Groundwater, as the name implies, is water found below the surface of the Earth. Water from rains and snowfall infiltrates soil and other permeable surfaces.
Groundwater can be divide into two major zones: unsaturated and saturated.
The unsaturated zone is the section between the surface of the Earth and the top of the water table. This zone is also known as the vadose zone after the Latin word for ‘shallow’.
The capillary fringe is a section of the unsaturated zone where water from the saturated zone seeps up.
The saturated zone is the section below the surface of the Earth that is completely saturated with water. This means that all of the pores, cracks, and spaces between rock particles are completely filled with water.
The saturated zone is also known as the phreatic zone after the Greek word for “well” or “spring”.
The top of the saturated zone is known as the water table.
Aquifers are underground areas that store water.
How Much of the Earth’s Water is Groundwater?
Despite the fact that groundwater constitutes a small percentage of total water on Earth, it accounts for a significant portion of total freshwater.
Groundwater makes up roughly 1.7 percent of all water on the planet, and it also makes up around 30.1 percent of all freshwater.
On Earth, there are approximately 5,614,000 cubic miles of groundwater (23,400,000 cubic kilometers). 54% of groundwater is saline and exists mostly below oceans. The other 46% of groundwater is freshwater that exists below land surfaces.
Overwash Can Infiltrate Freshwater Groundwater
One concern with sea level rise is the infiltration of saline water into freshwater groundwater. Overwash, the flow of ocean water onto atolls, increases with sea level rise. The saline water from the over wash seeps into underground water storage.
Using Satellites to Measure Groundwater
Researchers can remotely sense groundwater levels by measuring small changes in Earth’s mass and its gravity field.
This map models the relative amount of water stored in underground aquifers in the contiguous United States using data from the Gravity Recovery and Climate Experiment (GRACE) satellites as well as other satellite and ground-based studies.
The wetness, or water content, shows how much groundwater was present in the United States on July 7, 2014, relative to the average from 1948 to 2009. Blue areas are water than the longterm average and red areas are drier.
As seen on the map, California, Nevada, New Mexico, Colorado, Texas, Arizona, Oklahoma, and Nebraska were in drought conditions and had declining groundwater levels.
Read next: Water on Earth
Carlowicz., M. (2014, July 24). Groundwater deficit out west. NASA Earth Observatory. https://earthobservatory.nasa.gov/images/84065/groundwater-deficit-out-west
Gleick, P. H. (1996) Water resources. In Encyclopedia of Climate and Weather, ed. by S. H. Schneider, Oxford University Press, New York, vol. 2, pp.817-823.
Storlazzi, C. D., Gingerich, S. B., Van Dongeren, A. P., Cheriton, O. M., Swarzenski, P. W., Quataert, E., … & McCall, R. (2018). Most atolls will be uninhabitable by the mid-21st century because of sea-level rise exacerbating wave-driven flooding. Science Advances, 4(4), eaap9741. DOI: 10.1126/sciadv.aap9741
Water Science School. (2018, June 18). Groundwater storage and the water cycle. USGS.gov. https://www.usgs.gov/special-topics/water-science-school/science/groundwater-storage-and-water-cycle