Developing crop coefficients for field-grown tomato (Lycopersicon esculentum Mill.) under drip irrigation with black plastic mulch

A 2 years field study was conducted to develop crop coefficients for field-grown tomato (Lycopersicon esculentum Mill.), a major irrigated crop in the Jordan Valley, under drip irrigation system with black plastic mulch. The area of the study field was 1.5 ha surrounded by many similar tomato fields. Actual crop evapotranspiration (ETC) was measured using eddy covariance technique which distinguishes this study from other previous studies conducted in the Jordan Valley that relied on the old indirect approach for ETC estimation based on the soil water balance. Grass reference evapotranspiration (ETO) was determined by using the FAO Penman-Monteith method utilizing the agrometeorological parameters measured at the study site. The crop coefficient (K-C) was determined as the ratio of ETC to ETO. The tomato crop coefficients were determined following the FAO crop coefficient model. The average crop coefficient during the midseason growth stage (K-C mid) was 0.82 which is far below the adjusted FAO crop coefficient of 1.19 by about 31%. Also, the late season crop coefficient (K-C end) was much lower than the adjusted FAO crop coefficient of 0.76 by about 40%. Moreover, the weighted average crop coefficient over the entire growing season (K-C GS) was 0.69, which is about 36% lower than the FAO corresponding value. In fact, the low K-C values obtained reflect the effect of practicing both localized drip irrigation and plastic mulch covering. This study showed that there is a big difference between the reported FAO crop coefficients and the one measured in the filed using a precise approach. These exact updated values of crop coefficients will enhance future estimation of crop water requirements and hence irrigation management of tomato crop which is the major irrigated crop in the Jordan Valley. (c) 2004 Elsevier B.V. All rights reserved.