| With the increasing shortage of water and energy resources,reducing the working pressure of sprinkler irrigation system and developing low-pressure sprinkler irrigation are the important approaches for the green development of agriculture.Compared with traditional sprinkler irrigation,the hydraulic performance of sprinkler,droplet characteristics,water application rate and irrigation uniformity of low-pressure sprinkler systems have changed significantly.Therefore,the droplet and water distribution characteristics,the water soil infiltration and transportation should be studied.Meanwhile,the effect of irrigation uniformity on crop yield need to be further explored for low-pressure sprinkler irrigation systems.This study aims at saving water and energy and takes the multi-process movement of water as principal line.The research was carried out by using indoor experiments,numerical simulation and theoretical analysis methods.The distributions of droplet characteristics,water application rate and kinetic energy for low-pressure sprinkler were clarified.The characteristics such as soil water infiltration and migration of water in soil were investigated.The calculation model of irrigation uniformity for low-pressure sprinkler irrigation systems was constructed.The influence mechanism of irrigation uniformity on crop yield was discussed.The reasonable value range of irrigation uniformity for low-pressure sprinkler irrigation systems was determined.(1)Effects of working pressure and sprinkler structure on droplet characteristics of low-pressure sprinklers were analyzed.The droplet sizes and velocities of fixed spray plate sprinklers(FSPSs)and rotating sprinklers were measured by a 2DVD,and the kinetic energy per unit volume was calculated.The results showed that the droplet size increased with the decreasing working pressure.And the droplet diameter was proportional to the nozzle size.The relationship between droplet velocity and droplet diameter was a logarithmic function.As the same working pressure and flow rate of sprinkler,the droplet diameters of circular nozzle for the rotating sprinkler were1.01–1.90 times of the non-circular nozzle.The droplet diameters of blue plate(with big grooves on the plate)for the FSPSs were 1.15–2.24 times of the yellow plate(with small grooves on the plate).The larger diameter and velocity of water droplets under low working pressure led to the larger kinetic energy per unit volume of water droplet.(2)Experiments were conducted to investigate the effects of working pressure and sprinkler structure on water application distribution of FSPSs and rotating sprinklers.The specific powers were calculated based on droplet sizes,droplet velocities and water application rates.Under the low working pressure of 200 k Pa,the water application rate and specific power of rotating sprinklers remained at high levels.The peak values of water application rate and specific power for circular nozzles were 1.76–1.93 times and 1.11–1.80 times those of non-circular nozzles,respectively.For FSPSs,the cumulative water application depth and kinetic energy were calculated under a linear moving system.The deviations between calculated values and measured values of yellow plate and blue plate were 5.52%and 20%,respectively.The effects of working pressure and sprinkler space were evaluated.For the mobile sprinkler irrigation system,when the sprinkler space was 1 m,the irrigation uniformity coefficients were 69.6%–99.2%for the blue plate,and those of the yellow plate were 85.5%–99.6%.For the solid-set sprinkler irrigation system,with the sprinkler space within 11 m,when the working pressure of non-circular nozzle was greater than 150 k Pa and the working pressure of circular nozzle was greater than 250 k Pa,the coefficients of irrigation uniformity were higher than 80%.Considering the hydraulic performance of low-pressure sprinkler,for the rotating sprinkler,the working pressure of the circular nozzle should not be less than 250 k Pa,and the working pressure of the non-circular nozzle should not be less than150 k Pa.(3)Effects of application rate and kinetic energy of spray water on physical and hydraulic characteristics of surface soil were clarified for the low-pressure sprinkler irrigation system.The soil tank test was conducted under sprinkler irrigation.The changes of surface runoff,soil erosion process and surface soil structure under different water application rate and droplet specific power were analyzed.The results showed that the occurrence time of runoff and the cumulative infiltration water depth before runoff have negative exponential power function relationships with water application rate,specific power and kinetic energy per unit area.When the specific power was greater than 0.4 W/m~2,runoff was rapidly occurred.The final runoff rate was 80%of the water application rate of sprinkler.The soil infiltration rate decreased when the runoff occurred,and the final soil infiltration rate was negatively correlated with the kinetic energy per unit area and specific power.The sediment yield and soil loss rate decreased with the irrigation time.The cumulative soil loss has a power function relationship with specific power,water application rate and kinetic energy per unit area.The crust bulk of surface soil after irrigation was positively correlated with specific power and kinetic energy per unit area.Considering the soil water infiltration rate and water loss of runoff,the application rate should not higher than 60 mm/h,and the specific power should not higher than 0.2 W/m~2 for the low-pressure sprinkler.(4)A soil water movement model under low-pressure sprinkler irrigation was established to clarify the infiltration and transport processes of water in different soil textures.The movement characteristics of soil water in three typical soils(clay loam,loam and sandy soil)were explored by numerical simulation with COMSOL and soil tank experiments.The results showed that soil texture and initial water content had significant effects on soil water movement under sprinkler irrigation.From the end of irrigation to 24 hours after irrigation,soil water movement was intense,and then soil water movement slowed down.Soil moisture has redistribution characteristics.When the coefficient uniformity of sprinkler irrigation was 40.74%,the uniformity of soil moisture content was more than 87%.The mathematical relationship between sprinkler irrigation uniformity,soil moisture uniformity and soil texture(soil particle size classification)was established.Considering the movement characteristics of soil water under low-pressure sprinkler irrigation,the working pressure of R33LP sprinkler should not less than 150 k Pa.(5)The uniformity calculation method of low-pressure sprinkler irrigation system was proposed,and the finite element hydraulic calculation model of sprinkler irrigation system was constructed.Based on the harmonic analysis method,a functional model of water distribution for single sprinkler was proposed.And the relationship among sprinkler working pressure,nozzle size,and coefficients of water distribution model was established.Considered the influences of sprinkler combination space and crop root on the irrigation uniformity of sprinkler system,the concept of effective irrigation uniformity was proposed.Combined with the finite element hydraulic calculation model for sprinkler irrigation system,the effects of working pressure,nozzle diameter and sprinkler combination space on the irrigation uniformity were analyzed.The results show that the trigonometric function model can effectively predict the water distribution of a single sprinkler.The weight coefficient can effectively predict the uniformity of multi nozzle combined sprinkler irrigation.The inlet working pressure,nozzle diameter and sprinkler irrigation system diameter have a significant impact on the combined sprinkler irrigation uniformity of mobile sprinkler irrigation system.(6)The relationship between sprinkler irrigation uniformity and crop yield was determined.The reasonable value range of low-pressure sprinkler irrigation uniformity was proposed.Based on crop water production function,a crop yield estimation model was established.Taking corn as an example,the effects of sprinkler irrigation uniformity on yield under different irrigation amount and rainfall conditions were analyzed.The results showed that with the increased irrigation amount of sprinkler,the crop yield decreased caused by non-uniform water distribution of irrigation.The effect of sprinkler irrigation uniformity on crop yield decreased with the increasing rainfall.Considered the crop yield and total cost of sprinkler irrigation uniformity,the effects of technical parameters on the output to input and irrigation uniformity of sprinkler irrigation system were analyzed.The results showed that the sprinkler number,sprinkler spacing and pipe diameter influenced the investment cost of sprinkler irrigation system.The system inlet pressure has a great effect on the operation cost.The sprinkler combination spacing has a great impact on system investment,operation cost and water bills of the sprinkler irrigation system.For the FSPSs with nozzle diameters of 1.79,4.76 and 7.94 mm,the recommended sprinkler uniformity coefficient ranges were 0.81–0.89,0.83–0.85,and 0.85–0.95,respectively. |
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