Computing air temperature and humidity for reference crop evapotranspiration calculation in passive Mediterranean greenhouses

The Penman-Monteith equation has been worldwide adopted as the standard for calculating daily reference crop evapotranspiration (ET o ) and standardised procedures for computing meteorological data (air temperature and humidity) have been recommended for its calculation. In passive greenhouses, which predominate worldwide, this ET o equation is also used to calculate irrigation water requirements, but the standardised climate data computation procedures for its calculation do not appear to be applicable. The mean daily greenhouse air temperature measured over five representative horticultural crop cycles in two representative Mediterranean greenhouse areas was overestimated when calculated as the average of maximum and minimum daily greenhouse values, especially in cold crop periods, while the mean daily air relative humidity was underestimated. Consequently, the daily greenhouse ET o was overestimated when calculated using standardised procedures of computing greenhouse air temperature and humidity data (the RMSE ranged between 0.1 and 0.3 mm day −1 ) throughout all the studied crops cycles in two representative Mediterranean greenhouse areas. Consequently, mean daily values of greenhouse air temperature and humidity have to be determined from hourly or shorter time-period measurements. • Mean daily greenhouse air temperature was overestimated when calculated as the average of maximum and minimum daily values • Mean daily greenhouse air relative humidity was underestimated when calculated as average of maximum and minimum daily values • Daily greenhouse ET o was overestimated when calculated with standardised procedures of computing climate data