SYNOPSIS The Paper considers the problem of how the Nile and its tributaries can be developed to the greatest advantage, the main object being to provide as much water for irrigation as possible. The solution has been reduced to a form that can be handled by an electronic digital computer. The basic idea is to simulate mathematically the behaviour of the river during the 48-year period under analysis, and then to find the answer by trial and error. Methods have been devised for dealing with time lags; transmission and evaporation losses; and initial contents of reservoirs. Overyear storage, virtual storage, and supplementary reserves are freely used, as also is the principle of a target value. Continuity equations, which express basic natural laws, are contrasted with empirical control equations, which are tailored to fit the physical facts as closely as possible. The efficiency of this method of attacking the problem depends largely on insight and judgement. Flow diagrams are used to summarize the steps in the calculation. They form an indispensable connecting link between the engineer in charge of the investigation and the mathematician in charge of the computer. There emerges from these calculations a Nile Valley Plan based on local topography and hydrology and on the assumption that irrigation and hydroelectric power are desirable; but it is not an economic analysis. It gives maxima and minima for reservoir contents and controlled discharges; evaporation losses; benefits from certain conservation works; volumes of water available for irrigation; and figures for hydroelectric power. In order to maintain flexibility the volume of water available for irrigation is allowed to vary slightly from year to year. The annual total is, however, little affected by the proportion in which it is divided between Egypt and the Sudan.

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