Estimation of Greenhouse-Grown Eggplant Evapotranspiration Based on a Crop Coefficient Model

Abstract

Accurate estimation of crop evapotranspiration (ETc) is important to determine crop water requirements in greenhouse agriculture and to improve the irrigation water use efficiency. Here, a 3-year (2017–2019) experiment with spring greenhouse-grown eggplant (Solanum melongena L.) was conducted to investigate variation in the crop coefficient (Kc) measured with a weighing lysimeter, adjust Kc based on the local climate (Kc,Adj), and estimate daily ETc using a crop coefficient model. The 3-years mean local Kc (Kc,Loc) were 0.23 ± 0.03, 0.62 ± 0.06, 1.05 ± 0.03 and 0.87 ± 0.03 at the initial, development, mid-season, and end-season stages, respectively. Significant linear correlation was observed between Kc,Adj and Kc,Loc in the 3 years (R2 = 0.873, 0.901, and 0.897 in 2017–2019, respectively). Compared with the FAO-56 recommended Kc value (Kc,FAO), the mean Kc,Adj and Kc,Loc in the 3 years were by 66.3% and 61.8% lower, respectively. The single crop coefficient model accurately estimated daily ETc for greenhouse-grown eggplant. The coefficient of determination (R2), mean absolute error (MAE), root-mean-squared error (RMSE), and index of agreement between measured ETc and that estimated by the single crop coefficient model were 0.94, 0.35 mm‧d−1, 0.26 mm‧d−1, and 0.98, respectively, for the means in 2018 and 2019. Therefore, the crop coefficient method reliably estimated evapotranspiration with adjustment for the actual environment and can serve as a useful tool to improve water use efficiency.