along streams, and some streams flow into natural and artificial ponds and lakes
that can impound water for many years.
Precipitation falling as rain, or falling as snow and then melting, infiltratessoil.
The amount involved is widely variable even over small areas because soils are so
heterogeneous. Some of the infiltrated moisture is held against gravity for days
and weeks in the soil pores and so becomes soil moisture. Soil moisture is one
key to life on Earth. Plants rooted in soil usually do not tap precipitation directly
from the sky. They have evolved to use soil moisture, which is a steady supply
as long as precipitation reliably fills the soil pores. Plants bring in the liquid water
through the roots and let it go in vapor form, usually from small openings in the
undersides of leaves; this path back to the atmosphere is known as transpiration.
Of course, some water evaporates from the soil itself. Evapotranspiration is the
combined name given to water vapor from all sources.
Sometimes, infiltrated liquid water passes entirely through the soil profile and
enters the joints, cracks, fissures, and other openings present in “solid rock.” In this
case, it is known asgroundwateras opposed to soil moisture. Given certain rock
types, especially sandstones, large quantities of groundwater can be stored. About
.5 percent of Earth’s water occurs as groundwater. The residence time can be mil-
lions of years and it is difficult for groundwater to find as convenient a path to the
sea. In selected places, groundwater can emerge at the surface in the form of
springs and that water can enter streams. More generally, the rocks containing
groundwater intersect with the levels of stream channels and provide steady base-
flow to streams. It is generally estimated that about half of Earth’s shallow ground-
water returns to streams in this way.
Hydrologic Cycle 183