Geoscience Reference
In-Depth Information
3
Precipitation:
rain falls on land
3
Precipitation:
Snow falls on land
2
Clouds carry
water over land
4
Ice accumulates
in glaciers
1
Evaporation:
from plants by
transpiration
1
Evaporation:
from surface
water
5
Ice melts and
forms streams
1
Evaporation:
from oceans
3
Precipitation:
rain/snow over seas
7
Runoff:
water returns to sea
from rivers and streams
6
Infiltration:
Water seeps into
ground, flows to sea
8
Water stored
in oceans
Total land precipitation:
Total land evaporation:
Runoff to sea:
107
-71
36
7
Water flows downhill
from rivers and streams
to the sea
Total sea evaporation:
Total sea precipitation:
Excess in air:
434
-
398
36
Figure 7.4 The global water balance.
The global water balance refers to the way that water moves within the hydrologic cycle
and the relationship among evaporation, precipitation, and runoff. Oceans are a net source of evaporation, whereas precipitation
processes dominate on landmasses. The system stays balanced because water runs off the land and returns to the ocean. Values
are in cubic kilometers (km
3
) and indicate average annual water flows to and from the Earth's land and ocean areas.
Humidity
5.
The hydrologic cycle refers to the movement of water from
one reservoir to another by various processes such as
precipitation, evaporation, and overland flow in streams.
Let's now look more closely at how water actually changes
physical state from vapor to liquid and the factors that govern
this process. In the atmosphere, this change of state ultimately
results in precipitation.
Maximum, Specific, and
Relative Humidity
This discussion begins with the concept of
humidity
, which is the
concentration of water vapor within the air. Vapor concentration
is expressed in three ways:
maximum humidity, specific humidity
,
and
relative humidity
. Although at first it may seem confusing
that three different humidity types are recognized, their defini-
tions and interrelationships are really quite easy to follow. Here is
a simple analogy to keep in mind as you work your way through
this section. Imagine that you are about to fill a drinking glass to
some level with water. Given that the glass can hold only so much
water, you probably fill it to some level below the maximum
amount of water that the glass can hold. If you try to put more
KEY CONCEPTS TO REMEMBER ABOUT
THE PHYSICAL COMPOSITION OF
WATER AND THE HYDROLOGIC CYCLE
1.
Water molecules in the liquid or solid state are attracted
to one another through hydrogen bonding.
2.
When water changes its physical state, latent heat
energy is either lost or gained.
3.
The vast majority of water on Earth (the hydrosphere) is
stored in the oceans.
4.
Humans collectively depend on less than 1% of all
water on Earth.
Humidity
A measure of how much water vapor is in the air. The
ability of air to hold water vapor is dependent on temperature.
