative steady infiltration rate change was 2.94%. To simplify the calculation of cumulative infiltration under film hole irrigation, only irrigation water depth was considered for its effect on the relative steady infiltration rate. The relationship between vertical infiltration coefficient and lateral infiltration coefficient was established. The simplified calculating model for cumulative infiltration capacity of film hole irrigation was proposed, including the film hole diameter, irrigation water depth, sorptivity of vertical one-dimensional infiltration and soil saturated hydraulic conductivity. The film hole infiltration parameters could be determined by using one-dimensional vertical infiltration parameters. The model was verified by infiltration experiments of one-dimensional vertical infiltration, experiments of film hole infiltration and the other researcher’s experiment results. The results showed that the calculated values of experimental equation were consistent with the measured values(R²≥0.995, P<0.001), and the average relative error was about 5%(4.84%~6.81%). The equation proposed here only have 2 undetermined coefficients: vertical one-dimensional infiltration characteristics and film hole infiltration characteristics. It was simple enough for accurately predicting film hole infiltration characteristics.