Geoscience Reference
In-Depth Information
Oregon
Douglas-fir Forest
Wyoming
Lodgepole Pine Forest
Precipitation
237
Precipitation
66
Transpiration
Transpiration
Evaporation
Evaporation
Evaporation and
Sublimation
Evaporation
22
218
37
4
5
40
24
Canopy
storage
Canopy
storage
Snow
Snow
14
4
5
18
181
2
Litter
Litter
198
38
43
25
Rooting
zone
Rooting
zone
155
Outflow
13
Outflow
Fig. 12.6. Annual hydrologic budgets for stands of rain-
dominated Douglas-fir in oregon and drier, snow-dominated
lodgepole pine in Wyoming. the boxes represent major stor-
age compartments; the arrows are processes affecting water
movement through the ecosystem. Wide arrows indicate
processes that affect relatively more water than processes
with narrow arrows. numbers are centimeters of water (for
example, 25 cm is the volume of water required to cover any
area 25 cm deep; to convert centimeters to inches, divide by
2.54). canopy interception is higher in the oregon forest,
because more rainfall and a higher leaf area index occur there
compared to Wyoming forests. interception and subsequent
evaporation of summer rains is important in Wyoming, as is
sublimation of snow during winter. the figure for Douglas-fir
is adapted from Sollins et al. (1980); data for lodgepole pine
are from Knight et al. (1985) and Biederman et al. (2012).
ests. 15 Forests dominated by spruce and fir in the Rocky
Mountains have considerably more leaf area than do
lodgepole pine forests, and furthermore, the spruce
and fir use water at higher rates per unit of leaf area
than either pine or aspen. All things equal, streamflow
would be less from watersheds where spruce-fir forests
are common.
Aspen forests are different because the leaves are
deciduous and absent during most of the snowmelt
period, precluding most early-spring transpiration.
Moreover, the leaves are shed in the fall, eliminating
the possibility of foliar transpiration during subsequent
warm days. Also, there is less potential for intercept-
ing snow on leaf surfaces, from which evaporation
and sublimation can occur. Some land managers have
noticed that the flow of mountain streams can increase
after aspen leaf fall. the net effect of forest domina-
tion by deciduous trees is that less water is transpired
and a larger portion of annual precipitation is available
for streamflow. Aspen forests may change to evergreen
coniferous forests during long periods without distur-
bance (see chapter 11), with streamflow decreasing as
this shift occurs. 16
Reductions in leaf area stemming from tree cutting,
fire, and some other disturbances reduce evapotranspi-
ration and, therefore, potentially increase streamflow.17 17
Streamflow may continue at an elevated level for up to
60 years, depending on forest type, intensity of distur-
bance, and rate of vegetation regrowth, before declin-
ing to its predisturbance volume after the original leaf
area has been restored. 18 n otably, for reasons described
below, the tree mortality caused by bark beetles does
not necessarily lead to an increase in streamflow.
Soil Water-Holding Capacity
Another variable affecting the amount of surface runoff
is the amount of water that the soil can store, that is,
the amount that is not lost to downward flow caused
by gravity. At some point, the soil becomes saturated
 
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