Water-Resources Primer Chapter 2: The Global Water Cycle The following pages represent chapter 2, The Global Water Cycle, of a water-resources primer prepared by the Education and Outreach Committee of the Verde River Basin Partnership. The purpose of the primer is to provide well-explained, documented source material for interested citizens, public decision makers, teachers, and presenters of water-resource information. The primer is intended as a source of information from which targeted and sometimes simplified presentations can be generated. Chapter 2 reviews the global water cycle, in which the sum of Earth’s water—as liquid, vapor, or ice—has been constantly recycled for billions of years through Earth’s atmosphere, oceans, streams, glaciers, groundwater, and even the Earth’s interior. Our supply of readily usable fresh water occurs as rain catchment and in groundwater, rivers, streams, and lakes. Groundwater represents approximately .626 percent—about 2 million cubic miles—and streams, rivers, and lakes approximately .0091 percent—about 31,000 cubic miles—of the Earth’s water. This fresh-water supply, while less than one percent of Earth’s total water supply, is vital to the sustenance of virtually all land-based life on Earth.
Chapter 2: The Global Water Cycle Earth’s water is always in motion, and it is always changing states—from liquid to vapor to ice and back again. The water cycle has been working for billions of years and the lives of countless living things depend upon it.
Figure 2.1. The Global Water Cycle www.ga.water.usgs.gov/edu/watercycle.html
Motion of the Earth’s water is a constant process—through the atmosphere, the oceans, streams, lakes, glaciers, and in the groundwater. The motion is driven by the heat of the sun and gravity. The Earth’s water supply is essentially a closed system; little water is either added to the system or removed from it. Thus, the same water has cycled for billions of years around the globe, being used over and over again to support life. A component of the Earth’s water is even recycled from the oceans into the Earth’s mantle miles below the Earth’s surface and redelivered to the surface by volcanic eruptions. Evaporation, which is driven by the heat of the sun, converts water at the surface of the oceans, streams, and lakes to the invisible gas that we call water vapor. Ice and snow can be converted directly to water vapor by the process of sublimation. Evapotranspiration, commonly denoted as ET, refers to the combination of water evaporated from the soil plus water transpired by plants.
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Sublimation: A change directly from the solid to the gaseous state without becoming liquid; for example, a change directly from ice to water vapor without production of liquid water.
Evapotranspiration (ET): Evaporation of water from the soil plus transpiration—the release of water vapor from plants to the atmosphere.
Much of the precipitation, of course, falls directly back into the oceans. In some areas—though not in the Verde River watershed—precipitation falling as snow accumulates as ice caps and glaciers, which can store water for thousands of years. In Arizona, snow accumulates at higher elevations during the winter. Some of it sublimates, sending water vapor directly to the atmosphere, some infiltrates into the ground during temporary winter thaw periods, and the rest thaws in the spring. The snowmelt flows overland, downhill under the influence of gravity, as part of the annual runoff. Runoff: Water from rain or snowmelt that flows over the land surface and is neither evaporated nor absorbed into the ground, instead flowing into streams, lakes, or the ocean.
During warmer weather in Arizona—largely during the summer monsoon months—the precipitation occurs mostly as rain. Like snowmelt, the rain-produced runoff also flows overland. In Arizona’s dry, warm climate, part of the runoff evaporates, forming water vapor, which rises directly into the atmosphere. Runoff, whether from snowmelt or rain finds its way into gullies, streams, and rivers. In the Verde watershed, the runoff may get trapped in reservoirs such as Watson and Willow Lakes in Prescott or Horseshoe and Bartlett Reservoirs on the lower Verde River north of Phoenix. During wet years, runoff that reaches the Verde River may exceed the capacity of the reservoirs and be permitted to flow downstream to the Salt River to then join the Gila River. The Gila River joins the Colorado River near Yuma, and, in exceptionally wet years, runoff may exceed the capacity of reservoirs on these rivers and flow all the way to the ocean entering the Gulf of California. Some of the runoff soaks into the ground as infiltration. Gravity carries it downward, and whatever part of the infiltrating water gets past the root zone of plants moves on downward, in response to gravity, to become part of the groundwater. Large amounts of groundwater may have infiltrated during much wetter climatic regimes. Infiltration: The downward movement of water from the land surface into soil or porous rock.
Groundwater: Water below the ground surface that completely fills (saturates) all of the void spaces in porous sedimentary deposits or rock, and is capable of supplying springs and wells.
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Groundwater is stored in voids between grains of gravel, sand, silt, and clay in unconsolidated sedimentary deposits and in pore spaces or fractures in consolidated rock. Groundwater flows continuously, although usually slowly, under the influence of gravity from the areas of infiltration to the areas where the groundwater exits to wetlands, springs, streams, lakes, or the ocean. Thus, the groundwater is one component of the Earth’s never-ending water cycle.
Global Distribution of Water Fresh Water 0.009% Groundwater 0.626% Ice Caps & Glaciers 2.15%
Oceans Fresh Water Groundwater Ice Caps & Glaciers
Oceans 97.2%
Figure 2.2 Distribution of the Earth’s Water (Arizona Watershed Stewardship Guide, University of Arizona and Arizona Cooperative Extension, revised 2005; original can be found here: www.cals.arizona.edu/watershed/resources/docs/guide/(8)hydrology.pdf for purposes of readability, this chart was modified by the Verde River Basin Partnership.
Various estimates of the distribution of the Earth’s water differ only slightly. The estimate illustrated in the pie chart above shows the oceans containing about 97.2 percent of the Earth’s water—about 329 million cubic miles of water—and covering about 70 percent of the Earth’s surface. About 2.15 percent—or approximately 7 million cubic miles—of the Earth’s water is currently stored in ice caps or glaciers. Water in the atmosphere, wetlands, soil moisture, and in living organisms (not shown in graphic above) comprises about .015 percent or about 50,000 cubic miles of the Earth’s water. Our supply of fresh water occurs as rain catchment and in groundwater, rivers, streams, and lakes. Groundwater represents approximately .626 percent—about 2 million cubic miles—and streams, rivers, and lakes approximately .0091 percent—about 31,000 cubic miles—of the Earth’s water. This fresh-water supply, while less than one percent of Earth’s total water supply, is vital to the sustenance of virtually all land-based life on Earth. For more information about the Verde River Basin Partnership and water resources please visit www.vrbp.org. 4