Fluvial systems: catchments and rivers - Fundamentals of the Physical Environment - page 313

GENERATION OF CHANNEL FLOW

Net landward transfer
+39 (7%)

Catchment and water cascade

+110 (21%)

-71 (14%)

The catchment or drainage basin converts water, snow
and ice input (precipitation + in fluent ground water) to
output as river flow, evapotranspiration and ef fluent
groundwater flow. Catchment climate and landsystems
(soils, geology, slopes, ecosystems, and human structures
and activities) determine the volume, routing and
time scale of these transfers. This terrestrial hydrological
system is the vital link in the global cycle between its
principal atmospheric and ocean components. It processes
water evaporated from oceanic and terrestrial sources
in the ratio of approximately one to two. We cannot
overestimate the importance of net landward advection
of evaporated ocean water to the continents, for its
equivalent return to the oceans drives Earth's river flow.
From the moment precipitation arrives in the system,
water may be transmitted through a series of in-line stores
or exit any one of them directly or indirectly to a channel
( Figure 14.3 ).

+406 (79%)

-445 (86%)

-39 (8%)

Global annual transfer 516 w 10 3 km 3

Figure 14.2 Global hydrological cycle, showing sources of
the annual total of 516,000 km 3 of evaporated water and its
distribution and eventual return to the oceans. Glacial growth
or shrinkage respectively reduces or increases surface run-off.
Source: Data based in part on L'vovich (1979)

compared with 10 4 yr via continental ice sheets. Rivers are
the principal route for water, sediment and energy
transfers from the continents.

Overland flow

Watershed

Capillary
flow
Infiltration

Stream flow

Groundwater
effluence

Depression
storage

Aeration
zone

Urban water
supply

Urban
effluent

Throughflow

Seepage
to
channel

Flood
storage

Groundwater
influence

Water
table

Groundwater
recharge

AQUIFER

Streamflow

Figure 14.3
The catchment landscape.

AQUICLUDE