2017 ASHS Annual Conference

The state of California accounts for over 90% of the processing tomatoes grown in the U.S. The production is mostly concentrated in the Central Valley where irrigation provides the primary and often the only water supply. Given the extent of tomato production in the state and its heavy reliance on irrigation, it is essential to accurately determine the water use of the crop to optimize water applications and improve water use efficiency. Crop water use (CWU) is most accurately determined through measurements of crop evapotranspiration (ET_c) using weighing lysimeters. ET_c generated by lysimeters can then be utilized to derive crop coefficients (K_c) that are needed for developing irrigation schedules. Studies have suggested that K_c can be influenced by crop varieties and that expressing K_c as a function of growing degree days (GDD) can account for the climatic conditions of a specific growing season (heat units). Therefore, the objectives of this study were to: a) determine the seasonal differences in ET_c and K_c for processing tomato (Solanum lycopersicum L.) grown over a three-year period; and b) develop ET_c and K_c curves as a function of both time and GDD. The research study was conducted at the University of California Westside Research and Extension Center in Five Points, CA. Results indicated that seasonal ET_c ranged from 407 to 598 mm and that midseason K_c reached 0.8-1.2. Daily GDD accumulation varied from 73 to 136 heat units among years. Maximum canopy cover was achieved at 84 -118 DAT during the three-year study, which represented 1206 to 1638 GDD units.