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.
2
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.
3
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