Groundwater - The portion of the rainfall that sinks into the earth is called groundwater or underground water. It penetrates all rocks to a great depth, passing through the cracks, crevices and the pores of the rock. It moves down by force of gravity, capillarity, and pressure. A part of the groundwater finds its way to the surface again, some of it quickly, some of it after a long period of time and some of it remains below the surface indefinitely.
Depth of groundwater.
The lower limit of the groundwater occurs at depths of five or six miles below the surface, where the pressure from the overlying material is so great that even the finest pores are closed and the rock becomes too dense for the water to find its way through it.
The water tahle.
The successive rainfalls through the ages past have filled the rocks with water from the lowest limit up to a place where there is a balance between the annual supply from the rains and the loss through escape to the surface. This upper limit of the zone of saturated rocks is known as the water table, the level of permanent groundwater qr the permanent water plane. It serves to mark the upper boundary of the water zone or the zone of permanent groundwater, or the zone of saturation.
In some places the water table is at the surface, in some places a few inches or a few feet below the surface, while in others it is several hundred feet below the surface. The depth of the water table depends upon the surface features, the climate and the rocks. It lies nearer the surface in the valleys and plains than it does in the mountains and plateaus.
One reason why the former are more productive than the latter is that in the one in many places the water table is near enough to the surface to furnish moisture to the plant roots in dry seasons, whereas in the other during a prolonged rainless season the water near the surface is evaporated and the water table sinking too deep to be touched by the roots, the vegetation withers and dies.
The level below which the moving water does not sink in dry weather is the permanent water table. In wet seasons this level rises towards, sometimes to the surface, to sink again during the dry season. This fluctuating upper surface of the water zone that varies with the seasons is known as the temporary water table.
Aquifer.
While the rocks below the water table are saturated, some of the layers are so fine grained and dense that the water can move through them only with extreme slowness, while other layers are coarse grained and permit free movement of the water. The latter are called aquifers (aqua, water; fero, to bear) and are very important economically since they are the beds which supply the wells and springs.
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