Study On Water Savings Efficiency Of Plain Reservoirs Covered With Spheres In Arid Area

Plain reservoirs in arid areas typically have large water areas,shallow water depths and considerable evaporation loss.This work adopts spheres(high-density polyethylene)with diameters of 100 mm as plain reservoir anti-evaporation materials.At present,research on anti-evaporation and water conservation under the sphere coverage is still in its infancy.This study combined the wetting rate of the sphere surface in the wind wave environment and the evaporation loss rate from the 9%pore water surface between the spheres to establish a model for calculating the evaporation inhibition rate and water-saving efficiency of spheres.The energy balance and aerodynamics were used to consider the physical process of evaporation from water surfaces covered with spheres.The evaporation model suitable for the water surface under the sphere coverage was established by combining the physical evaluation results and common natural water surface evaporation models.Finally,the layout and benefits of the anti-evaporation structure of spheres in the reservoir was analyzed.The main conclusions of this study are as follows:(1)The homogeneous spheres were selected as the anti-evaporation material for the plain reservoir in arid area.Combined with the evaporation loss rate caused by the pores between the spheres and the wetting rate of spheres in the wind wave environment,the calculation models for the evaporation inhibition rate and anti-evaporation amount of the spheres were proposed,and the annual water saving efficiency caused by the homogeneous spheres covering a unit water surface was obtained.The results showed that the annual water saving efficiency per unit water surface were 64.21%,66.35%,68.84%and 70.96%,when the water surface covered by the homogeneous spheres were 1m2,2m2,3m2,and 4m2.(2)The counterweighted spheres were selected as the anti-evaporation material for the plain reservoir in arid area.On the monthly scale,according to the frequency of occurrence of different wind speeds and the calculation models for the evaporation inhibition rate of the spheres,the calculation model of the monthly water saving rate was established.Finally,the change curve of monthly water saving rate was obtained.The results showed that during the non-freezing period,the water saving efficiency per unit area reached the maximum in July,76.6%,78.1%,79.6%and 81.2%respectively,and reached the minimum in April,51.9%,54.0%,58.2%and 61.3%respectively.During the freezing period,the water saving efficiency per unit water surface were 91%.The annual water saving efficiency per unit water surface under the counterweighted spheres were 6.62%,6.24%,5.95%,and 5.53%higher than that of the homogeneous spheres.(3)The main factors affecting water surface evaporation(mass transfer coefficient and evaporation driving force),microclimate,and energy balance components were evaluated when water surface covered by black and silver-gray spheres.The results showed that the evaporation inhibition rates(67.9%)of silver-gray spheres was slightly higher than that(66.7%)of black spheres.Sphere coverage significantly decreased the mass transfer coefficient and significantly increased the evaporation driving force.Thus,decrease of the evaporation from the water surface covered by spheres was not caused by the increase of the evaporation driving force,but by the decrease of the mass transfer coefficient.The latent heat flux of black spheres and silver-gray spheres decreased by 66.7%and 67.9%,the net radiation increased by 33.0%and 32.9%,and heat storage flux decreased by 63.5%and 94.1%.The sensible heat flux changed from 43.92W/m2to-158.28W/m2 and-153.62W/m2.(4)On the monthly scale,the changes of the evaporation driving force,mass transfer coefficient,evaporation inhibition rate,and energy balance components of the natural water surface and the water surface covered by silver gray spheres during the irrigation period were studied.Results revealed that during the irrigation period,the average evaporation reduction rate under the coverage of counterweighted spheres equaled 70.6%,average growth rate of the evaporation driving force equaled 19.2%,and average reduction rate of mass-transfer coefficient equaled 75.3%.The spheres changed the original energy balance system of the water surface and established a new energy balance system.33.6%of the net radiation was used for water surface evaporation,and 66.4%of the net radiation was transferred to the atmosphere through heat transfer.(5)Combining the monthly evaporation from the uncovered water surface and the covered water surface in the physical process assessment,the simplified form of the Dalton formula,Penman formula and the Priestley-Taylor formula were used to simulate the evaporation of the water surface.Results showed that the Priestley-Taylor formula had a good simulation effect on the evaporation from the uncovered water surface and the covered water surface.The value ? of the natural water surface was 1.16,and the value a of the covered water surface was 0.34.(6)The feasibility of anti-evaporation technology from two aspects of structural layout and benefits were studied.The results showed that the sphere with a diameter of 100mm was an ideal anti-evaporation material.The combination of the anti-evaporation structure and the original structure of the reservoir can avoid the interaction during operation.Compared with agricultural water,when the water saved was used for industrial,the investment recovery period was short and the economic benefit was significant.According to the calculation formula and example of the average salinity of water in the reservoir established in this article,the use of anti-evaporation technology could effectively reduce the salinity of water in the reservoir,thereby alleviating salinization degree of the downstream irrigation area.