Evaporation from the reservoir of the High Aswan Dam, Egypt: A new comparison of relevant methods with limited data

Summary

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 billion m³ yr⁻¹ at the highest storage level, is nearly one-eighth the share by treaty of Egypt, and more than one-third of the share of the Sudan. It is also more than the estimated increase of the annual water 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 Sudan quite difficult. This paper compares the relevant methods of estimating evaporation from the limited data available.

These methods are:water-balance, energy budget, bulk aerodynamic (Dalton),combination (Penman) andComplementary Relations Lake Evaporation (CRLE) model (Morton). The new estimates have a much narrower range, from 5.70 mm d⁻¹ to 7.05 mm d⁻¹, or only a bit more than 4% of the annual Nile flow below the High Aswan Dam. The average of these annual estimates of evaporation, after excluding the bulk aerodynamic method because of its severe limitations, is 6.0 ± 0.3 mm d⁻¹ or 20% less than the 7.5 mm d⁻¹ adopted by the irrigation authorities in Egypt and the Sudan. This difference corresponds to 3 billion m³ yr⁻¹ at the highest storage level or more than 5% of the annual outflow from the reservoir. Even when the higher estimates from the bulk aerodynamic method and from the Penman method with its usual wind function are included, the new average is still 15% less than the figure of 7.5 mm d⁻¹. The monthly distribution of the annual evaporation varies more widely with the method applied. Similar comparative studies in future, aiming at obtaining improved estimates of evaporation, require all the data relevant to all the methods to be collected properly for a common period of several years at relatively stable lake level.