Environmental Engineering Reference
In-Depth Information
cooled to dew-point temperature on contact with cold surfaces (dew, hoar frost and rime
ice). This also identifies precipitation by
type
as a function of temperature and creates
patterns determined by climatic zone, altitude, synoptic conditions and exposure. All
precipitation is measured in water-equivalent terms. This accommodates substantial
density differences between water (1 gm cc
−3
), fresh snow (0·01-0·06 gm cc
−3
) and the
subsequent evolution of snowpack by densification to
firn
(0·4-0·5 gm cc
−3
) and glacier
ice (0·85-0·9 gm cc
−3
) (Plate 14.1). The proportion of precipitation in liquid and solid
forms strongly influences the timing and amount of subsequent water transfers and
dependent catchment processes.
Table 14.1 Continental water balances (km
3
yr
-1
)
Stream flow
Total
Q
Surface
(flood)
Stable
(base
flow)
% land
area
%
global
Q
Continent
P
E
Q
E
Q
F
Europe
7·17
4·06
3·11
2·05
1·06
7·7
6·5
43·2
65·8
Asia
32·69
19·50
13·19
9·78
3·41
32·0
30·0
40·3
74·1
N. America
13·91
7·95
5·96
4·22
1·74
18·6
12·6
42·8
70·1
S. America
29·36
18·98
10·38
6·64
3·74
12·9
26·5
35·3
63·9
Africa
20·78
16·56
4·22
2·76
1·46
22·3
18·7
20·4
65·2
Australasia
6·41
4·44
1·97
1·50
0·47
6·5
5·7
30·7
76·1
Total
110·32
71·49
38·83
26·95
11·88
100·0
100·0
35·5
69·4
Source
: In part after L'vovich (1979).
Notes
:
P
, precipitation;
E
, evaporation;
Q
, stream flow;
Q
E
, percentage of precipitation discharged
as stream flow
Q
F
, percentage stream flow discharged as surface flood or quickflow. Total values
of
Q
E
and
Q
F
are global mean values.
Broad distinctions exist between humid-temperate catchments with transient snow cover
and cool-temperate/cold or alpine catchments with enduring seasonal snowpack and rapid
spring melt. Fluvial processes are severely disrupted in glacial catchments (Figure 14.4).
Temporal aspects of precipitation relate its duration, intensity and frequency to an
annual regime. This is crucial in determining the balance between storage and onward
transfer, as set out below. Intensity is inversely proportional to duration and frequency
and determines catchment water input within single events or annual budget.
Evapotranspiration reduces the amount of water available for catchment processes by its
return directly to the atmosphere. It is two processes, not one, subject to different controls
beyond the principal requirement of free energy capable of liberating water molecules
from a surface (see Chapter 5).
Evaporation
occurs from intercepted water
films
(coating
plants, soil particles, impervious rock and artificial surfaces) and standing water
bodies
(rivers, lakes). It may also tap subsurface water where
vapour pressure deficit
and
capillary action overcome gravity and surface tension.
Transpiration
from plants results
from their maintenance of internal nutrient flow and cell turgidity. It has to overcome
