Developing crop coefficients and pan coefficient for greenhouse-grown tomato under drip irrigation

Abstract Because of the special environment in the greenhouse and the partial wetting character of drip irrigation, the water requirement of crop was difficult to estimate or measure accurately. Water requirement of drip irrigated tomato grown in a solar-heated greenhouse in North China Plain was determined using a water balance method in this research. The continuous soil moisture status in the wetted soil volume with respect to crop spacing and root effective depth over the tomato growing season was monitored with a combination of volumetric and tensiometric sensors to calculate the water content variation and drainage. Based on the actual water requirement of tomato, the crop coefficients and pan coefficients were updated. Results indicated that the drainage occurred in the root effective depth (40 cm) obviously lagged behind the irrigation event because of the small amount of water applied by drip irrigation. The seasonal average percentage of drainage in the application amount was 13%. Water requirement increased with tomato growing and then decreased at the late season growth stage, resulting in water consumption intensities ranging from 1.5 to 4.2 mm·d-1. The measured Kc values were 1.01 and 0.61, being 5% and 8% lower than the Kcmid and Kcend values recommended by FAO, respectively. The ratio between 20 cm diameter pan coefficient and class A pan / sunken Colorado pan coefficient reported by FAO was 1.19 and 0.89, respectively. The experiment recommended that a crop-pan evaporation coefficient of 0.47, 0.85 and 0.59 for development stage, midseason stage and late season stage of tomato, respectively.