Validation of the FAO methodology for computing ET_o with limited data. Application to south Bulgaria

Abstract

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

Estimativa da evapotranspiração de referência a partir de dados meteorológicos limitados

“…PM ea , método padrão, com uso da pressão de vapor estimada pelas temperaturas máxima e mínima, e pela umidade relativa do ar; PM Krs , método padrão, com uso da radiação solar estimada pelo balanço entre ondas curtas e longas; HS, método de Hargreaves-Samani; e PM u2 , método padrão, com uso da velocidade de vento constante (2,0 m s -1 ). (Popova et al, 2006;Sentelhas et al, 2010).…”

Evapotranspiração Horária X Diária Utilizando Penmanmonteith Para O Polo De Desenvolvimento Petrolinape/Juazeiro-Ba

“…Tal resultado assemelha-se aos obtidos por Popova et al (2006) que obtiveram R 2 0,96; 0,85 e 0,98 com a metodologia dos mínimos quadrados, ao comparar estimativas com dados meteorológicos para obtenção de ETo em uma estação de referência para três localidades do sul da Bulgária.…”

Spatial modelling of reference evapotranspiration using adjusted Blaney-Criddle equation in an arid environment

“…where T max and T min are the maximum and minimum air temperature, respectively ( • C); and k R S is an adjustment factor, equal to 0.16 for interior locations where land mass dominates and air masses are not strongly influenced by a large water body, and 0.19 for coastal locations situated on or adjacent to the coast of a large land mass and where air masses are influenced by a nearby water body (Popova et al 2006). The calculation of R a followed the procedure outlined in Allen et al (1998).…”