PREDICTING ACTUAL EVAPOTRANSPIRATION: APPLICATION TO A TOMATO FIELD CROP

Actual evapotranspiration of tomato plants (Lycopersicum esculentum Mill.) row-planted in a sandy soil was measured using a simplified aerodynamic method as proposed by Itier (1981). Evapotranspiration was obtained after observations of net radiation, sensible heat flux and soil heat flux. It was determined during two sucessive irrigation seasons, for a total of 143 days. Field plots were located in the Sorraia valley, in central Portugal. The linear coefficient (C) between actual evapotranspiration and the energy term was analysed. The energy term explained 76% of actual evapotranspiration variation and leaf area index explained 57% of C variation. This dependance was further considered in the model to estimate actual evapotranspiration for low crops, proposed by Kanemasu et al (1979) based on Ritchie (1972). The model gives predictions with an error acceptable for practical applications and it can be improved if a coefficient for reduction of transpiration during soil drying is considered.