INVESTIGATION OF FAO PENMAN-MONTEITH REFERENCE EVAPOTRANSPIRATION OVER THE TERRITORY OF BULGARIA

Since many years calculation of crop evapotranspiration has been done by the Delibaltov-Hristov-Tzonev equation. It reads the impact of one only meteorological factor on the evapotranspiration – the temperature sum. Recently a 20-50% error has been established in its estimates. Since 1998, FAO Penman-Monteith method has been recommended by FAO as the most accurate among all existing ones because of comprising all climatic and meteorological aspects of environments and localizations. Validation of this method has not been completed in Bulgaria yet. The reason is that meteorological data with the needed all-over-the-country representativeness is hardly available. In this paper 30-year averages of the reference evapotranspiration, calculated by FAO Penman-Montheit method for 30 sites in the arable territory of Bulgaria, as well as summarized results for the climatic regions are presented. 30-year sequences of meteorological factors values of the period 1971-2000 have been processed. The dynamics of the potential vegetation period reference evapotranspirarion and its typical winter and spring crop vegetation period values, the irrigation season July-August inclusive, are presented too. It has been established that Petrich-Sandanski Climatic gets the highest daily values and the greatest water losses by evapotranspiration, followed by the Climatic Region of Eastern Middle Bulgaria. The Climatic Region of the High Fields of Western Bulgaria gets the lowest values. The same parameters are higher in the Continental-Mediterranean Climatic Region than in European-Climatic one. The results, presented in the paper, first may be used by the scientists for establishing crop factors Kc for each irrigated crop in our country. Afterwards, the standardized values of the reference evapotranspiration, supplied with Kc, may be used for future irrigation net design. Each year, current ETo values, supplied with Kc, can be used by the farmers from all over the country to practice the necessary irrigation scheduling for their cultivated crops. The results of this investigation can be used by the authorities to plan adaptation and even mitigation measures to future drought.