Geology Reference
In-Depth Information
The movement of rainwater through the soil layers into sediment and rock layers below
the earth's surface is called infiltration. The amount of water that infiltrates the ground
depends on how much space exists between particles. The upper surface of the earth is
like a sponge in this way; generally, enough space exists between particles to allow wa-
ter to infiltrate. But that's not always the case. For example, layers of tiny clay particles
are often so tightly packed together that there is no space between them for water to fill.
Three factors can increase the infiltration of sediments:
Increasing the space between particles: Creating more space between the sedi-
ment particles allows more water to fill it. For example, the growth of tree roots in-
to the soil creates more space and increases infiltration. Similarly, space can be
created by the activity of animals in the soil.
Decreasing the slope of the ground: A ground surface that is flat encourages the
rainfall to sink in. If the ground surface is steeply sloped, rainfall runs off as sur-
face or streamflow before it has a chance to infiltrate the surface and become
groundwater.
Lowering the intensity of rainfall: Rainfall resupplies groundwater, but there can
be too much of a good thing. Too much rain that falls too quickly may fill only the
upper layers of sediment, leaving additional water to run off in streams. Think
about when you water your lawn or garden: The plant roots deep in the soil get
much more water if you use a gentle sprinkler for many hours than if you turn the
hose on full blast for 10 minutes, creating large surface puddles but little infiltra-
tion.
Measuring porosity and permeability
Two measurements describe how much water a rock or sediment layer will hold:
Porosity: A measure of the amount of space between particles, porosity is ex-
pressed as the percentage of the total volume.
Permeability: Describes how easily water can move through the rock or sediment.
It is determined by both the porosity and how well the spaces are connected (to
allow flow from one space to another).
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