Environmental Engineering Reference
In-Depth Information
however, the air temperature in the layers below the cloud is also below the freez-
ing point, solid precipitation can occur. Types of such solid precipitation are,
among others, snow, snow grass, frazil ice, soft hail, and hail. Snow can either
occur as "ordinary snowfall" or as snow showers, and consists of snow stars or
other types of ice crystals which fall down either separately or merged together
into flakes /2-24/.
Dew or rime, both forms of precipitation, are caused by condensation or depo-
sition of gaseous water, if the temperature of a surface very close to the ground is
below the dew or rime point. Additionally, fog or clouds can form water deposits
or, at temperatures below the freezing point, form various types of frost deposits
/2-24/.
From precipitation to flow.
The water precipitating on an area of land within a
defined period of time is stored in the ground, evaporates again or flows off in
streams and rivers. With the water resources equation (Equation (2.21)), the total
water available in the course of a particular period and for a particular area can be
described (Fig. 2.37).
P
S
is the precipitation on the surface
S
of a particular area,
F
a,S
is the flow above ground and
F
b,S
the flow below ground from this area during
the investigated period of time
∆t
.
E
S
is the evaporation from the surface of this
area and
R
S
the retention (i.e. the proportion of the precipitation stored in the
ground as groundwater) during the same period of time.
P
=
(
F
+
F
)
+
E
±
R
(2.21)
S
a
,
S
b
,
S
S
S
Above ground, precipitation could be snow, ice and/or water (i.e. rain), how-
ever below ground exclusively water occurs. If the time span
∆t
is large enough,
retention can be neglected as it is sufficiently balanced.
P
P
S
Ocean
E
S
F
a,S
E
Ocean
F
b,S
Ocean
S
Fig. 2.37
Natural water cycle with precipitation
P
, evaporation
E
and flow
F
below (
b
) or
above (
a
) ground in an area with the surface
S
(see /2-23/)
Search WWH ::
Custom Search