— 53 — It will be seen that, although the evaporation is greater as the temperature increases, the variations from day to day are due tg the strength and duration of the air movements.

As a rule the daily curve of evaporation is steepest from 8 a.m. till 2 p.m., although should the wind retain its force throughout the day, evaporation continues at a correspondingly high rate.

Evaporation from Free Water Surface.—The readings of the Piche Evaporimeter are mainly of use for comparative purposes, as they furnish no evidence as to the amount of evaporation from a free water surface. It is obvious, however, that the water requirements of the crop, and the losses of irrigation water by evaporation, will vary directly with the readings of the instrument.

By the courtesy of the Egyptian Irrigation Service we are able to reproduce the readings from a tank evaporimeter placed in the stream of the White Nile some 100 metres above its junction with the Blue Nile.

Evaporation from Free Water Surface.

Daily Means in Millimetres.

Month.