PRECIPITATION - Hydrology: An Introduction - page 81

Table 3.1 Annual precipitation recycling ratio for different regions

Square root of
area (km) Region Recycling ratio Reference

Amazon 2300 0.25 Brubaker et al . (1993)

Amazon 2500 0.25-0.35 Eltahir and Bras

(1994)

Mississippi 1800 0.10 Benton et al . (1950)

Mississippi 1400 0.24 Brubaker et al . (1993)

Eurasia 2200 0.11 Budyko (1974)

Eurasia 1300 0.13 Brubaker et al . (1993)

Sahel 1500 0.35 Brubaker et al . (1993)

water vapor originates from evaporation outside the region, while the remainder results
from evaporation within the region in question. The precipitated water produced by the
evaporation inside the region can be referred to as recycled water. Water recycling has been
the subject of intensive investigations (see, for example, Eltahir and Bras, 1996; Gong and
Eltahir, 1996). Table 3.1 shows some estimates for a few regions of the world. Recycling
of precipitation, or lack thereof, resulting from the soil moisture conditions can be a strong
feedback mechanism leading to persistence of weather and climate patterns.

3.1.2 Types of precipitation

Precipitation can reach the ground surface in different forms.

Drizzle is a very light, usually uniform, precipitation consisting of numerous
minute droplets with diameters in excess of 0.1 mm but smaller than
0.5 mm.
Rain is precipitation consisting of water drops larger than 0.5 mm. It can be
classified as light rain when the intensity is smaller than 2.5 mm h − 1 , mod-
erate when it is between 2.5 and 7.5 mm h − 1 , and heavy when it exceeds
7.5 mm h − 1 .
Snow is precipitation in the form mainly of branched hexagonal or star-like ice
crystals, resulting from direct reverse sublimation of the atmospheric water
vapor; snow particles can reach the ground as single crystals, but more often
than not they do so after agglomerating as snowflakes. These flakes tend to
be larger at temperatures close to freezing. The specific gravity of snow can
vary over a wide range (Judson and Doesken, 2000), but as a rule of thumb
for fresh snow it is often taken around 0.1.
Sleet (North American usage) is precipitation consisting of fairly transparent
pellets or grains of ice, formed as a result of the passage of raindrops
through a layer of colder air near the ground. In British usage the word sleet
refers to precipitation consisting of melting snow or a mixture of snow and
rain.
Glaze or freezing rain is ice deposited by drizzle or rain on cold surfaces.