Skip to main content

Currently Skimming:

3 Forest Disturbance and Management Effects on Hydrology
Pages 45-74

The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.


From page 45...
... These changes can affect nearly all components of forest ecosystems, including surface erosion, slope stability, nutrient cycling, channel morphology and aquatic organisms, and the quantity and quality of water. This chapter defines forest hydrology, describes the factors that produce change in forests, and lists the general principles of forest hydrology (Figure 31)
From page 46...
... Changes in 3. Changes in forest flowpaths in water, soil structure soil and subsoil chemistry Specific hydrologic responses Hydrologic responses within forests: Interception & transpiration Infiltration & overland flow Water flowpaths in soil and subsoil Changes in watershed outputs: Water yield Floods Lowflows Sediment Chemistry Temperature Managing forests for water FIGURE 3-1 Forest hydrology examines the flowpaths and storage of water in forests, and how forest disturbance and management modify hydrologic responses.
From page 47...
... The change in the relationship of streamflow and water quality between the treated and control watersheds before and after treatment is defined as the effect of the forest treatment. Most paired watershed studies in forest hydrology were begun in the 1940s through the 1960s, and many of these studies had been abandoned by the 1980s.
From page 48...
... Department of Agriculture Forest Service budget is now being devoted to fire suppression and fuels management, which is nearly double the proportion allocated in the 1980s and 1990s. Hydrologic Effects of Changing Species Composition in Forests The composition of forest species changes as a result of natural disturbance and forest management.
From page 49...
... Hydrologic Effects of Timber Management and Silviculture Since the 1950s and 1960s, when timber harvesting expanded on federal forest lands and industrial forestry developed on private lands, an extensive literature has examined the effects of forest management. Forest hydrology studies have addressed the effects of silvicultural practices (such as site preparation, herbicide treatment, and thinning)
From page 50...
... . Overgrazing in forests was associated with decreased infiltration capacity, increased overland flow and surface erosion, increased peak flows, and increased sedimentation in streams (Johnson, 1952; Dissmeyer, 1976; Anderson et al., 1976)
From page 51...
... modify water flowpaths, increase overland flow, and deliver over land flow directly to stream channels 8 Impervious surfaces increase surface erosion. 9 Altered hillslope contours and modified water flowpaths along roads increase mass wasting Hydrologic Response to Application of Chemicals 10 Forest chemicals can adversely affect aquatic ecosystems especially if they are applied directly to water bodies or wet soils 11 Forest chemicals (fertilizers, herbicides, insecticides, fire retardants)
From page 52...
... In North American forests, evapotranspiration accounts for 40 to more than 85 percent of gross precipitation. A reduction in leaf area from forest harvest, fire, or insect or disease outbreaks reduces evapotranspiration and increases water available for runoff.
From page 53...
... If soils are compacted to the extent that infiltration rates are lower than rainfall or snowmelt rates, the resulting overland flow can greatly increase runoff rates and surface erosion. CHANGES IN WATERSHED OUTPUTS Forest hydrology science describes direct changes in watershed outputs resulting from fire, timber harvest, and roads and trails (Table 3-2)
From page 54...
... affect extreme events Low flows High-severity fire Increase low flows in Little or no effect increases low flows; short term; deficits little effect of low- may develop as severity fire forests regrow Erosion, land- High-severity fire Increase surface Increase surface slides, sedimenta- increases erosion erosion, landslides, erosion (road surtion and sedimentation in and sedimentation; faces and gullies streams; less effect effects may be long- below culverts) and from low-severity lived landslides; increase and prescribed fire sedimentation in streams Water temperature Increases water Increase water tem- Deliver road and chemistry temperature due to perature due to ripar- chemicals (e.g.
From page 55...
... . In the Colorado Front Range, for example, summer thunderstorms with 60 mm of rainfall per hour often produce no surface runoff or erosion, but after a highseverity fire, surface rainfall intensities of only 10 mm per hour can generate overland flow (Moody and Martin, 2001; Kunze and Stednick, 2006; Wagen
From page 56...
... . The greatest increases in peak flows occur in areas with summer thunderstorms or fall rains, where burning has altered infiltration and overland flow processes.
From page 57...
... The chemistry of unburned organic matter varies with plant species, underlying geology, time elapsed since the last disturbance, and atmospheric deposition of elements such as mercury and lead. Fires usually affect water quality by the indirect pathway of increasing stream water temperature and two direct pathways, atmospheric deposition and surface runoff.
From page 58...
... In most cases the adverse effects of forest fires on chemical water quality persist for no more than two or three years. Forest Harvest Timber Management, Silviculture, and Water Yield Dozens of paired watershed forest harvest experiments have demonstrated that forest removal increases water yield (Bosch and Hewlett, 1982; Hornbeck et al., 1993; Ice and Stednick, 2004; Jones and Post, 2004; Brown et al., 2005)
From page 60...
... versus percent basal area removal for 21 paired watershed forestry experiments in the conterminous United States. The trend line, shown for comparison, was taken from Figure 1 of by region were developed by Stednick (1996)
From page 61...
... . Thus, in both eastern and western forests, water yield increases after forest harvest often occur during seasons when water is abundant, not scarce (Harr, 1983; Troendle et al., 2001)
From page 62...
... . Timber Management, Silviculture, and Low Flows Relative to peak flows or annual water yields, few studies have examined the effects of forest harvest on low flows.
From page 63...
... . Recent compilations of studies examining forest management effects of peak flows show wide variability in the magnitude of peak flow response to forest harvest (Austin, 1999; Moore and Wondzell, 2005; Grant et al., 2008)
From page 64...
... Understanding the role of forest management in moderate and large floods requires a clear definition of terms and careful consideration of the various processes by which forest management can affect the size of peak flows. Floods are variously defined by scientists and affected populations.
From page 65...
... . In effect, forest harvest changes the snowmelt hydrograph from one peak to two, with the first peak coming from the harvested openings and the second peak from the remaining mature forest.
From page 66...
... In rain events, forest harvest affects peak flows directly through changes in soil water. In events involving snow, the effect of forest harvest on peak flows depends on how forest harvest changes snowpack size and snowmelt, as well as soil moisture (Verry et al., 1983; Troendle and King, 1985; Jones, 2000; MacDonald et al., 2003; MacDonald and Stednick, 2003; Schnorbus and Alila, 2004)
From page 67...
... After forest harvest on steep slopes, decreasing root strength and increased soil moisture and pore water pressures contribute to decreased slope stability and can increase the likelihood of shallow landslides (debris avalanches) during precipitation events.
From page 68...
... . After forest harvest, concentrations of nitrate-nitrogen (nitrate-N)
From page 69...
... . Elevated peak flows and surface erosion after forest harvest may increase phosphorus delivery to streams.
From page 70...
... Roads, Trails, and Water Yield Roads have their greatest impact on hydrologic effects on infiltration and overland flow (see preceding section)
From page 71...
... Roads can contribute to an increase in the size of peak flows by increasing the amount of surface runoff from impervious surfaces, intercepting subsurface stormflow, and speeding the delivery of this runoff to the stream network through ditches or gullies (Megahan, 1972; Wemple et al., 1996; Wemple and Jones, 2003)
From page 72...
... Hence peak flow responses to forest harvest often include the effects of forest removal, the effects of roads, and the interaction between them (Jones and Grant, 1996)
From page 73...
... Although in principle forest harvest can increase water yield, in practice a number of factors make it impractical to manage forests for increased water. Water yield increases from vegetation removal are often small and unsustainable, and timber harvest to augment water yield may diminish water quality.


This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More information on Chapter Skim is available.