Study On Water-heat Exchange Characteristics And Coupling Model Of Rice Field Under Irrigation

Rice is one of the three major grain crops in China and occupies an important position in grain production and national economy.With the change of industrization and urbanization,China's rice production is facing more and more challenges in the new era.Traditional rice cultivation requires plenty of water for irrigation,but its water use efifiency is very low.In the future,understanding the water and heat's dynamic changes,finding the law of water consumption,breeding high-yield types of rice will be the main targets of rice research around the world.In this paper,the water-heat transfer coupling model was established on the systematic research on the microclimate change,water exchange process and heat balance of the irrigated rice field.The main conclusions are as follows:(1)The temperature and humidity change process of rice field under irrigation.The effects of irrigation on the temperature and humidity of canopy were mainly expressed as,reducing the daily maximum temperature,increasing the daily minimum temperature.Also,the effects were more pronounced at noon than in the early morning.By the way,the water condition in the rice field was significantly changed.The overall relative humidity was improved,but the humidity daily variation was significantly reduced.The closer to the water surface,the greater to the difference in temperature and humidity.Compared with the lower part of the canopy,the effect of irrigation on the upper part of the canopy was weak.And the effect at night was less marked than the day.In general,the effect of ' 10cm' on cooling and humidification was better.However,for the upper part of the canopy,the cooling and humidification effect of ' 5cm '? was better than ' 10cm' in the short term.(2)The water exchange process of rice field under irrigation.In 2017,four treatments were set up in the irrigation experiment,and the water consumption in each treatment area was significantly different.The difference was showed as 10cm>5cm(?)>5cm(I)>0cm,and the water consumption increased with the increase of irrigation water.The water consumption of rice paddy is composed of two parts:evapotranspiration and leakage.In 2017,the treatment plots'(0cm,5cm ?,5cm ?,10cm)cumulative evaporation were 375.8 mm,422.7 mm and 406.2 mm and 422.7 mm.And the cumulative leakage were 137.6mm,167.8 mm,155.4mm and 209.6mm respectively.Under the same traditional irrigation treatment,the evapotranspiration and seepage activity would increase with the irrigation thickness.When the thickness of the water layer of the spray irrigation and the traditional irrigation were the same,the water-saving effect of the spray irrigation was better.Obviously it could inhibit the evaporation activities of the paddy field and reduce the leakage of water in the rice field.(3)The heat balance process of rice field under irrigation.The daily variation of latent heat and sensible heat flux in irrigated rice field wre consistent with the net radiation flux.The water layer allowed some part of the heat to be trapped and adsorbed into the aquifer.There was a phase difference between the peak value and heat transfer direction of water reservoir,soil heat storage and soil heat flux.Compared with soil thermal flux,water reservoir heat and soil heat storage had less participation in energy distribution.The results of the path analysis showed that the main factors which affected the sensible heat and latent heat flux of irrigated rice field on a daily scale were net radiation,relative humidity and maximum temperature.The overall effect decreased after lowering the time scale.In addition to meteorological factors,the thickness of the water layer was also an important factor affecting the flux variation.The ratio of latent heat flux to net radiation increased with the thickness of the water layer.Compared with latent heat flux,it had less influence on the sensible heat flux in the daytime.The ratio of the apparent heat flux to the net radiation was lower than that of the aquifer,but the difference between shallow water layer and deep water layer was small.(4)The results of water and heat transfer coupling model in irrigated rice field.The model had achieved good results in temperature simulation of the air,water and leaves.The root-mean-square error between the temperature simulation value and the measured value is 0.12,0.08 and 0.07 respectively at the upper,middle and lower temperatures of the canopy.The root-mean-square error between the simulated values and the measured values of water is only 0.04.At the same time,the model showed good effect on blade temperature simulation at 120cm height.The simulation results obtained by adjusting the irrigation parameters showed that it's effective to decrease the overall temperature and leaf temperature in the rice field when the water layer is shallow,and its cooling effect was decreasing from the bottom up.The temperature adjustment effect of additional irrigation was negligible when the water depth was greater than 0.25m.Also when the irrigation temperature was less than 25 ??the overall water temperature and air temperature would accelerate with the increase of the irrigation temperature,and then the change tended to be stable.After the irrigation temperature increased to 30 ?,the changes of leaf temperature in the canopy gradually leveled off.Therefore,the high temperature should reduce the inlet temperature and improve the temperature regulation effect in the canopy.