Abstract: Based on the modified Penman-Monteith equation, and through the analysis of the relationships between crop coefficient and cumulative temperature, a new model for estimating the water requirement of greenhouse tomato under drip irrigation was built. The model was validated with the measured data of plant transpiration and soil evaporation in May 2-13 (flowering-fruit-developing stage) and June 9-20 (fruit-maturing stage), 2009. This model was suitable for the estimation of reference evapotranspiration (ET₀) in greenhouse. The crop coefficient of greenhouse tomato was correlated as a quadratic function of cumulative temperature. The mean relative error between measured and estimated values was less than 10%, being able to estimate the water requirement of greenhouse tomato under drip irrigation.

Key words: greenhouse, drip irrigation, tomato, water requirement, estimation model