VALIDATION OF FAO-56 METHODOLOGY FOR COMPUTING ETO WITH MISSING CLIMATE DATA APPLICATION TO SOFIA FIELD | ВАЛИДАЦИЯ НА МЕТОДОЛОГИЯТА НА ФАО 56 ЗА ИЗЧИС- ЛЯВАНЕ НА ЕТАЛОННАТА ЕВАПОТРАНСПИРАЦИЯ ЕТо – РМ
2011
Ivanova, M., N. Pushkarov Institute of Soil Science, Sofia (Bulgaria) | Popova, Z., N. Pushkarov Institute of Soil Science, Sofia (Bulgaria)
Calculation of reference evapotranspiration ETo with the FAO Penman-Monteith method eq. 1 requires maximum and minimum air temperature Tmax and Tmin, actual vapour pressure ea, net radiation Rn, namely computed from sunshine duration or short wave solar radiation Rs, and wind speed at 2 m u2 However, some of these weather variables, especially solar radiation, are often missing in the irrigation practice. To overcome these problems, procedures to estimate ETo with missing climate data are proposed as part of the FAO-PM methodology. It is important to assess the accuracy of these procedures for different climate conditions. This is the objective of the study relative to the Sofia field, South West Bulgaria, where data relative to two meteorological stations and three periods are analysed. All stations are located within a homogenous 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 eq. 3 Procedures to estimate missing humidity relate to the use of minimum temperature data eq. 4 and those for the wind speed data refer to adopting regional average data. The accuracy of all different procedures analysed is assessed by regressing ETo values calculated with the FAO-PM equation computed with limited data versus ETo values computed with complete data sets fig. 1, 2, 3, 4. Statistical parameters of the regression though the origin and the computed standard errors of estimates, RMSE, are used tables 1, 2, 3, 4. It could be concluded that the procedures proposed by FAO are accurate providing small RMSE including when only maximum and minimum temperature data are used, which yields lower RMSE than using the Hargreaves equation 5, which tends to overestimates ETo for the observed conditions.
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