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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. This method was proven to be simpler and more accurate than that based on the Penman-Monteith equation.