TABLE 3-2 Magnitude and Duration of Direct Effects on Watershed Outputs of Three Sets of Processes That Modify Hydrology in Forests: Fire, Forest Harvest and Silviculture, and Roads and Trails
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Processes That Modify Hydrology in Forests
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Watershed Output
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Fire
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Forest Harvest and Silviculture
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Roads and Trails
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Water yield
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High-severity fire increases annual water yields; little effect of low-severity fire
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Increase water yield; magnitude and duration of response varies (see text)
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Little or no effect
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Peak flows
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High-severity fire increases peak flows; effect is short-lived
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Increase peak flows; magnitude and duration of response varies (see text)
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Increase peak flows; effects may be long-lived and affect extreme events
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Low flows
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High-severity fire increases low flows; little effect of low-severity fire
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Increase low flows in short term; deficits may develop as forests regrow
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Little or no effect
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Erosion, landslides, sedimentation
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High-severity fire increases erosion and sedimentation in streams; less effect from low-severity and prescribed fire
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Increase surface erosion, landslides, and sedimentation; effects may be long-lived
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Increase surface erosion (road surfaces and gullies below culverts) and landslides; increase sedimentation in streams
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Water temperature and chemistry
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Increases water temperature due to riparian forest removal; fire retardants and ash affect chemistry; effects are short-lived
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Increase water temperature due to riparian forest removal; effects of fertilizer mostly small and short-lived; short-lived post-harvest increases in nitrate
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Deliver road chemicals (e.g. salt, oil) to streams
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Research gaps
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Uncertainty about effects beyond a few years; magnitude and persistence of downstream effects; effects of salvage logging
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Uncertainty about effects beyond one or two decades; magnitude and persistence of downstream effects; effects on habitat and aquatic ecosystems
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Uncertainty about road effects on extreme floods and in watersheds >1 km2
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NOTE: These are general effects, not predictions, so qualifying adjectives such as “may,” “usually,” etc., are omitted. See text for factors that influence when, where, and to what extent these effects apply.
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