Partitioning of Evapotranspiration Using Lysimeter and Micro‐Bowen‐Ratio System

The present study estimated evaporation E and transpiration T losses separately during various growth stages of a tomato crop (Lycopersicon escallen mill. UC 82 variety), in Davis, California. Micro‐Bowen‐ratio systems were used during the summer of 1984 to determine net radiation and temperature, and humidity gradients close to the soil surface. Transpiration was determined by subtracting E from crop evapotranspiration ET as measured by a large weighing lysimeter. Results show considerable effects of soil moisture on evaporation and above‐soil net radiation during early growth stages, but the effects diminished greatly as crop cover approached a maximum. With crop cover less than 50%, soil‐surface evaporation one day after, irrigation was much higher than for two or three days later. Increased transpiration occurred as the soil surface dried, apparently in response to decreased evaporation. Irrigation timing and crop cover substantially affected partitioning of ET. At 34% crop cover, ET was evenly split into E and T. Later, at 43% and 85% cover, calculated E/ET values were 0.28 and 0.05, respectively.