Running Water - The Changing Earth: Exploring Geology and Evolution - page 303

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Stages of the hydrologic cycle

1 Evaporation from the sea 6 Precipitation

2 Evaporation from streams, rivers, lakes, and soil 7 Runoff of rainfall, snow, and ice-melt back to the sea via rivers

3 Evapotranspiration from plants 8 Infiltration of some surface waters

4 Movement by wind of moisture-laden air masses 9 Groundwater flow back toward the sea

5 Condensation into clouds as air rises 10 Emergence of some groundwater as springs that feed streams and rivers

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Figure 12.3 Stages of the Hydrologic Cycle Water is recycled from the oceans to land and back to
the oceans.

Fluid Flow
Solids are rigid substances that retain their shapes un-
less deformed by a force, but fl uids—that is, liquids and
gases—have no strength, so they fl ow in response to any
force, no matter how slight. Liquid water fl ows downslope
in response to gravity, but its fl ow may be laminar or turbu-
lent. In laminar fl ow, lines of fl ow called streamlines paral-
lel one another with little or no mixing between adjacent
layers (

soil before runoff begins. Regardless of the initial condi-
tion of surface materials, once they are saturated, excess
water collects on the surface and, if on a slope, it moves
downhill.

RUNNING WATER
The term running water applies to any surface water that
moves from higher to lower areas in response to gravity. We
have already noted that running water is very effective in
modifying Earth's land surface by erosion, and that it is the
primary geologic process responsible for sediment transport
and deposition in many areas. Indeed, it is responsible for
the tiniest rills in farmer's fi elds to scenic wonders such as
the Grand Canyon in Arizona, as well as vast deposits such
as the Misssisippi River delta.

Figure 12.4a). All flow is in one direction only,
and it remains unchanged through time. In turbulent fl ow,
streamlines intertwine, causing complex mixing within the
moving fl uid (Figure 12.4b). If we could trace a single wa-
ter molecule in turbulent fl ow, it may move in any direc-
tion at a particular time although its overall movement is
in the direction of fl ow.
Runoff during a rainstorm depends on the infi ltration
capacity, the maximum rate at which surface materials ab-
sorb water. Several factors control the infi ltration capac-
ity, including intensity and duration of rainfall. If rain is
absorbed as fast as it falls, no surface runoff takes place.
Loosely packed dry soil absorbs water faster than tightly
packed wet soil, and thus more rain must fall on loose dry

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Sheet Flow and Channel Flow
Even on steep slopes, fl ow is initially slow and hence causes
little or no erosion. As water moves downslope, though, it
accelerates and may move by sheet fl ow , a more or less con-
tinuous fi lm of water fl owing over the surface. Sheet fl ow is