| Subsurface drip irrigation(SDI)system is buried in the soil,and the outflow of the emitter is influenced by various factors such as the hydraulic characteristics of the emitter and the physical properties of the soil.The cavity at the outlet of the emitter is an important factor affecting the flow rate of the emitter.In the hydraulic design of the SDI system and the numerical simulation of soil moisture movement,it is necessary to accurately estimate the infiltration boundary of the buried point source,i.e.the size of the cavity.However,existing studies often consider the cavity radius as a constant and overlook the influence of infiltration boundaries.This article focuses on two types of soil,clay and loamy sandy soil,and analyzes the influence of the development of the cavity at the emitter outlet on the emitter flow rate and soil water positive pressure through a combination of indoor experimental research and numerical simulation.The influence of the value of the cavity radius on the simulation results in Hydras-2D simulation is also analyzed.The research results can provide some theoretical references for the rational design of underground drip irrigation systems.The main work and conclusions are as follows:(1)A SDI soil box experiment was conducted indoors to study the changes in the flow rate of the emitter and the soil back pressure at the emitter outlet during the irrigation process.The results showed that during SDI,due to the formation of a cavity at the emitter outlet,the size of the cavity would change with the progress of irrigation,and the soil moisture content around the cavity would increase,As a result,the flow rate of the emitter and the soil back pressure at the outlet of the emitter constantly change with the progress of irrigation,but it will reach a relatively stable state with the progress of irrigation.(2)By comparing the variation law of emitter flow and soil back pressure under the conditions of different soil unit weight,soil type and emitter rated flow,it is found that for emitters with the same rated flow and the same type of soil,the larger the unit weight,the greater the soil back pressure generated at the emitter outlet,and the smaller the emitter flow.The flow rate of the emitter in clay is smaller than that in sandy soil,and the soil back pressure at the emitter outlet is greater than that in sandy soil.During the irrigation process,the changes in flow rate and soil back pressure are greater than those in sandy soil.The larger the rated flow rate of the emitter in sandy soil,the greater the soil back pressure;In clay,there is no clear relationship between the soil back pressure and the rated flow rate of the emitter.(3)The irrigation process was simulated using Hydras-2D software.Explore the influence of the size of the cavity radius at the emitter outlet on the simulated soil back pressure and emitter flow rate at the emitter outlet under constant flux boundary and constant head boundary conditions,respectively.When the boundary condition is constant flux,the larger the cavity radius,the smaller the soil back pressure generated by the flux boundary inside the cavity.When the boundary condition is a constant head,the larger the cavity radius at the outlet of the emitter,the greater the flow rate of the emitter.By combining this law with experimental data,the cavity radius in clay is inverted,and the inverted radius value is substituted into the existing flow calculation formula to improve the accuracy of the calculation.(4)The influence of cavity radius on soil water movement under different soil type,different soil bulk density meters and different water heads was studied in the hydrous-2D simulation.The results indicate that it is very important to accurately select the size of the spatiotemporal cavity for simulating the outflow of underground drip irrigation emitters using Hydrous-2D software.If the radius of the cavity is too large,the simulated wetting peak migration speed will increase,and the water content around the cavity will also be too high.If designed according to this standard,it may cause insufficient irrigation and cannot meet the water demand of crops.If the radius of the cavity is too small,the simulated wetting peak migration speed will decrease,and the water content around the cavity will also be low.If designed according to this standard,it may cause excessive irrigation,affect the normal growth of crops,or waste water resources due to deep leakage.(5)By using nonlinear regression analysis method to analyze and fit the experimental and simulation data of clay and loamy sandy soil,simplified formulas for soil back pressure and droplet flow rate at the emitter outlet under clay and loamy sandy soil conditions were obtained,as well as the calculation formula for the cavity radius at the emitter outlet under clay conditions.By analyzing,comparing,and validating the calculated values of the proposed model with the measured experimental data,the results indicate that the model has high accuracy. |
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