Energy Balance Characteristics And Unclosed Correction Method In Summer Maize Field In North China Plain

The surface energy imbalance has been a challenge in the study of material and energy exchange in ecosystems over the past 30 years.Solving the problem of energy imbalance provides a theoretical basis for further development of climate forecasting and material and energy transfer models,and guides farmland irrigation and agricultural production.This article took the summer maize in the North China Plain as the research object,analyzed the variation law of temperature and humidity in the canopy atmosphere and soil under sunny,rainy and sunnny weather conditions,elucidated the changes of farmland energy balance elements and heat storage flux on a monthly scale and under cloudy,rainy and sunny weather conditions,evaluated the closure degree of farmland energy at different time scales,clarified the impact of heat storage flux on energy balance under cloudy,rainy and sunny weather conditions.A correction method for farmland energy unclosure had been established.The main research contents and results are as follows:（1）Under cloudy,rainy,and sunny weather conditions,there were significant diurnal variations in soil profiles at different depths and within canopy.Additionally,there were temperature and humidity inversion phenomena at different heights within the canopy,which further affected the surface energy balance.The vertical distribution of soil temperature varied slightly from the upper layer to the lower layer,with little difference in humidity between different soil layers ranging of 0.2-0.6%.The amplitude of the sine curve distribution of soil temperature between 5-15 cm on sunny day was stronger than on cloudy day,and the soil temperature gradually decreased over time on rainy day.On sunny and cloudy day,the soil moisture at different depths was roughly in the order of deep>shallow>surface.The daily variation amplitude of soil moisture at different depths on sunny day was within the range of 0.7-0.9%for each planting density.On cloudy day,the daily variation amplitude of average soil moisture at different depths decreased with the decrease of planting density,while on rainy day,there was no obvious pattern for each planting density.There was a temperature gradient difference of 5-15 cm under different weather conditions.On sunny and cloudy day,the soil layer released heat during the day and stored heat at night.On sunny day,more heat was released than on cloudy day,and less heat was stored than on cloudy day.On rainy day,the soil mainly absorbed heat.The daily variation range of air temperature and humidity was sunny day>cloudy day>rainy day.From 8:00 to 16:00 each month,the air temperature in the lower layer of the canopy was prone to inversion,while the middle and upper layers of the canopy were prone to inversion.（2）Under cloudy,rainy,and sunny weather conditions,the energy elements and thermal energy storage terms exhibited significantly different inverted "U" and multi peak curve distributions,with a lag of 1-2 hours compared to the net radiation peak.The daily variation of net radiation（R_n）,sensible heat flux（H）,latent heat flux（LE）,and soil heat flux（G）all exhibited an inverted "U" curve distribution.The average daily maximum values of R_n,LE,H,and G were 533.34,291.61,83.42,and 11.07 W m^-2,respectively.H,LE,and G showed a lag phenomenon compared to the peak of R_n.As the growth period progressed,the daily variation amplitude of R_n in each month was not significantly different,and the daily variation amplitude of H decreased first and then increased,While LE was opposite to H,G gradually increased.The daily changes of enthalpy change of air humidity（S_a）and canopy heat storage（S_c）in each month were similar,while the daily variation of enthalpy change of air humidity（S_q）,was opposite to S_a.The daily variation of soil heat storage（S_s）showed an inverted "U" shape.As the growth period progressed,the daily variation amplitude of S_q and S_c gradually increased,while the daily variation amplitude of S_s in each month gradually decreased.On sunny day,all energy fluxes showed an inverted "U" shaped single peak variation,while on cloudy and rainy day,the variation curves showed a multi peak pattern.The magnitude of R_n,LE and H in different weather conditions showed sunny day>cloudy day>rainy day.G absorbed heat overall on sunny day and released heat into the atmosphere on cloudy and rainy day.The variation of S_a and S_q which were farther away from the soil surface and the S_s which were closer to the soil surface,were more intense in different weather conditions.The variation range of S_a and S_q were shown as cloudy day>sunny day>rainy day,S_c was shown as sunny day>cloudy day>rainy day,and S_s was shown as sunny day>cloudy day>rainy day.The higher the planting density,the more heat S_a,S_q,S_c absorbed or released on sunny day,while the S_s was the opposite.Different planting densities on cloudy and rainy day had no obvious pattern.（3）The farmland showed an energy unclosure state on the half hour and daily time scales.There were differences in compensation contributions between air temperature changes in enthalpy change of air humidity（S_a）,canopy heat storage（S_c）,enthalpy change of air humidity（S_q）and soil heat storage（S_s）to the residual replenishment of surface energy unclosed under cloudy,rainy,and sunny weather conditions.There was a phenomenon of energy unclosure in this study area,with a higher energy closure ratio on sunny and cloudy day than on rainy day.As the time scales of half an hour,hour,day,and month increased,the correlation between heat storage flux and energy balance residuals at different heights of the canopy and depths of the soil decreased.The correlation between atmosphere heat storage（S_a+S_q）,soil heat storage（S_s）,and energy residuals at the bottom of the canopy was the highest.As the planting density decreased,the more significant was between atmosphere heat storage and energy residuals at different heights inside the canopy,and the smaller the correlation between soil heat storage and energy residuals in different soil profiles.The correlation between soil heat storage and energy residuals at different depths under sunny,cloudy,and rainy conditions was significantly higher.The correlation between atmosphere heat storage and energy residuals at different heights within the canopy was more significant on cloudy and rainy day.Considering different weather conditions,soil heat storage at different depths had a better effect on improving energy closure ratio.On this basis,considering atmosphere heat storage within the canopy on cloudy and rainy day,the effect on improving energy closure ratio was more significant.The average compensation for surface energy balance residuals by S_s on sunny and cloudy day was 1.88 W m^-2 higher than that on rainy day,while the average compensation for S_a,S_q and S_c on rainy day was 0.64～0.85 W m～2 higher than that on sunny day,which was 0.02～0.30 W m^-2.The higher the planting density,the greater the contribution of S_a,S_q and S_c,and there was no obvious pattern of S_s.The effect of S_s on energy closure was 11.39%,the range of S_c action was 1.13-2.10%,and the range of S_a action was 0.51-0.94%.S_q may reduced the energy closure ratio to some extent.After compensating for each energy storage flux,the average energy closure rate of different treatments increased from 64%to 81%.