The calculation of reference evapotranspiration ET o with the FAO Penman-Monteith method (FAO-PM) requires maximum and minimum air temperature (T max and T min ), actual vapour pressure (e a ), net radiation (R n ), namely computed from sunshine duration or short wave solar radiation (R s ), and wind speed at 2 m (u 2 ). However, some of these weather variables, especially solar radiation, are often missing in irrigation practice. To overcome these problems, procedures to estimate ET o with missing climate data are proposed as part of the FAO-PM methodology and it is important to assess the accuracy of these procedures for different climate conditions. This is the objective of this study relative to the Trace plain, south Bulgaria, where data relative to five meteorological stations in the Trace plain are analysed. All stations are located within a homogeneous physiographic region where the air masses governing the weather are the same. The procedures to estimate the missing radiation data refer to the use of the maximum and the minimum daily temperature differences and data from a neighbouring station. Procedures to estimate missing humidity data relate to the use of minimum temperature data and those for the wind speed data refer to regional average data. The accuracy of all different procedures analysed was assessed by regressing ET o values calculated with the FAO-PM equation computed with limited data versus ET o values computed with complete data sets. Statistical parameters of the regression through the origin and the computed standard errors of estimates (SEE) were used. It could be concluded that the procedures proposed by FAO are accurate providing small SEE, including when only maximum and minimum temperature data are used, which yields lower SEE than when using the Hargreaves equation, which tends to overestimate ET o for the observed conditions. Copyright C'est l'objectif de cette étude relative à la Plaine

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