A COMPARISON OF FOUR EVAPOTRANSPIRATION MODELS IN A GREENHOUSE ENVIRONMENT

Advances in evapotranspiration modeling have led to a variety of formulas and terms used in irrigation control research. Four models that represent the progression from outdoor, “big leaf” estimates to indoor, greenhouse–specific formulations are compared in this article. Each of the models was algebraically reduced to common and consistent terms. The data from a lysimeter study were used to compare evapotranspiration combination models using evapotranspiration (ET) rates of a ‘Red Sunset’ Red Maple tree (Acer rubrum ‘Red Sunset’) grown in a controlled–environment greenhouse. The measured ET was compared with two empirical climatic factors (solar irradiance and vapor pressure deficit) and with calculated ET based on four evapotranspiration models: (1) Penman, (2) Penman–Monteith, (3) Stanghellini, and (4) Fynn. A measure of the model performance is given by the Nash–Sutcliffe R2, or model efficiency. The relationship between measured and calculated ET for the Stanghellini model was calculated to have a model efficiency of R2 = 0.872, while the other models yielded correlations of R2 = 0.214, R2 = 0.481, R2 = –0.848 for Penman, Penman–Monteith, and Fynn, respectively. The differences between the models are discussed, revealing the importance of the leaf area index factor and a sub–model for irradiance in the plant canopy. Based on the coefficient of determination, vapor pressure deficit alone yielded a good linear correlation to measured ET (r2 = 0.884), while solar irradiance alone resulted in r2 = 0.652.