Research On Solute Concentration Change In Drip Fertigation Of Differential Pressure And Its Effect On Water And Nitrogen Distribution

Drip fertigation of differential pressure is one of the most widely used fertigation technology in China.An important characteristic of this method is that the solute concentration would constantly decline during the fertigation,while related research on this method was not systematic.In this paper,further researches including the change characteristic of the solution concentration,preliminary design of the intelligent control device of the drip fertigation of differential pressure and the distribution of water and fertilizer in the drip irrigation pipe network as well as that in the soil of fertilizer point-source infiltration under different attenuation conditions were carried out through experimental and simulation methods.The main results are as follows:(1)An experiment about the solution concentration change was carried out by using the chemical reagent K2SO4 as the fertilizer.The experiment includes two different flow rates of 0.6 m3/h and 1.2 m3/h,and two different fertilizer amounts of 900 g and 1800 g.The results showed that:the solution concentration decayed quickly in the early stage,and then tended to be gentle.The flow rate was the main factor for the speed of concentration attenuation,the fertilizer amount was a secondary one.The attenuation curve of solution concentration had a better correlation with the theoretical curve and conformed to the power function model.Meanwhile,the fertigation duration can be estimated with theoretical formula.The duration of fertigation would be 11?12 min when the flow rate was 0.6m3/h,and 5.5?6min when the flow rate was 1.2 m3/h.(2)An cylinder with the volume of 9.42 L was simplified as a model of the differential pressure tank,and two working conditions of 1.425 m3/h and 0.71 m3/h were considered.The change characteristic of the solution concentration was simulated by using the computational fluid dynamics(CFD)technology.The results showed that:the simulated curves achieved by CFD coincided well with the theoretical curve.The absolute values of the concentration deviation are less than 0.07 and 0.03 under the two working conditions above.Meanwhile,the fertigation duration calculated by CFD coincided well with the results of theoretical formula.The duration simulated by CFD are 93 seconds and 186 seconds under the flow rate of 1.425 m3/h and 0.71 m3/h,respectively.The deviation compared with the results calculated by theoretical formula are only 2.31%and 2.83%,respectively.(3)Two intelligent control technology and devices were carried out preliminarily to solve the problems as the low intelligence level of the differential pressure tank.One was a mobile drip fertigation device of differential pressure.In this solution,the inlet flow was measured by a flow meter and the duration was calculated by a control interface based on the theoretical formula,then instructions were given to the solenoid valve to realized an automatic fertilization.This solution could be used in small drip irrigation system and liquid fertilizer.The other was the remote control drip fertigation device of differential pressure.This solution based on the existing intelligent irrigation control technology,realized a real-time monitoring on fertilizer concentration by using the conductivity sensor,then perform remote control to the solenoid valve through an mobile APP.It could be used in large scale drip irrigation system.(4)The water and fertilizer distribution of the drip irrigation pipe network under different attenuation conditions was studied by an experiment with two different length of pipe network laying(45 m and 90 m),and three different network operating pressure(0.06 MPa,0.10 MPa and 0.14 MPa),as well as the differential pressure operation mode(the different flow rate and the different fertilizer amount).The distribution of the drip irrigation amount,the drip discharge,the solute concentration and the fertilizer amount were further discussed.The results showed that the spatial uniformity of the drip irrigation amount and the drip discharge was better than that of the solute concentration and the fertilizer amount.The distribution of the solute concentration and the fertilizer amount had a high fluctuation,especially in the rear of the pipe network.The drip irrigation amount and the drip discharge decreased with the increase of laying length while the value of the solute concentration and the fertilizer amount increased gently.The drip irrigation amount and the drip discharge could be greatly affected by the length of pipe network laying and the drip network operating pressure.By contrast,the spatial distribution of the solute concentration and the fertilizer amount was not only affected by the length of pipe network laying and the drip network operating pressure,but also affected by the flow rate and the fertilizer amount.The Christiansen Uniformity Coefficient CU,the Distribution Uniformity DU,the Variation Coefficient Cv and the Statistical Uniformity Us of the four indexes above were calculated,and the results showed that:For the drip irrigation amount,the value of CU was higher than 97%,the value of DU was higher than 0.96,the value of Cv was 0.02?0.03 and the value of Us was 97%?98%,respectively.But for the fertilizer amount,the value of CU was 90%-97%,the value of DU was 0.86-0.95,the value of Cv was 0.05-0.15 and the value of Us was 85%?95%,respectively.The uniformity value of the drip irrigation amount was higher than the fertilizer amount.Meanwhile,the uniformity were assessed by the ASAE standard EP458,and the results showed that the grade of the drip irrigation amount could be estimated as "Excellent",while the fertilizer amount could rarely be "Excellent",majorly between "Excellent" and "Good".(5)The distribution of soil water and nitrate nitrogen under different fertilizer concentration and attenuation conditions were studied by the point source infiltration experiment.The results showed that the soil water content would reduce from the inside to the edge of the wetting body.The solution concentration and its attenuation course had no significant effect on the soil water content.The soil nitrate nitrogen concentration was low inside of the wetting body,while accumulated at the edge.The higher of the solution concentration the higher of the soil nitrate nitrogen concentration inside the wetting body,while no relationship between them at the edge of the wetting body.The HYDRUS-2D model was calibrated and validated by observed data of soil water content and the soil nitrate nitrogen concentration,and the effect of the initial soil nitrate nitrogen concentration on its distribution was analyzed by the constructed HYDRUS-2D model.The results showed that the model was good at soil water content simulation,and well reflected the variation trend of the soil nitrate nitrogen concentration.While the model was not accurate enough for simulating soil nitrate nitrogen concentration,especially at the edge of soil wetting body.The soil nitrate nitrogen concentration variation was highly affected by the initial concentration.