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are relatively few well - controlled studies on the effects
<br />of fires on runoff, but the combination of process -based
<br />studies and field observations provides a reasonable ba-
<br />sis for understanding and prediction.
<br />2.2. Effects of Changes in the Forest COO DV on Runoff-
<br />2.2. 1. Overall water balance
<br />A useful starting point for understanding the effects of
<br />forest management on runoff is the water balance equa-
<br />tion. In simplest terms:
<br />Runoff = Precipitation - Evaporation - Transpiration
<br />+ Change in Storage
<br />In many instances the evaporation and transpiration
<br />terms are combined into a single evapotranspiration (ET)
<br />term, as in practice it is very difficult to distinguish the
<br />water being lost to the atmosphere by evaporation from
<br />the water being lost to the atmosphere through the sto-
<br />mata of plants by transpiration. However, it is useful to
<br />separate these two components, as changes in vegetation
<br />type and density can have different effects on these two
<br />processes under different conditions. It is also important
<br />to note that evaporation includes the loss of water by
<br />interception, which is the evaporation or sublimation of
<br />water captured on plant surfaces during or immediately
<br />after a precipitation event (Dunne and Leopold, 1978).
<br />In many forested areas the change in storage can be
<br />ignored because the amount of water stored as soil and
<br />ground water shows little change from one year to the
<br />next (Hewlett, 1982). This assumption is generally valid
<br />for Colorado, as the amount of stored water in undis-
<br />turbed forests generally reaches a similar minimum val-
<br />ue at the end of each summer dry season (Troendle and
<br />Meiman, 1986). Changes in the amount of stored water
<br />are often significant over shorter time scales (e.g., sea-
<br />sonal, monthly), but on an annual time scale the change
<br />in storage in undisturbed forests is usually small relative
<br />to the errors in the other terms in equation 1.
<br />If we assume constant precipitation from year to year,
<br />equation 1 indicates that runoff will be directly propor-
<br />tional to the amount of evaporation and transpiration. It
<br />follows that a reduction in vegetation cover will reduce
<br />the amount of interception and transpiration. If there is
<br />not a corresponding increase in the amount of evapora-
<br />tion from the soil, annual runoff must increase. In rela-
<br />tively dry areas or in dry years a reduction in vegetation
<br />has little or no effect on runoff because the water that is
<br />"saved" by the reductions in interception and transpira-
<br />Table 2.1. Summary of data from paired- watershed experiments in Colorado and northern Arizona, including pre -
<br />and post- treatment water yields, elevation, and percent of vegetation removed by forest harvest.
<br />Initial
<br />Mean
<br />increase
<br />Veg.
<br />Mean annual
<br />annual
<br />in
<br />Area
<br />Elevation
<br />Vegetation
<br />removed
<br />precipitation
<br />(inches)
<br />ch f)
<br />C
<br />runoff
<br />Source
<br />Watershed
<br />(mil)
<br />(feet)
<br />type
<br />(%)
<br />6.1
<br />1.1
<br />Bates and
<br />Wagon Wheel Gap, south-
<br />9,300
<br />Spruce
<br />100
<br />21
<br />Henry, 1928
<br />central Colorado
<br />50
<br />30
<br />8,7
<br />3.2
<br />Troendle and
<br />Fool Creek, Fraser
<br />9,600
<br />Spruce -fir,
<br />lodgepole pine
<br />King, 1985
<br />Experimental Forest (FEF)
<br />30
<br />NA
<br />22.5
<br />3.6
<br />Troendle and
<br />Deadhorse Creek, FEF Upper
<br />0.3
<br />9,400-
<br />11,600
<br />Spruce -fir,
<br />lodgepole pine
<br />36
<br />32.3
<br />15
<br />2.4
<br />King, 1987
<br />Troendle and
<br />Basin
<br />North Fork
<br />King, 1987
<br />6.6
<br />g,g00-
<br />Lodgepole pine,
<br />24
<br />34.3
<br />17.4
<br />3.0
<br />Troendle et al.,
<br />2001
<br />Coon Creek, south - central
<br />11,000
<br />spruce -fir
<br />Wyoming
<br />Beaver Creek, northern Arizona
<br />5,600 -8,500
<br />Ponderosa pine
<br />21.7 -31
<br />24,3
<br />5,9
<br />2.4
<br />Baker, 1986
<br />Watershed 12
<br />0.7
<br />7,100
<br />Ponderosa pine
<br />100
<br />77
<br />28.6
<br />8.1
<br />2.5
<br />Baker, 1986
<br />Watershed 17
<br />0.5
<br />6,900
<br />Ponderosa pine
<br />33
<br />27.4
<br />6.7
<br />2.9
<br />Baker, 1986
<br />Watershed 8
<br />2.8
<br />7,300
<br />Ponderosa pine
<br />68
<br />27.7
<br />5.3
<br />2.8
<br />Baker, 1986
<br />Watershed 16
<br />0.4
<br />7,100
<br />Ponderosa pine
<br />57
<br />25.6
<br />4.6
<br />1.3
<br />Baker, 1986
<br />Watershed 14
<br />2.1
<br />7,200
<br />Ponderosa pine
<br />31
<br />25.4
<br />6.1
<br />1.0
<br />Baker, 1986
<br />Watershed 9
<br />1.8
<br />7,200
<br />Ponderosa pine
<br />32.8
<br />3.3
<br />0.9
<br />Hibbert and
<br />Workman Creek, central
<br />0.5
<br />6,600 -7,800
<br />Douglas -fir,
<br />79
<br />Gottfried, 1987
<br />Ponderosa pine
<br />Arizona
<br />
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