Most of the water under the ground does not just sit there—it is constantly in motion, although rates are typically only an inch or two (2-5 cm) per day. The rates of movement are controlled by the amount of open space in the bedrock or regolith, and how the spaces are connected. The groundwater system also includes water beneath the ground that is immobile, such as water locked in soil moisture, permafrost, plus geothermal and oil-formation water.
Porosity is the percentage of total volume of a body that consists of open spaces. Sand and gravel typically have about 20 percent open spaces, while clay has about 50 percent. The sizes and shapes of grains determine porosity, which is also influenced by how much they are compacted, cemented together, or deformed.
In contrast, permeability is a body's capacity to transmit fluids or to allow the fluids to move through its open pore spaces. Permeability is not directly related to porosity. For instance, all the pore spaces in a body could be isolated from each other (high porosity), and thus the water may be trapped and unable to move through the body (low permeability). Molecular attraction, the force that makes thin films of water stick to objects instead of being forced to the ground by gravity, also affects permeability. If the pore spaces in a material are small, as in a clay layer, then the force of molecular attraction is strong enough to stop the water from flowing through the body. When the pores are large, the water in the center of the pores is free to move.
After a rainfall much of the water stays near the surface, because clay in the near-surface horizons of the soil retains much water due to molecular attraction. This forms a layer of soil moisture in many regions able to sustain seasonal plant growth.
Some of this near-surface water evaporates and is used by plants. Other water runs directly off into streams. The remaining water seeps into the saturated zone, or into the water table. Once in the saturated zone it moves slowly by percolation, from high areas to low areas, under the influence of gravity. These lowest areas are usually lakes or streams. Many streams form where the water table intersects the surface of the land.
once in the water table the paths that individual particles follow vary. The transit time from surface to stream may vary from days to thousands of years along a single hillside. Water can flow upward because of high pressure at depth and low pressure in streams.
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