A Simplified Model for Substrate-Cultivated Pepper in a Hexi Corridor Greenhouse

The aim of this study was to investigate the method of estimating actual crop evapotranspiration (ETcact) in a greenhouse using other measured meteorological parameters when solar radiation (Rs) data are missing. The study estimated ETcact of greenhouse green peppers by combining solar radiation estimation models with the Penman–Monteith (PM) model and evaluated model performance. The results showed that the prediction accuracy of the temperature-based solar radiation model was higher than the model based on sunshine hours in the Hexi Corridor region. The effect of the insulation cover on the incident solar radiation in the greenhouse is modeled by introducing a ramp function. In terms of crop coefficients (Kcb), the initial Kcb value of green peppers in the 2023 growing season was generally consistent with the updated FAO-56 standard values, whereas the initial Kcb values (0.17) were higher than the standard values in the 2023–2024 growing season. During the two growing seasons, the mid-stage Kcb values were 1.01 in the 2023 growing season and 0.82 in the 2023–2024 growing season. The study also found that PM–RT4, PM–RT5, and PM–RT6 models were all able to accurately predict the ETcact of greenhouse green peppers during the 2023 growing season. The PM–RT4 model performed well in both growing seasons, with R2 = 0.8101 in the 2023 growing season and R2 = 0.7561 in the 2023–2024 growing season. Our research supports the PM–RT4 model as appropriate to estimate green pepper actual evapotranspiration in Gobi solar greenhouses (GSGs) and may be further used to improve irrigation scheduling for green peppers grown in GSGs.