Environmental Engineering Reference
In-Depth Information
The leakage might be very small, though, with some parts of the
country perhaps getting only a few inches a year of recharge.”
Here's an excerpt of the USGS's “brief” (as in 20 pages of expla-
nation) summary of the water cycle:
. . . We begin in the oceans, since that is where most of Earth's
water exists. The sun, which drives the water cycle, heats water in
the oceans. Some of the water evaporates as vapor into the air.
Ice and snow can sublimate (transform) directly into water vapor.
Rising air currents take the vapor up into the atmosphere, along
with water from evapotranspiration (a combination of evapora-
tion and transpiration), which is water transpired from plants and
evaporated from the soil. The vapor rises into the air where cooler
temperatures cause it to condense into clouds. Air currents move
clouds around the globe; cloud particles collide, grow, and fall out
of the sky as precipitation. Some precipitation falls as snow and
can accumulate as ice caps and glaciers, which can store frozen
water for thousands of years.
Snowpacks in warmer climates often thaw and melt when
spring arrives, and the melted water flows overland as snowmelt.
In some cases, especially in the arid West, dry winds blow over
snow and ice and change it directly to the atmosphere as water
vapor. Most precipitation falls back into the oceans or onto land,
where, due to gravity, the precipitation flows over the ground as
surface runoff. A portion of runoff enters rivers in valleys in the
landscape, with stream flow moving water towards the oceans.
Runoff, and groundwater seepage, accumulate and are stored as
freshwater in lakes.
Not all runoff flows into rivers, though. Much of it soaks into
the ground as infiltration. Some water infiltrates deep into the
ground and replenishes aquifers. Some infiltration stays close
to the land surface and can seep back into surface-water bodies
(and the ocean) as groundwater discharge, and some groundwa-
ter finds openings in the land surface and emerges as freshwater
springs. Over time, though, all of this water keeps moving, some
to re-enter the ocean, where the water cycle continues all over
again.
23
