DRY BEAN EVAPOTRANSPIRATION RATES ESTIMATED BY PENMAN-MONTEITH EQUATION IN VENTILATED GREENHOUSE

ABSTRACT: Dry bean crop (Phaseolus vulgaris L.) evapotranspiration rates were estimated by the Penman-Monteith (1981) equation (ETcal) and measured in a weighing minilysimeter (ETwg). The experiment was carried out in a greenhouse at Embrapa Milho e Sorgo (EMBRAPA/CNPMS), Sete Lagoas, MG, Brazil. The dry bean was grown in 15-liter vases which also served as weighing minilysimeter containers. Seven vases formed the experimental plots; six were set in a circle as a border, with one at the center from which data was collected. Two plants were maintained per vase in under conditions for optimum development to obtain the crop maximum evapotranspiration rates (ETmax). The ETmax values were obtained by weighing the system daily at 30 minute intervals by means of a precision digital electronic scale. The aerodynamic resistance was calculated using the equation proposed by Thom and Oliver (1977), and SMITH et al. (1991) to obtain ETcal values. A linear regression equation was fitted through the origin, and a standard error of estimate (SEE) was calculated together with the standard error of estimate for the adjusted linear regression equation (ASEE). The estimated aerodynamic and plant canopy resistance for complete crop development were 14.1 and 4.9 sm-1, respectively. The average evapotranspiration values obtained between 57 and 62 days after planting (DAP) were 7.21 and 6.08 mmd-1 for ETwg and ETcal, respectively. The following regression equation was adjusted between measured and estimated evapotranspiration values ETwg=1.0285ETcal. This equation presented a r2 equal to 0.788, SEE equal to 0.0605 and ASEE equal to 0.0110, based on 30 minute data averages. The equation for ra estimated for low wind speed conditions (U < 1.0 ms-1), proposed by Thom and Oliver (1977), presented better results, when compared to the equation recommended by SMITH et al. (1991).