Crop evapotranspiration of processing tomato in the San Joaquin Valley of California, USA

Yield of processing tomato has increased by 53% over the past 35 years. Thus, concerns exist about the current seasonal crop evapotranspiration requirements of processing tomato compared to the past published requirements, which were about 645 mm. Also, the mid-season crop coefficient for processing tomato developed 35 years ago with sprinkler irrigation was 1.25, while a mid-season coefficient developed 20 years ago with subsurface drip irrigation was 1.05. Because of the age and variability of crop coefficients and the long-term yield increase, a study was conducted to determine the seasonal crop evapotranspiration and crop coefficients of processing tomatoes using the Bowen ratio energy balance method in eight commercial fields from 2001 to 2004. Measurements were made in both furrow- and drip-irrigated fields. Results showed seasonal crop evapotranspiration to range from 528 to 752 mm with an average of 648 mm. No statistical differences were found between furrow and drip irrigation. Mid-season crop coefficients varied between 0.96 and 1.09 with statistically similar values between furrow- and drip-irrigation for a given year. Current evapotranspiration rates were similar to those of the early 1970s, indicating that the water use efficiency of processing tomato increased substantially with time during the past 35 years.