Evapotranspiration and influencing factors of greenhouse tomato

Crop evapotranspiration under plastic mulching film in greenhouse is a critical factor to schedule irrigation and evaluate water use efficiency, in North China in particular. Therefore, a field experiment on tomato evapotranspiration was done in Ningjin County, Hebei Province from October 2018 to January 2019. Sap flow, soil matrix potential, soil volumetric moisture content and microclimate factors such as total radiation, temperature, relative humidity in the greenhouse were measured, to characterize tomato evapotranspiration and sap flow rate, and their correlation with meteorological factors, and to evaluate water use efficiency after comparison of evapotranspiration and irrigation water. Sap flow was found to begin mostly at 08:00, before increased quickly to maximum at 13:00, then decreased quickly and stopped at 21:00. Diurnal curve of sap flow in sunny days showed a single peak, but in cloudy days multiple peaks were found. With sufficient soil water, sap flow was linearly related to total radiation, air temperature and vapor pressure deficit, with a Pearson correlation coefficient of 0.972, 0.949 and 0.970 respectively. Total daily evapotranspiration was linearly related to total radiation, daily average temperature, daily maximum temperature and vapor pressure deficit, with a Pearson correlation coefficient of 0.970, 0.828, 0.922 and 0.965 respectively. During fruit expansion, crop coefficient K_c in greenhouse was found to be 1.5, much higher than 1.15 recommended by FAO-56. Therefore crop coefficient as recommended by FAO-56 should probably be adjusted for the cultivated greenhouse tomato in North China. Total tomato evapotranspiration in the experiment was found to be 78 mm, which was 31% of total irrigation (255 mm), therefore approximately 69% of irrigation water was lost indicating over-irrigation. Irrigation is therefore recommended to be reduced to save water and improve water-use efficiency.