Measurements and estimates of potential evapotranspiration over Egypt

Mehanna (1976) estimated potential evapotranspiration (PE) for a number of stations in Egypt, using Penman's method with adjustment of the constants of the radiation term and the aerodynamic term, such that they would agree with measurements of radiation in Egypt and with estimations by Omar (1971a,b) of PE in a large field at Giza, to the southwest of Cairo. Measurements of PE were made at Bahtim, about 20 km to the north of Cairo, by evapotranspirometers.In the FAO Irrigation and Drainage Paper No. 24 on “Crop Water Requirements” (Doorenbos and Pruitt,1977) four methods were given for the calculation of PE; Blaney—Criddle, radiation, Penman (adjusted) and pan evaporation. It was considered that the Penman method was the most accurate of those given.The main features of comparison of the Mehanna and FAO's estimates of PE with measurements at Bahtim were that Mehanna, and three of the FAO's estimates (Blaney—Criddle, radiation and pan evaporation) are within ±10%0 of the measurements while the Penman estimate was 15% higher. An estimate of PE at Aswan was based on measurements of pan evaporation and estimates of evaporation from a large water surface at both Bahtim and Aswan, together with measurement of PE at Bahtim. The estimates by the methods of Mehanna, Blaney—Criddle, radiation and Penman lie in the limits 0—6% of the estimate based on measurements while the pan evaporation estimate is markedly lower. Comparison of the estimates at eight stations over Egypt shows that the Penman estimate is higher than the other estimates in Northern Egypt. The pan evaporation estimate is lower than the other estimates except at Bahtim where it is generally higher than Mehanna's and Blaney—Criddle's estimates. The low values of the pan evaporation estimates at Assuit and Aswan are remarkable.