Io WATER SUPPLIES IN THE SUDAN history, since the earliest dynasty in about 3200 B.C., has not been subject to any great change and consequently that the rainfall in the Nile basin has also been similar to that of recent times. There have been periods with more rain and others with less but the average has remained fairly constant. We have lately been experiencing one of the dry periods.

Much research has been carried out with the object of ascertaining the factor.on which the rain depends and perhaps enabling the height ° of the Nile flood to be predicted. It seems to be agreed that the rainfall over the Sudan depends on evaporation in the South Atlantic and on the temperature and atmospheric pressure over the Nile basin.

A monthly chart is issued with diagrams of the principal Nile gauge readings and also tables recording the volumes of flow.

EVAPORATION.

This meteorological factor, though hardly considered in Northern European countries, is of very great importance in the Sudan especially in considering the losses from the surfaces of reservoirs and tanks. The measurements are usually made by means ofa Piché tube exposed in an ordinary meteorological screen and the readings of these instruments are given in tables. It has been found that the results of the Piché tube observations have to be multiplied by the factor 0.58 to give figures representing evaporation from open water surfaces such as rivers or large reservoirs in the neighbourheod. The evaporation from a small area of water, such as a hafir or a fula must be relatively greater but we have not yet any direct measurements. The following are results obtained at Khartoum and are the means of observations extending over a period of 16 years. “ The figures represent the depth of water, in millimetres, lost daily by evaporation :— = --: : 5) | 5 A . 5 rae bee aes a Be es sl2/2)2\2)2/212 | 2 ee A 12.9|15.6|18.4|19.6.18.6|16.1/13..2 10.21, 34, 4heaeaiee ee B 7-5] 9-1\T0.7|11.4.10.8) 9.4) 7.6) 5.9 6.5, 8.4) 8.4] 7.2 The first line gives the mean Piché readings.

The second line shows the estimated evaporation from large sheets of water on the basis of the reduction factor 0.58. In estimating the loss likely to occur in a hafir, it can be assumed that about I cm, depth is evaporated daily. Therefore that a depth of Im. of water would last about 3 months if it is not subject to any other losses. This rate of evaporation decreases towards the more humid eos in the South but can be regarded as applying over the Central udan.

(1) Climate of Khartoum, p. 60.