Study of evapotranspiration and crop coefficients for eggplant in a Venlo-type greenhouse in South China

Determination of greenhouse evapotranspiration (ET_c) and the response mechanism of environmental factors are important for accurate irrigation management and greenhouse environmental control. Models for predicting ET_c under greenhouse conditions are extremely scarce in comparison to those under open field conditions. Meteorological data inside and outside a Venlo-type greenhouse were measured from April to July 2015 to estimate the reference evapotranspiration (ET₀) based on four approaches (the Penman–Monteith model, a revised Penman–Monteith model, a modified Priestley–Taylor approach and the Irmark–Allen fitting approach). The daily ET₀ values inside the greenhouse calculated from these four methods presented similar variation trends but were somewhat different. The ET_c for eggplant in the greenhouse was measured by lysimeters to estimate the crop coefficient with the FAO-56 method (K_c = ET_c/ET₀), and the response mechanism of environmental factors on the ET_c was analysed. The results showed that the crop coefficients of the eggplant were different, with values of 0.94–1.97, 0.83–1.87, 0.64–1.34 and 0.58–1.21 based on the ET₀ from the Penman–Monteith model, revised Penman–Monteith model, modified Priestley–Taylor approach and Irmark–Allen fitting approach, respectively; the net radiation showed the highest correlation with the ET_c; the secondary parameters were the air temperature and solar radiation, with R² values equal to 0.77 and 0.75, respectively; and the relative humidity and soil temperature showed different extents of influence on the ET_c. The results of this study provide more options for greenhouse cultivars to accurately determine the ET_c of eggplants using the FAO-56 method based on the availability of different meteorological data.