Response Characteristics Of Radiation Frost In Tea Fields And Energy Quantitative For Frost Protection

In order to improve the frost protection effect and the cost efficiency,this research is mainly focused on the response characteristics of radiation frost in tea fields and energy quantitative for frost protection.It has huge application value for agricultural frost protection practice.The objective of this research is to?1?analyze the response characteristics of micrometeorology and physiology to the radiation frost;?2?clarify the energy transfer process in the frost night;?3?provide frost prediction model based on support vector machine;?4?build and verify a new energy transfer model for frost night;?5?provide a new control strategy based on the energy allocation.This research was mainly funded by the key R&D project of Jiangsu province?BE201634?.The main research work and results are shown as follows:?1?Micrometeorological characteristics of frost night and microstructure response to cold damageWeather information station was constructed in the tea field to collect the long-term micrometeorological data in the frost night.Three intensities of typical radiation frost events happened in the autumn,winter and spring were selected for analyzing the dynamic change with time.Energy transmission process was used to reveal the formation of radiation frost.Response of microstructure and ultrastructure of tea leaves to cold damage were observed by scanning electron microscopy?SEM?and transmission electron microscopy?TEM?methods.Micrometeorological characteristics observation results showed that the temperature inversion is obvious in the radiation frost night.In the winter light frost,spring and autumn heavy frost nights,the temperature inversion is higher than that of light and moderate frost.Relative humidity at six different heights were all higher than 90%.Wind speed at 2.5 m above the canopy was stable and kept less than 1.0 m·s^-1 in different frost nights.There was no obvious change for the stomatal resistance and canopy resistance which provided a stable underlying surface for the atmosphere-canopy-soil system.Soil temperature decreased slowly during the frost nights and the total sensible heat accounted to net radiation was nearly 32.2%⁶8.4%.For the latent heat,it was nearly1.5%¹7.9%.The external energy mainly enters the tea canopy through the medium of air in the form of sensible heat.As the temperature decreased to-2.0?,the membrane system of tea leaf was the first to be damaged and the cell walls became fuzzy.With the temperature decrease to-10.0?,the organelles began to collapse and were destroyed at the temperature of-15.0?.The cooling process showed that the cells of tea leaf suffered from low temperature stress at the temperature of-2.0? and the organelles were destroyed when temperature gradually decreased to-15.0?.The irreversible cell damage of cell can be considered as the response characteristics to cold damage.?2?Radiation frost prediction using support vector machinesMultidimensional and nonlinear micrometeorological data enhances the difficulty in predicting the radiation frost.Support Vector Machines?SVMs?,a type of algorithms,can be supervised learning which widely be employed for classification or regression problems in research of precision agriculture.This research is the first attempt of using SVMs to build prediction models for radiation frost.32 kinds of micrometeorological parameters were collected from November,2016 to July,2018.Six combinations inputs were used as the basis dataset for testing and training.Three types of kernel functions,such as linear kernel,radial basis function kernel and polynomial kernel function were used to develop the SVMs models.Five-fold cross validation was conducted for model fitting on training dataset to alleviate over-fitting and make prediction results more reliable.The results showed that an SVM with the radial basis function kernel?SVM_BRF?model with all the 32 micrometeorological data obtained high prediction accuracy in training and testing sets.When the single type of data?temperature,humidity and radiation data?was used for the SVM without any functions,prediction accuracy was better than that with functions.The SVM_BRF model had the best prediction accuracy when using the multidimensional and nonlinear micrometeorological data.Considering the complexity level of the model and the accuracy of prediction,micrometeorological data at the canopy height with the SVM_BRF model has been recommended for radiation frost prediction in Yangtze River Delta.?3?Energy transfer model in frost nightA new energy transfer model was established by the eddy diffusion theory to quantize the enteral energy transfer in the atmosphere-canopy-soil system.Nine typical radiation frost nights selected from autumn,winter and spring was classified into three intensities.The energy closure was used evaluate the accuracy of latent heat and sensible heat calculated by the energy transfer model and single layer model.The results showed that energy flux?sensible and latent heat flux?estimated by two models had good linear relation in different frost nights.But the numerical size was different.Both H_fmax and LE_fmax were lower than H_max and LE_max.EBR_f was higher than EBR_s which showed that the energy transfer model was better than the single-layer model on estimating the energy flux.System enteral energy changed with time in different frost nights.The value of net radiation under canopy(R_ns)was relatively low in the night and it transferred from soil to canopy which provided energy for the soil-canopy subsystem.LE_s was lower than H_s and there was no significant sensible heat exchange between the soil surface and canopy.The sum of canopy net radiation(R_nc-sum)was negative and R_nc transferred from canopy to atmosphere.The external energy mainly entered the tea canopy through the medium of air in the form of sensible heat?H_c?,but it was in a low level.Otherwise,energy loss in the canopy-atmosphere subsystem was in the form of latent heat?LE_c?.Similarly,H_f was negative and LE_f was positive in the frost night.?4?Modification of the energy transfer model based on eddy covariance method To improve the accuracy of the energy flux calculated by the energy transfer model,an eddy covariance observation system was established in the tea field.Light,moderate and heavy frost nights were selected to analyze the underlying surface stability in the experimental tea field.Eddy covariance method was used to determine the calibration factors for modifying the energy transfer model on estimating the latent heat and sensible heat.During the radiation frost night,the underlying surface of the tea field system was stable.H_f and H_ec had good linear relation in the light,moderate and heavy frost night which was same to LE_f and LE_ec.H_f was lower than H_ec and LE_f was higher than LE_ec.Sensible heat flux calibration factors k_H and i_H was 1.82 and-1.65,2.80 and-6.94,3.06 and-15.10,respectively for light,moderate and heavy frost night.For latent heat,the k _LE and i _LE was 2.78 and 10.61,2.44 and 4.34,2.51 and 11.94,respectively.Compared with H_ecc and LE_ec,modified sensible and latent heat flux(H_fre and LE_fre)had the same trends and amplitudes.?5?Quantification of frost protection energyTo conduct the frost protection experiments in the tea field,a wind machine and sprinklers coupled frost protection system was established.Three typical radiation frost nights was selected for verifying the protection efficiency of wind machine method,sprinklers method and coupled method.Modified energy transfer model was used to evaluate and quantize the frost energy in respective frost protection methods.When started wind machine frost protection system,LE reduced and LE and H were alternately converted.When the system is closed,there was no extra sensible heat energy applied in the system and H decreased rapidly.Meanwhile,aerodynamic resistance decreased resulted in the rapid increase of LE.Before the system was started,the canopy temperature(T_1.5)in the frost protection area was basically the same as that in the control area(T_ck).During the system operation period,T_1.5 slowly decreased to0?and maintained above 0?.The temperature rise??T?gradually raised and kept above 4.0?.After the system was closed,canopy temperature rapidly increased.In the operation period of sprinklers frost protection system,LE was converted into H.The canopy energy loss was in the form of net radiation and sensible heat flux.But the ice on the leaf inhibited the long wave effective radiation out the canopy and reduced the net radiation flux.After the system was closed,the ice on the canopy continued provide the latent heat for the system,and LE and H were maintained almost the same.During the system operation period,?T kept above 0?which was a significant temperature rise.After the system was closed,canopy temperature was kept around 0? which could avoid secondary damage caused by the rapid temperature rise.Wind machine and sprinklers coupled frost protection method also had a significant efficiency.In the early period operation of the system,canopy energy loss was in the form of net radiation,sensible heat and latent heat.However,LE alternated between positive and negative.With the increase amount of the spraying water,the canopy was completely frozen and the LE was negative.After the system was closed,H and LE almost had no change.That is because the solidification of the spraying water and continuously provide energy for the canopy.The canopy temperature remained around0? after the system was closed which could avoid the secondary damage on tea leaf caused by the rapid temperature rise.