This section is from the book "Commercial Gardening Vol1", by John Weathers (the Editor). Also available from Amazon: Commercial Gardening, A Practical & Scientific Treatise For Market Gardeners.
We have already seen (p. 118) that the deeper a soil is cultivated the more water it will absorb, no matter what its character may be. This water sinks down and down until it comes to a level where water is always standing. This water level or water table may be from 3 to 150 ft. beneath the surface, and may be taken to represent the reserve supply locked up in the soil. It is obvious that all the rain that falls does not reach the water table, because it is waylaid en route and absorbed by the particles of soil. And it must be remembered that although more rain falls in the hilly districts, the soil on the hillsides is not moistened so deeply as that in the lowlands and valleys. In the latter places, apart from the natural annual rainfall, a good deal of extra water is obtained when the rivers and streams overflow their banks at floodtime. When the fields and meadows are flooded to a depth of 1, 2, or 3 ft., many hundreds of tons of water are thus spread over the land, and a very large quantity of it must sink downwards to the water table if the soil is in a porous condition.