Flowing water at the land surface - Fundamentals of the Physical Environment - page 416

Figure 14.1 Global water resources.
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. Glacier
growth or shrinkage respectively reduces or increases
surface run-off.
Source: Data based partly on L'vovich (1979).
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, routeing 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).
Catchments range in scale from single first-order streams (see below) less than 1 km 2
in area to major trunk rivers, such as the Thames (9950 km 2 ) or Mississippi (3,270,000
km 2 ). River flow is measured from single precipitation event responses, in hours or days,
to the annual water balance year . The extent to which actual river discharge flow differs