Estimation and verification of autumn lettuce evapotranspiration in greenhouse based on single crop coefficient method

【Objective】 The variation in evapotranspiration is an important parameter in irrigation design. In this paper, we propose and verify a modified single crop coefficient method to calculate the evapotranspiration of greenhouse lettuce. 【Method】 The experiment was conducted in autumn from 2021 to 2022 in a greenhouse grown with lettuce. The crop coefficient was calculated using the formula recommended by FAO-56, with the calculated crop coefficient for initial growth stage being modified based on ET0 and irrigation frequency, while for other growing stages it was modified based on meteorological factors and plant height. The modified single crop coefficient was then used to calculate the evapotranspiration of the lettuce, with the calculated results being validated against measured data from weighing lysimeters. 【Result】 The mean reference crop evapotranspiration (ET0) of the lettuce varied between 1.18 to 2.44 mm/d, but showed a decreasing trend from 2021 to 2022. The modified crop coefficient and measured crop coefficient both peaked at the initial stage, with their associated two-year average being 0.74±0.13 and 0.76±0.13, respectively. As the crop grew, both crop coefficients asymptotically decreased to 0.5~0.6 in the middle and later growing stages. The modified crop coefficient was closer to the measured crop coefficient than that calculated from the formula recommended by the FAO-56, with the relative error between them ranging from -6.7% to 5.4%. The mean estimated and the measured evapotranspiration of the lettuce in autumn were 1.310 mm/d and 1.283 mm/d, respectively. The determination coefficient, the mean absolute error, root mean square error and the index of the agreement between the measured and estimated evapotranspiration were 0.975, 0.176 mm/d, 0.222 mm/d and 0.955, respectively. 【Conclusion】 The modified single crop coefficient is more accurate than that calculated from the formula recommended by the FAO-56 for estimating the evapotranspiration of greenhouse lettuce.