Environmental Engineering Reference
In-Depth Information
Chapter
5
Precipitation and evapotranspiration
PRECIPITATION
For those who live in the humid regions of the world, precipitation is normally so
frequent that it is taken for granted. During times of drought the importance of
precipitation and its role in feeding the hydrological system and providing water for
human use and plant life become all too apparent. In recent years the problems raised by
drought in areas such as Afghanistan, Somalia, Australia and even parts of north-west
Europe have become publicized in the media as demand for water has increased at a time
when some areas have experienced below average precipitation.
Precipitation represents the vital input of water to the surface hydrological system. It is
the nature of this input - the character and distribution of precipitation - which we will
consider in the first part of this chapter.
TYPES OF PRECIPITATION
To most people three types of precipitation come immediately to mind: rain, snow and
hail. If we looked at these more closely, the distinction between them is not always clear,
and the terms mean different things in different areas. Moreover, they are not the only
forms in which moisture is input to the surface. Dew, fog-drip and rime all transfer water
from the atmosphere to the ground (Table 5.1). Their contribution, however, is usually
small.
RAIN
We have already discussed the main processes of rainfall generation in Chapter 4. Here
we are concerned with the nature of the rainfall after it has fallen from the cloud.
Typically rainfall consists of water droplets that vary in size. Where the rain is
produced by thin, low-level stratiform clouds, droplets tend to be small, with a majority
in the range from 0·2 mm to 0·5 mm in diameter. Where the clouds are thicker, strong
updraughts hold the droplets in the atmosphere longer, so the number of collisions
increases and the rain is composed of larger droplets, often several millimetres in
diameter. The diameter of a droplet is also affected by events during its fall through the
atmosphere. In general, the droplets reaching the ground show a logarithmic size
distribution, with a large number of small droplets and a much smaller number of large
drops.
The size of the droplets has considerable significance, for, together with the strength
of updraughts in the air, it controls the fall velocity of the rain. In still air the fall speed of
a droplet 0·2 mm in diameter is about 70 cm s
−1
; for a drop of 2 mm diameter, it is about
