Mechanism And Control Technology Of Late Frost Protection For Tea Plant (Camellia Sinensis L.) Through Air Disturbance

Major tea growing areas in the south of the Yangtze River, China have been suffering from severe late spring frost damage in consecutive several years, even though both China's tea cultivated area and yield have firmly ranked the first in the world. The frost damage has resulted in sizable economic losses and unstable tea product supply, which is becoming one of the main bottlenecks to hold back the sustainable healthy development of tea industry. However, only conventional frost protection measures are available in China so far, such as covers, flood irrigation and smudge, which have disadvantages of low protection effectiveness, waste of time and labor or potential environmental pollution. Yet mechanized or automated equipment and technology have not been developed or put into use. Some foreign advanced frost protection methods are not always suitable to our national conditions, and moreover the technology blockade is extremely strict. Therefore, it is of great importance to conduct the fundamental research on the mechanism and control technology of late frost protection for tea plant, and to develop own technological system of frost protection with independent intellectual property and practical applicability, which not only improves the labour productivity, mechanization and automation of frost protection, but also promotes the development of control technology on China's agro-meteorological disasters. In this dissertation, perfect frost protection mechanism based on air disturbance was disclosed through the analysis and experimental study on the occurrence and development of frost damage and the feature of near-ground thermal inversion in tea farm. And then the technological system of frost protection base on air disturbance was established and simulated. The energy-saving and reliable control strategy of the system was made on the basis of biometeorology. Finally, the wind machine prototype was developed and tested through practical application in the field, and the overall evaluation of frost protection effect and comprehensive performance were conducted among several models of the wind machine. Major research work and findings are as follows.(1) Forecast of frost events and identification of freeze damage to tea plantsFirstly, in macro-scale of time from 1979 to 2010, the historical weather data was applied to seasonal calamities grey prediction in order to establish and validate the forecast models of the first and last frost date, which are most serious damage to tea production. The occurrence probability of late frost is 96.67% for the past 32 years and the occurrences are unsteady with wide spans.Grey prediction model was established to forecast the first/last frost days in Zhenjiang, which would be the closest to now. Secondly, on a short-time scale weather observations were used to establish the minimum temperature model on potential frost nights, and then air temperature throughout the frost night was predicted based on the typical variation trend of temperature, which ranges within the minimum temperature and the sunset temperature. It is proved that the minimum temperature has a linear relation with the initial temperature and the best initial time is one hour after the sunset. The correlation coefficient of the linear wquation is 0.86. The frost night temperature changes as a descending curve of natural logarithm. The above models help provide frost warning and support the decision-making on frost control. Thirdly, diffuse reflectance spectra of tea leaves frozen at different low temperatures were collected to set up a calibration model for identifying the degree of freeze damage to tea plants. Consequently, the analytical method could be a potential way of determining critical damage temperature by comparing the normal and the slightest frozen leaves. Diffuse reflectance spectrum of the surface of tea leaves was collected after the sample leaves were frozen at different low temperatures. Then pre-processing of the spectra was conducted with the combination of normalization, moving average filtering and second-order differentiation. The best prediction model was established through the algorithm of partial least square combined with artificial neural network, with 7 principal components,0.1512 for RSMEC and 0.9950 for the correlation coefficient between the true and predicted values. The average relative error of the prediction model is the minimum at 5.0725%. The model could be used for rapid identification of normal leaves and the slightest frozen ones and produces a new method of determing the critical temperature, consequently provides parameters for frost protection control.(2) Mechanism of late frost protection for tea plant based on air disturbance The features of microclimate, especially thermal inversion in tea fields were discussed from the perspective of energy and meteorology. Under frost conditions thermal inversion experiments were conducted to reveal the variation characteristics of air temperature field near above the ground, and subsequently to explain different frost protection mechanisms based on air disturbance on different occasions.The test results indicate that frosts tend to be occurring during cloudless calm nights in early spring, when the air becomes vertically stratified with temperature increasing with height. The vertical distribution of temperature field near above ground changed as a quadratic nonlinear curve. The temperature at the height of 9 m was 6～8℃higher than that of canopy. And there was an obvious thermal inversion at the height of 6～8 m. The lowest temperature arose at the depression, where frost damage was the severest. For a individual tea plant the temperature was not uniform throughout the whole canopy, and on the top of the northwest canopy was the lowest temperature, which could be used as one of the frost control parameters. Therefore, frost damage to the tea plant can be avoided or reduced by convecting the thermal inversion layer with a fluid machinery (fan), which pushes the top warmer air downward to the canopy and mixes with the cold air to increase the canopy temperature. This is the frost protection mechanism for the thermal inversion stage. The thermal inversion was gradually disappearing around the sunrise, and air temperature aloft becomes lower than that of tea plant canopy, when tea leaves could not be damaged since their temperature rise is delayed if air disturbulance is made. This is the first time to come up with the frost protection mechanism for anticyclonic inversion. After sunrise the canopy temperature increased very fast, but the temperature on the east side of the row was higher than that on the west side.Computational fluid dynamics simulation of the frost protection was carried out to show that its coverage is pear-shaped and symmetric and major area is prevented from frost damage with remarkable temperature rise of 4℃.(3) Control technology of frost protection through air disturbanceControl technology is the key to frost protection through air disturbance. The technological defect was found after the overall investigation on current situations of overseas frost protection control. The thermal inversion-driven control strategy based on critical temperature(short as TCSCT) was first put forward from the compound perspective of tea plant's hardiness and thermal inversion features. TCSCT makes the wind machine run with low energy consumption. During the thermal inversion, that the canopy temperature is less than the critical temperature is the sufficient condition, and that thermal inversion difference is higher than a certain threshold is the necessary condition control the sytem for start and stop, avoiding the possible misoperation or empty run. After the thermal inversion disappears and the frost thaws, air temperature and period of time were subject to the control of delaying the operation in order to mitigate the defrosting and prevent the further damage. The hardware and software were developed and tested. Meteorological Information acquisition and wireless transfer apparatus for tea farm were deloped based on AT89C52 and wireless communication module TC35i, and application was programmed to complete the data collection and wireless transfer. The apparatus has the simple structure, convenient maintanence and high reliability of realtime data acquisition and wireless transmission, which provides the information support for microclimate monitoring and early warning and control for late frost damage to the tea plant.(4) Frost protection test of developed wind machine and its performance evaluationThe elevated frost protection wind machine was independently designed and developed base on the study on mechanism and technology of frost protection through air distrubance. The machine has simple structure and convenient maintainance without any pollution, and especially suitable for tea farms and orchards in hilly regions. Performances and frost protection effects were investigated both at the lab and in the field, and compared between self-developed machines and similar products imported from Japan. The results of fuzzy comprehensive evaluation based on improved analytic hierarchy process show that the self-developed model WT3000 achieved the best technology economy, which has the rated power of 3 kw, functioning area of 1000 m2 and could increase the canopy temperature by 2.86℃and raise the yield by 47.7% during the heavy frost season。The control system was running reliable and stable, and could save energy consumption by 20%, compared with Japanese products.Based on the occurrence and development of frost damage to tea plants and the thermal inversion measurement, mechanism and control stategy of frost protection through air disturbance was presented. The elevated wind machine and the automatic controller for frost protection were developed with our own intellectual property rights, and put into use in tea fields with excellent results. It is of theoretical and practical significance for the mechanism and control stategy to improve the technology of disaster reduction and elimination for China's agricultural frost/freeze damage.