Surface permeability and water on Earth
Sightly more than 70% of the earth is covered by water. Almost all of that is contained in oceans. Water also exists in lakes and rivers, in vapor in the atmosphere and in solid form in glaciers and ice in polar regions. On land water lies below the surface in ground moisture and aquifers. Water is also contained, although in a very miniscule proportion of the total, in all living things (from bacteria and archaea, which total about half the earth’s biomass to the most advanced species on the planet, the ant, which accounts for about 27 times the total biomass of humans). If all this water was added up, the amount of water on Earth would total about 1,386 million cubic kilometers (332.5 million cubic miles), according to the US Geological Survey (USGS). Since a cubic meter equals 7396.817 gallons, the water on Earth amounts to 1.0251 x 1022 or
The USGS graphically shows the total amount of water in the illustration at the right. The ball of water has a diameter of 860 miles (1,385 kilometers). If this water covered only the United States, it would reach a height (depth?) of 90 miles (145 kilometers).
Only a small proportion of this water is fresh water. Fresh water lakes and rivers represent less than 1% of the Earth’s total water budget. About 12,900 cubic kilometers (3,100 cubic miles) of water is in the atmosphere at any one time. Most of the fresh water on Earth is currently (pending results of warming) locked in glaciers and ice caps. This frozen water amounts to 29,200,000 cubic kilometers (7 million cubic miles). In addition, there is water locked in permafrost (although that amount is also dwindling owing to global warming). Of liquid fresh water, by far the largest proportion is underground. In fact, underground fresh water represents more than 30% of all fresh water in whatever form. The chart below shows how the Earth’s supply of water is proportioned.
|Water source||Water volume, in cubic miles||Water volume, in cubic kilometers||Percent of
|Oceans, Seas, & Bays||321,000,000||1,338,000,000||—||96.54|
|Ice caps, Glaciers, & Permanent Snow||5,773,000||24,064,000||68.6||1.74|
|Ground Ice & Permafrost||71,970||300,000||0.86||0.022|
|Source: Igor Shiklomanov’s chapter “World fresh water resources” in Peter H. Gleick (ed.), Water in Crisis: A Guide to the World’s Fresh Water Resources (NY: Oxford University Press: 1993.|
All of this water is of course interconnected in the global water cycle. The USGS illustrates the cycle in the diagram below and provides a helpful outline with links explaining each of the 16 components it divides the cycle into.
The step by which water goes from rain (and melting snow and ice) to underground storage is called infiltration. A number of factors influence infiltration, the greatest of which of course is the amount, intensity and duration of precipitation. Soil characteristics determine whether the water can penetrate or will run off. The three soil types, clay, silt and sand, are arranged in that order from slower to faster absorption of water. Soils with slower absorption rates see more water runoff to streams and rivers. Other factors include soil saturation, land cover, biological activity (root and worm holes) and land slope.
The soil level of the earth surface can be divided into unsaturated and saturated zones. In the upper, unsaturated zones the pore spaces (i.e., the areas not occupied by minerals or organic matter) are filled with both air and water. (Soils with more pore spaces are said to have higher porosity, which is measured as a fraction between 0 and 1, typically ranging from less than 0.01 for solid granite to more than 0.5 for peat and clay.) Water can escape this area through plant activity (roots absorb water, which is eventually transpired into the atmosphere) and directly by evaporation. In the lower saturated area, all voids are filled by water, as in the USGA illustration below.
Permeability in general is the measure of a porous material’s ability to transmit fluids. In this case the question is the ability of the soil and rock layers of the surface to transmit water.
In Tom Gleeson, Leslie Smith, Nils Jansen, Jens Hartmann, Hans H. Dürr, Andrew H. Manning, Ludovicus P. H. van Beck & A.M. Jellinek, “Mapping permeability over the surface of the earth,” 38 Geophysical Research Letters L02401 (January 2011) (abstract; paper behind pay wall), Tom Gleeson, a postdoctoral fellow in the Department of Earth and Ocean Sciences, University of British Columbia and colleagues analyzed world-wide rock data from researchers at the University of Hamburg and Utrecht University to depth up to 100 meters. This is vastly deeper than heretofore permeability was mapped and amounts to the first global map of permeability. (The map is at the top of this post.) According to the press release by the University of British Columbia Gleeson hopes that the map can help “evaluate sustainable groundwater resources as well as the impact of groundwater on past, current and future climate at the global scale.” The study’s maps include a global map at a resolution of 13,000 kilometres squared and a much more detailed North American map at a resolution of 75 kilometres squared. The North American map is below: