Determinação da transpiração da cana-de-açúcar por métodos térmicos
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2010-07-28
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Universidade Federal de Viçosa
Resumo
O objetivo principal foi avaliar a aplicabilidade de métodos térmicos para medição do fluxo de seiva da cana-de-açúcar sob condições ambientais distintas. Para a avaliação dos métodos (balanço de energia e pulso de calor), foram realizadas campanhas de medição da transpiração e do fluxo de seiva da cana-de-açúcar em uma casa de vegetação. O experimento foi conduzido na área experimental do setor de Meteorologia Agrícola, pertencente ao Departamento de Engenharia Agrícola da Universidade Federal de Viçosa, Minas Gerais. O monitoramento dos dados meteorológicos no interior da casa de vegetação foi realizado por meio de um sistema computadorizado de aquisição de dados. Ele foi capaz de monitorar o fluxo de seiva da cana-de-açúcar e de controlar a potência elétrica aplicada nos terminais resistivos da fonte de calor. Constatou-se que as necessidades hídricas da cultura da cana-deaçúcar, em escala horária e diária, podem ser determinadas, de forma adequada, por meio de medições do fluxo de seiva com o método do balanço de energia em um segmento de caule. No entanto, em decorrência do grande volume de seiva armazenada nos colmos e da baixa velocidade de deslocamento da seiva em comparação com outras culturas, o método do pulso de calor não pôde ser implementado por meio da metodologia clássica proposta na literatura. Somente com a aplicação de pulsos mais prolongados, em escala de minutos ao invés de segundos, foi possível detectar variações de temperatura que pudessem ser mensuradas pelo sistema de aquisição de dados. Desta forma, para que possa ser aplicada para a cana-de-açúcar, a teoria do método do pulso de calor precisa ser adaptada a partir de estudos específicos. Notou-se que, tanto a temperatura da seiva acima, quanto a temperatura abaixo da fonte de aquecimento comportam-se de maneira distintas após a aplicação do pulso de calor nos diversos horários do dia, respondendo às variações da transpiração para um dia de céu claro. Foi observado também que o componente do armazenamento de calor no caule não pode ser desconsiderado no balanço de energia como foi proposto por alguns autores para medições realizadas em outras culturas. Por outro lado, devido ao diâmetro expressivo dos colmos, é necessário instalar quatro termopares para obtenção da temperatura da seiva nos níveis acima e abaixo da fonte de aquecimento, sendo dois inseridos no centro e outros dois na superfície do colmo. Um termopar adicional deve ser colocado no centro da manta de isolamento térmico, ao nível da fonte de aquecimento, para quantificação da condução axial e radial de calor. A transpiração de plantas individuais de cana-de-açúcar foi ligeiramente subestimada pelo método do balanço de energia em escala horária (RMSE = 14,6 g planta-1 h-1; MBE = -4,7 g planta-1 h-1; r = 0,9065; d = 0,9432) e diária (RMSE = 97,1 g planta-1 d-1; MBE = -56,2 g planta-1 d-1; r = 0,9369; d = 0,9488), com diferenças da ordem de 4% em relação aos valores máximos de transpiração medidos. Com a realização de novas pesquisas para aprimoramento desta técnica, o método do balanço de energia tem grande potencial para se tornar a técnica de referência na calibração de outros métodos utilizados para quantificação do fluxo de vapor d’água da cana-de-açúcar para a atmosfera.
The main objective of this research was to evaluate the applicability of thermal methods for measuring sugarcane sap flow under different environmental conditions. For the evaluation of methods (energy balance and heat pulse) measurement events were conducted in a greenhouse according to the transpiration and the sugarcane sap flow under cloudy skies and partly cloudy conditions. The experiment was conducted at the experimental field of Agricultural Meteorology sector from the Department of Agricultural Engineering, Federal University of Viçosa, Minas Gerais. The monitoring of meteorological data within the greenhouse was performed by a computerized data acquisition. It was able to monitor the sap flow from the due sugarcane culture and control the electric power applied to the resistive terminals of the heat source. Based on the results, was noticed that the water needing for the sugarcane culture in hourly and daily scale, may be adequately determined by means of sap flow measurements for using the method of energy balance according to the segment of stem. However, even with the large volume of stored sap in the stems and the low sap flow speed compared to other cultures, the heat pulse method could not be implemented through the methodology proposed on a classical knowledge literature. Only with the application of longer pulses in a scale of minutes instead of seconds, it was possible to detect temperature variations that could be measured by the current data acquisition system. Thus such in order to be applied to the studied sugarcane, the theory of heat pulse method needs to be adapted from specific studies in that matter. It was noticed that, both as the low stream temperature, as the high stream temperature of the heat source show differences after the application of heat pulse in different times of the day, responding with transpiration changing compared to a day of clear skies. It was also observed that the component of heat storage in the stem cannot be disregarded for the energy balance as proposed by some authors for measurements performed in other cultures. Moreover, considering the significant diameter of the stems is necessary to install four thermocouples to obtain the temperature of the sap in the levels above and below from the heat source, two from those inserted in the center and other two in its surface. An additional thermocouple should be placed in the center of the blanket insulation, at the heat source, to quantify the axial and radial conduction of heat. The transpiration of individual sugarcane plants was slightly underestimated by the method of energy balance in the hourly scale (RMSE = 14,6 g planta-1 h-1; MBE = -4,7 g planta-1 h-1; r = 0,9065; d = 0,9432) and daily (RMSE = 97,1 g planta-1 d-1; MBE = -56,2 g planta-1 d-1; r = 0,9369; d = 0,9488), with differences about 4% compared to maximum values of transpiration measured. With the procedure of the new research for improvement of this technique, the energy balance method has great potential to become a referential technique for calibration of other similar methods in order to quantify the water flow of vapor from sugarcane to the atmosphere.
The main objective of this research was to evaluate the applicability of thermal methods for measuring sugarcane sap flow under different environmental conditions. For the evaluation of methods (energy balance and heat pulse) measurement events were conducted in a greenhouse according to the transpiration and the sugarcane sap flow under cloudy skies and partly cloudy conditions. The experiment was conducted at the experimental field of Agricultural Meteorology sector from the Department of Agricultural Engineering, Federal University of Viçosa, Minas Gerais. The monitoring of meteorological data within the greenhouse was performed by a computerized data acquisition. It was able to monitor the sap flow from the due sugarcane culture and control the electric power applied to the resistive terminals of the heat source. Based on the results, was noticed that the water needing for the sugarcane culture in hourly and daily scale, may be adequately determined by means of sap flow measurements for using the method of energy balance according to the segment of stem. However, even with the large volume of stored sap in the stems and the low sap flow speed compared to other cultures, the heat pulse method could not be implemented through the methodology proposed on a classical knowledge literature. Only with the application of longer pulses in a scale of minutes instead of seconds, it was possible to detect temperature variations that could be measured by the current data acquisition system. Thus such in order to be applied to the studied sugarcane, the theory of heat pulse method needs to be adapted from specific studies in that matter. It was noticed that, both as the low stream temperature, as the high stream temperature of the heat source show differences after the application of heat pulse in different times of the day, responding with transpiration changing compared to a day of clear skies. It was also observed that the component of heat storage in the stem cannot be disregarded for the energy balance as proposed by some authors for measurements performed in other cultures. Moreover, considering the significant diameter of the stems is necessary to install four thermocouples to obtain the temperature of the sap in the levels above and below from the heat source, two from those inserted in the center and other two in its surface. An additional thermocouple should be placed in the center of the blanket insulation, at the heat source, to quantify the axial and radial conduction of heat. The transpiration of individual sugarcane plants was slightly underestimated by the method of energy balance in the hourly scale (RMSE = 14,6 g planta-1 h-1; MBE = -4,7 g planta-1 h-1; r = 0,9065; d = 0,9432) and daily (RMSE = 97,1 g planta-1 d-1; MBE = -56,2 g planta-1 d-1; r = 0,9369; d = 0,9488), with differences about 4% compared to maximum values of transpiration measured. With the procedure of the new research for improvement of this technique, the energy balance method has great potential to become a referential technique for calibration of other similar methods in order to quantify the water flow of vapor from sugarcane to the atmosphere.
Descrição
Palavras-chave
Cana-de-açúcar, Transpiração, Fluxo de seiva, Sugarcane, Transpiration, Sap flow
Citação
BOEHRINGER, Davi. Determination of transpiration of sugarcane by thermal methods. 2010. 114 f. Dissertação (Mestrado em Agrometeorologia; Climatologia; Micrometeorologia) - Universidade Federal de Viçosa, Viçosa, 2010.