Study On Field Infiltration Experiment And Determination Method Of Infiltration Parameters Of Film Hole Irrigation

Film hole irrigation is a kind of surface irrigation technology developed on the basis of agricultural film mulching.In the process of irrigation,water is transported on the film and infiltrated into the root zone soil through the crop hole or special irrigation hole（collectively referred to as film hole）,which has the advantages of saving water,increasing temperature and preserving soil moisture.The irrigation method has the advantages of low investment,high efficiency and simple operation,and has good application prospect in the field of agricultural water-saving irrigation.The study of film hole infiltration law is the basis for optimizing the technical elements of film hole irrigation in the field,and the accurate estimation of soil infiltration parameters is the key to quantitatively characterize the film hole infiltration law.Based on this,this paper takes field film hole irrigation as the research object,selects 12 typical soil textures,numerically analyzes the law of film hole infiltration under different combination of influencing factors,constructs the estimation model of cumulative infiltration per unit film hole area and wetting front migration distance,and uses field test results to verify the reliability of the estimation model.Its findings are as follows:（1）According to the characteristics of film hole irrigation in the field,a test device-cylinder infiltrometer was designed to simulate the single point film hole infiltration process of film hole irrigation in the field.The water supply unit of the cylinder infiltrometer is integrated with the seepage unit.During the test,the pressure head is constant and the soil is undisturbed.It is convenient to directly observe the water level change and the surface wetting front migration distance at different times,which can provide experimental data for the determination of soil infiltration parameters in field film hole irrigation.（2）According to the classification standard of soil texture,12 kinds of soil were configured to determine the soil water characteristic curve,saturated hydraulic conductivity and field capacity under different soil bulk density conditions.The results show that the soil volumetric water content decreases with the increase of soil water suction.In the low suction stage,the curve slope is relatively flat.In the middle and high suction stage,the curve slope is relatively steep,and the curve of clay soil is gentler than that of sandy soil.Under the same soil texture and water suction,the soil volumetric water content increased with the decrease of bulk density.Under the same soil texture,soil saturated hydraulic conductivity and field capacity decreased with the increase of bulk density.Soil texture had a great influence on soil saturated hydraulic conductivity and field capacity,but did not show a significant change with clay or sand content.（3）Based on HYDRUS-2D/3D simulation software,the mathematical model of soil water movement under film hole irrigation was established.Using the field test data of cylinder infiltration instrument,the indexes such as cumulative infiltration and wetting front migration distance were selected for verification.The results show that the RMSE values of the measured and simulated values are very small,between 0.399 cm and 0.273 cm,PBIAS≤±10%,between-0.041%and 1.460%,and NSE≥0.908.It shows that the model is feasible and the solution method is reliable.HYDRUS-2D/3D can be used to simulate the soil water movement process of film hole irrigation in field.（4）Soil texture had significant effect on soil infiltration（rate）and wetting front transport distance of film hole irrigation.Under the same soil texture condition,the cumulative infiltration amount per unit film hole area and the infiltration rate per unit film hole area at the same time decreased with the increase of soil bulk density,initial water content and film hole diameter,among which the film hole diameter had the greatest influence,followed by soil bulk density and initial soil water content.Film hole diameter and soil bulk density are the dominant factors affecting the cumulative infiltration per unit film hole area and the infiltration rate per unit film hole area.The variation of infiltration rate per unit film hole area with time is similar to that of vertical one-dimensional infiltration.There are two stages of rapid infiltration and stable infiltration.When the film hole infiltration reaches 120 min,the infiltration rate per unit film hole area basically enters the stable infiltration stage.Under the condition of the same soil texture,the migration distance of soil wetting front at the same time increases with the increase of film hole diameter and soil initial moisture content,and decreases with the increase of soil bulk density.The influence degree is film hole diameter>soil bulk density>soil initial moisture content,and film hole diameter and soil bulk density are the influence of soil wetting front migration distance.（5）The cumulative infiltration per unit film hole area can be expressed by Kostiakov three-parameter infiltration model（I=Kt^α+f₀t）.Under different soil textures,the stable infiltration rate f₀can be regarded as a fixed point.During the simulation process,it was found that the infiltration indexαvalue in the 180 simulation schemes changed little（between0.28 and 0.35）,andα=0.32 could be taken.Under the same soil texture,there was a good power function relationship between infiltration coefficient K and membrane pore diameter D and soil bulk densityγ（K=aγ^bD^c）.The relationship between steady infiltration rate f₀and soil bulk densityγand film hole diameter D is power function（f₀=dγ^eD^g）.Based on this,the formulas of infiltration coefficient K and steady infiltration rate f₀are established,and a simplified prediction model of cumulative infiltration per unit film hole area including film hole diameter and soil bulk density is proposed.The model contains two undetermined parameters,only one set of cylinder infiltration test data can be inferred,high precision,can be film hole irrigation under different soil texture conditions per unit area of soil film hole cumulative infiltration forecast.（6）The relationship between soil wetting front migration distance and infiltration time can be expressed by power function（horizontal direction:X=mtⁿ+D/2;vertical direction:Z=pt^q）.In the horizontal direction,the infiltration coefficient m increases with the increase of the steady infiltration rate f₀,and there is a good power function relationship between the two（R²≥0.919）,and the infiltration index n changes little,n=0.35.In the vertical direction,infiltration coefficient p and steady infiltration rate f₀have a good power function relationship（R²≥0.981）,infiltration index q changes little,q=0.32.Based on this,a simplified prediction model for the wetting front migration distance of film hole irrigation in the horizontal and vertical directions of different soil textures was established,and the predicted values were in good agreement with the measured values.The model is simple and can be determined by a set of field test data,which facilitates the end user to conduct rapid field evaluation.