Development of irrigation control method based on evapotranspiration from the plant canopy

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A simple heat balance equation for the plant canopy model from which the Penman-Monteith equation was derived is used to estimate evapotranspiration from a plant canopy. Incoming radiation downward and upward, air temperature and wind speed above the canopy are the only four factors to be measured. Estimated evapotranspiration is in good agreement with the values measured by an accurate scale in wet and dry conditions. The present method was simple and considered for irrigation control in greenhouses.It was found that the cosine law for a flat sensor causes an error since the surface of a plant canopy which receives the sun's energy is not flat. In order to develop an economical solarimeter to measure solar radiation received by the surface normal to the direct solar radiation, a photodiode was placed at the bottom of an opaque white ping-pong ball which had a hole for the photodiode at the bottom. It was concluded that this solarimeter can be used to form better control strategies for greenhouse environment control.It was proved that the system can be used without any problem in greenhouses. In the open field, the effect of precipitation should be taken into account. Four sensors to detect wetness have been used to determine when irrigation should be started after rain fall with a potentiometer to measure soil water content. It was concluded that the timing can be determined by the developed instrument itself by judging when evapotranspiration starts.