Page 115 - Biosystems Engineering

96 Cha pte r T h ree

see that more than 60 percent of the precipitation falling on the land
evaporates back into the atmosphere. Evaporation from the oceans
provides nearly 90 percent of atmospheric moisture. Details on some
of these processes and components of the hydrologic cycle that are
important to soil and water conservation are provided as follows.

3.3 Precipitation
Precipitation is the main water input to the earth’s lands and oceans.
The global average precipitation rate is about 1132 mm/year. This
includes precipitation falling on oceans. The average global precipita-
tion falling only on land is approximately 801 mm. However, precipi-
tation rates show very high spatial variations on the earth, varying
from less than 1 mm in the Arica Desert in Chile (during the past
59 years of instrumentation there was a period of 14 years that were
totally devoid of precipitation) to about 12 m in Mount Waialeale,
Hawaii. A natural question is what causes all these variations? Before
checking into that let us look at the conditions for precipitation to
occur. There are three general steps in the generation of precipitable
water in the atmosphere.
1. Creation of saturated conditions in the atmosphere. Satura-
tion is typically arrived by cooling of the lifting air. Tempera-
ture drops on average 1°C/100 m under dry conditions and
about 0.5 to 0.7°C/100 m in moist air.
2. Condensation of water vapor into liquid water. As air cools,
its capacity to hold water vapor decreases. When air becomes
fully saturated, water molecules start being attracted to small
particles such as clay and silt particles, smoke, sea salt, and
the like.
3. Growth of small droplets by collision and coalescence until the
droplets become large enough to precipitate due to gravity.

The foregoing three steps are only sufficient in generating precipita-
ble water. To have a sustained precipitation event, a lateral supply of
moist air is needed. One reason for the failure of cloud-seeding exper-
iments to fabricate precipitation events from rainfall clouds is the
lack of a lateral moist air supply.
Why do some locations on the earth receive a lot of precipitation
and some locations get very little to no rain? The average precipita-
tion on the continents of the world depends on many factors. In gen-
eral those factors are

1. Latitude: Precipitation is higher in areas with rising air and is
lower in areas with descending air. In general latitudes from
0 to 60° have rising air, and latitudes from 30 to 90° have
descending air.