CHAPTER IT, GENERAL PRINCIPLES GOVERNING GROUND WATER AND ITS MOVEMENTS.

DISPOSAL OF RAINFALL.

Rainfall on land is disposed of by evaporation, by absorption into the ground and by run-off. That which evaporates, concerns us no further. It is lost. Some. rain soaks directly into the ground. A great proportion of this may be subsequently drawn back to the surface and evaporate but a part may reach and increase the store of ground water. The third part collects into streams and runs away but in its course, some of it may be absorbed by the stream bed and thus adds materially to the ground water which supplies the wells. This is illustrated by the large number of wells that are situated near stream beds. In estimating the supply of water available frem a catchment basin it is important to know the proportion of run-off. Jn England about 25 % of the rainfall percolates into the ground. About 30% runs away in streams and the rest, about 45%, is lost by evaporation. In the Sudan the loss due to evaporation is much larger. The run-off of the Blue Nile has been estimated to be 18% of the rainfall. The Rahad River represents a run-off of about 13% and in the Northern Sudan there are large areas which contribute nothing to the streams and can be said to have no run-off at all.

PERMEABLE AND Porous ROCKS.

Wells and springs are fed as a result of the deposit or rock in which they occur, becoming partly saturated, and the nature of the materials and the manner in which they contain water will now be considered. To be favourable for water, the material of the geological formation must be porous and the pores must be large enough to allow the water to drain out under the influence of gravity. The water may occupy cracks and joints in solid, and otherwise impermeable, rock but commonly the materials are granular such as sand, sandstone, or rotten, crystalline rock, and the water fills the spaces between the grains. Sand may be reyarded as a typical example of porous material and we shall consider its properties for the purpose of illustrating the features of a water bearing formation. Ordinary sand, deposited either by wind or in water, occurs in layers each consisting of grains of fairly uniform size. This is a consequence of the transporting power of the stream which carried the sand being proportional to its velocity. It is simplest first to consider an ideal sand made up of spherical grains, all of similar size. The diagram P!.I. shows that the grains of such deposits may be packed in either of two ways. In one of these packings, a. of the diagram, the centres of the grains are at the corners of triangles and each grain would be in contact with twelve neighbours. This is the closest method of packing. In the other, b, the centres