The Forest Ecosystem - Mountains and Plains: The Ecology of Wyoming Landscapes - page 206

during most years, and additional water leaves the eco-
system as surface or subsurface runoff. thus, soil storage
capacity at any particular time and place depends on
the amount of water already in the soil, with deeper and
finer-textured soils holding considerably more water
than shallow, coarse-textured soils. the uptake of water
by plants leads to soil drying, enabling storage of addi-
tional water when it becomes available. the amount of
available storage generally increases during the sum-
mer, a function of the drying rate, rooting depth, soil
depth and texture, and the time since the last signifi-
cant wetting event—usually snowmelt. By the end of
summer, the soil moisture of lodgepole pine forests may
be depleted to depths of 6 feet or more, as the pines have
taproots extending that deep. 19

of less direct sunlight. in contrast, the snow on south-
facing slopes, especially at lower elevations, often melts
between snowstorms, with a considerable amount of
the soil moisture evaporating on relatively warm days
before additional snowfall occurs. Such periodic melt-
ing makes the snowfall events analogous to summer
showers, with each event producing too little moisture
to percolate through the litter and rooting zone to cause
substantial runoff. thus, north-facing slopes contribute
more to streamflow per unit area than do south slopes.

Percolation to Groundwater

Surface runoff in the form of streamflow is greatly
affected by the geologic substrate and by whether perco-
lation into aquifers is possible. Granites and other igne-
ous and metamorphic rocks are usually impervious to
water penetration, which enables more runoff. in con-
trast, watersheds underlain by sedimentary rock may
contribute significantly to the groundwater of inter-
mountain basins, which is frequently tapped by wells.
Plants growing at the juncture of impervious bedrock
above and permeable sedimentary rock below benefit
from the higher amount of runoff (see fig. 10.3).

Weather and Climate

Runoff from mountain watersheds is also affected by
weather. Gradual warming in the spring causes a pro-
longed snowmelt, which provides additional time
for early-spring transpiration, thereby reducing the
amount of water available for drainage to streamflow or
groundwater. in contrast, some springs are cold, becom-
ing warm only when day lengths are long. Under such
conditions, the snow melts late and rapidly, and there is
little time for spring transpiration. Moreover, the rate of
snowmelt occasionally exceeds the maximum infiltra-
tion rate, causing overland flow and a higher potential
for flooding, especially if the soil is already saturated
or has become compacted—as can happen when heavy
machines are used for timber harvesting. Water flowing
overland is not available for uptake by plants and can be
a significant cause of soil erosion.
Fall weather may be as important as that of the
spring. For example, heavy fall rains can saturate the
soil just before the onset of winter, when evapotrans-
piration rates are low. Little storage capacity then exists
during the following spring, leading to the potential for
greater spring runoff. Also, if snow falls before the soil
is frozen (and the snow is deep enough), the mountain
soils remain unfrozen for the entire winter and infiltra-
tion is enhanced.
Another factor causing variation in outflow is micro-
climate. For example, north-facing slopes in the north-
ern Hemisphere are more likely to retain snow because

Patchiness of Landscape Mosaic

Heterogeneity or patchiness in landscapes, caused by
disturbances and abrupt changes in various environ-
mental factors, has significant effects on watershed
hydrology. For example, a landscape where forests are
frequently interrupted by small meadows contributes
more water to streamflow than does an area of continu-
ous forest, primarily because wind causes snow drifting
in such openings, where there is little leaf area and less
evapotranspiration. 20 Furthermore, with more direct
sunlight, snowmelt is more rapid in meadows, reduc-
ing the time available for evapotranspiration. con-
sequently, more snow water flows down beyond the
rooting zone than in areas uniformly covered by forest.
Watershed managers sometimes recommend numer-
ous small patch cuts to increase the water yield from a
watershed. Based on research in northern colorado, the
largest increases in water yield are reached when 30-40
percent of a watershed is harvested in patches no larger
than 2-5 acres. Larger openings can have a different
effect, because the stronger winds in the opening blow