Crop coefficients for drip-irrigated processing tomato

Drip irrigation of processing tomato is increasing in the San Joaquin Valley of California (USA), a major tomato production area. Efficient management of these irrigation system(s) requires reasonable estimates of crop evapotranspiration (ET.) between irrigations. A common approach for estimating ET, is to multiply a reference crop evapotranspiration (ET.) by a crop coefficient. However, a review of literature revealed mid-season crop coefficients for processing tomato to range from 1.05 to 1.25. Because of this variability, uncertainty exists in the crop coefficients appropriate for drip irrigation in the San Joaquin Valley. Thus, a study was initiated to determine the ET, of processing tomato for drip irrigation in commercial fields and then calculate crop coefficients from the ET, and ETO data for the west side of the San Joaquin Valley. Crop ET, was determined at five locations using the Bowen Ratio Energy Balance Method (BREB). Canopy coverage was also measured using a digital infrared camera. Average crop coefficients ranged from about 0.19 at 10% canopy coverage to 1.08 for canopy coverage exceeding about 90%. A second order regression equation reasonably described a relationship between crop coefficient and canopy coverage. Generic curves describing crop coefficient versus time of year were developed for various planting times. (c) 2005 Elsevier B.V. All rights reserved.