Residence Time of Groundwater
The length of time water spends in the groundwater portion of the hydrologic cycle may be as little as days, or as much as 10,000 years or more. This is called “residence time.” For example: A raindrop may fall to the earth’s surface. It may seep down through the soil to a saturated zone or aquifer only to pumped back to the surface and sprayed from a garden hose and back down again.
Estimated depth and residence time of the
world’s water supply:
|Water||Equivalent depth (meters)||Residence Time|
|Soil moisture||0.13||~2 weeks-1 year|
|Groundwater||120||~2 weeks-10000 years|
|Ice caps/Glaciers||60||10-1000 years|
|Atmospheric water||0.025||~10 days|
|Biospheric water||0.001||~ 1 week|
Source: Freeze, R.A. and Cherry, J.A., 1979, p.5, Groundwater, Prentice-Hall.
How Much Water Exists?
The Earth is 70% Water. Less than 1% is groundwater. In fact, over 99% of all water is not available for our use. So where is all the water? The Earth’s water is all around you. Water is in streams, lakes, oceans and rivers. This water is called “surface water.” But … there is more to our water supply than surface water. There is plenty of water beneath our feet, in the ground as soil moisture and in aquifers.