The ’s Dynamic and the Earth System Global Hydrologic Cycle

By Thomas G. Huntington1 and Richard S. Williams, Jr.2

Of all the cycles and processes that that it is most dense (liquid) at 4°C and fast-flowing and ; if such operate on Earth, the most important is less dense as a solid (). This is critical ice adheres to the bottom, it forms anchor the hydrologic cycle (fig. 1), the complex to the survival of the aquatic organisms ice that can grow (from the bottom up) to process through which , in its three (in the biosphere) in marine and terrestrial many centimeters in thickness. Anchor ice phases—solid, liquid and vapor—moves environments of the when can affect the accuracy of measurements through all components and subcompo- floating ice forms. In , water freezes of a ’s winter discharge by altering nents of the geosphere and the biosphere. from the surface down; deeper lakes the cross section of the river. In temperate The physical and chemical properties and the therefore retain liquid and high-latitude regions, a convective of the water molecule make water water having below that circulation (“turnover”) occurs in lakes unique as a substance. Physically, it has of the ice cover, which is at or higher twice each year ( and fall) as the a high specific heat, meaning that much than 0°C. Under certain circumstances, water warms (or cools) to 4° C, thereby more energy is required to increase the however, frazil ice (special forms of ice oxygenating (adding oxygen to) the entire of water (4.186 joule/gram crystals) can form in the water column in water column. degrees Celsius (°C) than that of most common substances. Chemically, it is a universal solvent that varies with respect The Hydrologic Cycle to its pH value (negative logarithm of the concentration of the ion: Water storage Water storage (as vapor) less than 7, acidic; 7, neutral; more (as ice) in in the than 7, basic). Acid , for example, and Sublimation dissolves the surface of limestone blocks in buildings and of marble in sculptures, and it can lower the pH of lakes to render them devoid of aquatic organisms. The and snow meltwater runoff movement of water through the Earth to streams System also transports and releases energy. As water changes its phase from solid to Evaporation liquid, from liquid to gas, and from solid Spring G r Freshwater to gas (sublimates), thermal energy is o u storage n d wa absorbed (endothermic change); changes te Water storage (as liquid) r d is ch in oceans in phase in the opposite direction release arg e energy (exothermic change). Another unusual physical property of water is storage

Figure 1. The global hydrological cycle. Water evaporates from oceans and lakes 1 U.S. Geological Survey, Maine Water Science and precipitates on land as rain or snow. The cycle transports runoff from rivers and Center, Augusta, ME 04330 U.S.A. streams, by subsurface movement through , through animals, , and other 2 U.S. Geological Survey, Woods Hole Coastal and organisms, through the soil, and stores water in oceans, lakes, and glaciers. Modified Marine Science Center, Woods Hole, MA 02543- from U.S. Geological Survey, 2009, Water science for schools: The water cycle: accessed 1598 U.S.A. 23 February 2010, at http://ga.water.usgs.gov/edu/watercycle.html

U.S. Department of the Interior Professional Paper 1386—A Supplemental Cryosphere Note 2 U.S. Geological Survey December 2012 As water evaporates from oceans At present, eustatic level is rising and lakes and precipitates on land as rain at the rate of about 0.4 m per century or snow, the hydrologic cycle transports (about 3 to 4 millimeters per year) from liquid water by runoff of a combination of glacial meltwater and (rivers and streams); by subsurface an increase in volume (a steric rise) movement of groundwater (aquifers); from warming of the oceans. Under through animals (by respiration), plants conditions of global warming, the melting (by evapotranspiration), and other of glaciers is expected to accelerate. If this organisms; through the soil; and stores expectation is borne out, global water in oceans, lakes, and glaciers will rise rapidly. (cryosphere). The oceans (hydrosphere) A warmer will likely contain 97 percent of Earth’s water, and result in increased evaporation and glaciers store 2 percent. Therefore, less evapotranspiration that will intensify than 1 percent of Earth’s water remains or accelerate the hydrologic cycle. The as surface water, groundwater, and soil rate of exchange of water among its moisture to sustain most life forms in various phases is expected to increase terrestrial ecosystems. as it circulates through the atmosphere, The long-term exchange between cryosphere, hydrosphere, and through the glacier ice (cryosphere) and the oceans terrestrial biosphere. Climate projections (hydrosphere) determines global based on global circulation models (eustatic) sea level (fig. 2). In response (GCMs) indicate that wet regions are Figure 2. Ground photograph of glacial to variations in the volume of glacier likely to become wetter and dry regions meltwater plunging over the 60-m-high ice on the continents, sea level has may become drier. Although increases in Skógafoss waterfall, southern Iceland. repeatedly fallen and risen between precipitation are typically expressed as The glacial meltwater originates from the glacials and interglacials. Approximately increases in rainfall intensity (amount per Eyjafjallajökull and flows south 20,000 years ago, for example, sea level unit time), periods between precipitation into the North Atlantic . Photo- was about 125 meters (m) lower than events are likely to lengthen, even in graph by Richard S. Williams, Jr., U.S. at present (2009). If all of the present areas that receive more total precipitation. Geological Survey. glacier ice on land were to melt, sea is therefore likely to become level would rise an additional 75 m. more frequent.

URL Addresses: Percent by volume of the earth’s http://ga.water.usgs.gov/edu/watercycle.html [Modified from table in U.S. Geological Survey in U.S. Geological Survey Circular http://www.iwr.msu.edu/edmodule/water/cycle.htm 536 (http://pubs.usgs.gov/circ/1967/0536/report.pdf)] http://earthobservatory.nasa.gov Reservoir Percentage by Volume http://nasascience.nasa.gov/earth-science/oceanography/ Oceans 97.200 ocean-earth-system/ocean-water-cycle Freshwater: Glacier ice 2.150 Contact information Subsurface water 0.625

Surface water 0.017 Thomas G. Huntington, Atmosphere 0.001 e-mail: [email protected] Subtotal (fresh water) 2.793 Richard S. Williams, Jr., Total ~100.00 e-mail: [email protected]