Physiography of Flowing Water - Freshwater Ecology: Concepts and Environmental Applications - page 76

TABLE 5.3 Calculated Global Areal Cover of Seven Vegetative or Cover Classes and Associated
Runoff, Counts of Perennial and Intermittent Rivers, and Combined Vegetation Types a

Rivers

Coverage Runoff Perennial Intermittent

Water yield

3 year 1

(m year 1 )

Vegetation class km 2 10 6 % % Counts % Counts %

Broadleaf evergreen 13.4 9.0 14,663 29.8 1.09 16,269 10.4 407 1.1

forest
Grasslands-wooded 42.2 28.4 13,709 27.9 0.33 39,676 25.4 15,085 39.2

grasslands
Temperate forests, 28.7 19.3 9,438 19.2 0.33 57,985 37.1 3,069 8.0

seasonal forests
Cultivated land 13.3 8.9 4,377 8.9 0.33 14,820 9.5 4,141 10.8

Ice 15.9 10.7 3,838 7.8 0.24 1,446 0.9 0 0.0

Tundra 7.1 4.8 2,235 4.5 0.32 20,142 12.9 6 0.0

Shrub-desert 27.9 18.8 909 1.8 0.03 5,968 3.8 15,747 40.9

Total 148.4 100 49,169 100 156,306 100 38,455 100

a The seven classes are ordered by runoff (from Dodds, 1997a; reprinted by permission of the Journal of the
North American Benthological Society ).

the biological processes that occur in streams. For example, many stream
invertebrates rely on leaves and woody debris from terrestrial vegetation.
Also, the vegetation can alter the stream channel morphology. Streams in
pastures tend to be narrower than those in forests (Davies-Colley, 1997).
When streams are characterized by vegetation, most runoff occurs from
tropical evergreen forests. Grassland and temperate forests are important
sources of runoff as well (Table 5.3). Such classifications can be used to
quantify global fluxes of chemicals such as carbon that can be linked to
terrestrial vegetation types (Meybeck, 1993).

STREAM FLOW AND GEOLOGY

Important links occur between groundwater and streams. Most streams
are fed by groundwater for the majority of the time. Consider the headwa-
ter streams with which you are familiar; it rarely rains hard enough for wa-
ter to flow across the surface of the land (sheet flow). Rather, infiltration
through soil and subsurface sediments feeds into the streams to maintain
flow. This constant level of discharge in streams is called base flow .
Increased or prolonged rain events can cause rapid increases in dis-
charge or floods (Fig. 5.8). These events often occur randomly, but a prob-
ability can be calculated that an individual event of a specific magnitude will
occur given a certain amount of time (Fig. 5.9). Thus, we speak of a 10-
year flood, an event that on average will occur once every 10 years. In other
words, in any year there is a 1 in 10 chance of such a flood, regardless of
whether such a flood has not occurred for the past 20 years or whether it