EVAPORATION FROM THE RESERVOIR OF THE HIGH ASWAN-DAM, EGYPT - A NEW COMPARISON OF RELEVANT METHODS WITH LIMITED DATA

Previous estimates of average annual evaporation from the lake formed by the High Dam at Aswan, Egypt, fall in the range from 4.65 mm d(-1) to 7.95 mm d(-1). The difference between these limits, more than 7 bil lion m(3) yr(-1) at the highest storage level, is nearly one-eighth th e share by treaty of Egypt, and more than one-third of the share of th e Sudan. It is also more than the estimated increase of the annual wat er need for Egypt between 1990 and 2000. This state of affairs renders proper management of the river flow for the sake of Egypt and the Sud an quite difficult. This paper compares the relevant methods of estima ting evaporation from the limited data available. These methods are: w ater-balance, energy budget, bulk aerodynamic (Dalton), combination (P enman) and Complementary Relations Lake Evaporation (CRLE) model (Mort on). The new estimates have a much narrower range, from 5.70 mmd(-1) t o 7.05 mmd(-1), or only a bit more than 4% of the annual Nile flow bel ow the High Aswan Dam. The average of these annual estimates of evapor ation, after excluding the bulk aerodynamic method because of its seve re limitations, is 6.0+/-0.3 mmd(-1) or 20% less than the 7.5 mmd(-1) adopted by the irrigation authorities in Egypt and the Sudan. This dif ference corresponds to 3 billion m(3) yr(-1) at the highest storage le vel or more than 5% of the annual outflow from the reservoir. Even whe n the higher estimates from the bulk aerodynamic method and from the P enman method with its usual wind function are included, the new averag e is still 15% less than the figure of 7.5 mmd(-1). The monthly distri bution of the annual evaporation varies more widely with the method ap plied. Similar comparative studies in future, aiming at obtaining impr oved estimates of evaporation, require all the data relevant to all th e methods to be collected properly for a common period of several year s at relatively stable lake level.