Geoscience Reference
In-Depth Information
with severe drought, limiting water availability for
domestic and agricultural users. In a situation like
this the way that water is allocated (see Chapter 8)
literally becomes a matter of life and death, and
many economic livelihoods depend on equitable
allocation of a scarce water resource.
To try and overcome some of the difficulties in
interpreting the data in Figure 1.6 and Table 1.2
hydrologists often work at a scale of more relevance
to the physical processes occurring. This is frequently
the water basin or catchment scale (Figures 1.4 and
1.5).
and runoff), but it is possible to subdivide each into
different sub-processes. Evaporation is a mixture of
open water evaporation (i.e. from rivers and lakes);
evaporation from the soil; evaporation from plant
surfaces;
interception
; and
transpiration
from
plants. Precipitation can be in the form of
snowfall
,
hail, rainfall or some mixture of the three (sleet).
Interception of precipitation by plants makes the
water available for evaporation again before it even
reaches the soil surface. The broad term 'runoff'
incorporates the movement of liquid water above
and below the surface of the earth. The movement
of water below the surface necessitates an under-
standing of infiltration into the soil and how the
water moves in the unsaturated zone (
throughflow
)
and in the saturated zone (
groundwater flow
). All
of these processes and sub-processes are dealt with
in detail in later chapters; what is important to
realise at this stage is that it is part of one con-
tinuous cycle that moves water around the globe and
that they may all be operating at different times
within a river basin.
The catchment hydrological cycle
At a smaller scale it is possible to view the catchment
hydrological cycle as a more in-depth conceptual
model of the hydrological processes operating.
Figure 1.9 shows an adaptation of the global hydro-
logical cycle to show the processes operating within
a catchment. In Figure 1.9 there are still essentially
three processes operating (evaporation, precipitation
P
P
E
I
E
E
Q
Q
TF
Q
TTF
Ocean
Q
G
Figure 1.9
Processes in the hydrological cycle operating at the basin or catchment scale.
Q
= runoff; the subscript
G
stands for groundwater flow;
TF
for throughflow;
I
= interception;
E
= evaporation;
P
= precipitation.
