Geoscience Reference
In-Depth Information
z
z
TOP
TOP
dW
d
#
#
v
f
p
=
ρ
qdz
= −
d
$
(
ρ
qvdzEP
)
+ −
.
(9.9)
dt
dt
0
0
For the climatology, it is reasonable to assume that water vapor does not ac-
cumulate in the atmospheric column (
dW
/
dt
) 0 and Eq. 9.9 becomes
z
TOP
#
v
PE
−=−
d
$
(
ρ
qv dz
)
.
(9.10)
h
0
Review the precipitation and surface evaporation climatologies presented in
chapter 2
(
Figs. 2.25
-2.29
), and note again that their geographic distributions
are quite different so that, locally,
P
!
This dissimilarity means that the redis-
tribution of moisture in the atmosphere by the circulation plays an important
role in determining the precipitation distribution.
.
9.2 LAND SURFACE WATER BALANCE
An understanding of the processes that control the availability of water on the
land surface is important for agricultural applications, assessments of water
resources, and prediction of floods and droughts. The rate at which water ac-
cumulates in a volume of the land surface must account for input from precipi-
tation, and loss from evapotranspiration and runoff. Mathematically,
=−−−
(9.11)
SPERR
s
,
u
where
S
rate of storage of water (change in water content with time)
P
precipitation rate
E
evapotranspiration rate
R
s
surface runoff
R
u
underground runoff.
In Eq. 9.11,
P
includes water delivered to the surface in the form of rain, snow,
and ice. Evapotranspiration includes evaporation from a water surface or bare
land,
transpiration
, defined as the release of water vapor from plants, and
sub-
limation
, defined as the release of water vapor from ice and snow.
Flooding may occur when
S
is positive, that is, when the amount of wa-
ter on the surface increases with time, and droughts are related to persistent
negative values of
S
. Intense rainfall events that deliver high precipitation
rates that cannot be balanced by evaporation and runoff instigate most flood-
ing events.
For the climatology (denoted by overbars), and averaging over relatively
large regions (denoted by square brackets), the rates of water storage and
underground runoff are small and the dominant terms in the surface water
balance are
[] [] [ .
EPR
s
=−
(9.12)
