Go out after a drenching rainstorm and look for the answer to this question. The gullies along the street are full of muddy, running water. There are pools of standing water on level places, but on every slope the water is hurrying away. The ground is so sticky that wagons on country roads may mire to the hubs in the pasty earth. There is no use in trying to work in the garden or to mow the lawn. The sod is soft as a cushion, and the garden soil is water-soaked below the depth of a spading-fork.
The sun comes out, warm and bright, and the flagstones of the sidewalk soon begin to steam like the wooden planks of the board walk. The sun is changing the surface water into steam which rises into the sky to form a part of another bank of clouds. The earth has soaked up quantities of the water that fell. If we followed the racing currents in the gullies we should find them pouring into sewer mains at various points, and from these underground pipes the water is conducted to some outlet like a river. All of the streams are swollen by the hundreds of brooks and rivulets that are carrying the surface water to the lowest level.
Rain and wind are the sculptors that have carved these strange castles out of a rocky table
All the water in the seas, lakes, rivers, and springs came out of the clouds
So we can see some of the rainfall going back to the sky, some running off through rivulets to the sea, and some soaking into the ground. It will be interesting to follow this last portion as it gradually settles into the earth. The soil will hold a certain quantity, for it is made up of fine particles, all separated by air spaces, and it acts like a sponge. In seasons of drought and great heat the sun will draw this soil water back to the surface, by forming cracks in the earth, and fine, hair-like tubes, through which the vapour may easily rise. The gardener has to rake the surface of the beds frequently to stop up these channels by which the sun is stealing the precious moisture.
The water that the surface soil cannot absorb sinks lower and lower into the ground. It finds no trouble to settle through layers of sand, for the particles do not fit closely together. It may come to a bed of clay which is far closer. Here progress is retarded. The water may accumulate, but finally it will get through, if the clay is not too closely packed. Again it may sink rapidly through thick beds of gravel or sand. Reaching another bed of clay which is stiffer by reason of the weight of the earth above it, the water may find that it cannot soak through. The only way to pass this clay barrier is to fill the basin, and to trickle over the edge, unless a place is found in the bottom where some looser substance offers a passage. Let us suppose that a concave clay basin of considerable depth is filled with water-soaked sand. At the very lowest point on the edge of this basin a stream will slowly trickle out, and will continue to flow, as long as water from above keeps the bowl full.
It is not uncommon to find on hillsides, in many regions, little brooks whose beginnings are traceable to springs that gush out of the ground. The spring fills a little basin, the overflow of which is the brook. If the source of this spring could be traced underground, we might easily follow it along some loose rock formation until we come to a clay basin like the one described above. We might have to go down quite a distance and then up again to reach the level of this supply, but the level of the water at the mouth of the spring can never be higher than the level of the water in the underground supply basin.