Fire Effects on Water Temperature and Chemistry Fire can affect a series of water quality parameters see recent summaries by Landsberg and Tiedeman, ; Ranalli, ; Neary et al. The effects of fire depend in large part on the pre-fire composition of organic matter and the fire intensity. The chemistry of unburned organic matter varies with plant spe- cies, underlying geology, time elapsed since the last disturbance, and atmos- pheric 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. Extensive burning of the riparian forest canopy removes shade, increases the amount of solar radiation, and raises stream water temperatures. Increased organic carbon and temperature in streams can reduce concentrations of dissolved oxygen Neary et al.
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During a fire, gases and particulate matter are carried aloft and transported for varying distances before being deposited on water surfaces. In the Yellow- stone fires of , for example, increases in nitrogen in lakes and rivers were attributed to the diffusion of smoke into the water bodies under active fire condi- tions Spencer et al. Ash deposition can increase the pH of surface water and soil Neary et al. Post-fire pH values in stream water rarely exceed U.
Fire can cause a short-term in- crease in stream nitrate concentrations, and the delivery of ash and fine sediment. In most cases these in- creases do not exceed standards for drinking water Neary et al. During forest fires, chemical fire retardants are applied aerially to forests and inadvertently perhaps unavoidably to streams and rivers. The effects of these chemicals on water quality may be important, especially since recent stud- ies have shown that they persist for years after application Morgenstern, Fire retardants can contain nitrate and possibly sulfate, phosphate, and some trace elements Landsberg and Tiedemann, , which can contribute to eu- trophication, especially when applied directly to streams.
When these materials enter rivers, streams, and lakes, they react with sunlight to form compounds that are toxic to aquatic organisms e. Increased concentrations of other chemicals, such as manganese, sulfate, and mercury, also have been documented after forest fires. Elevated concentrations of both lead and mercury were detected in the post-fire runoff from the Bobcat fire outside Fort Collins, Colorado. Elevated post-fire concentrations of manganese and other constituents forced the Denver Water Board to initiate additional special- ized treatments to maintain drinking water quality.
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, ; Hornbeck et al. The magnitude of water yield increases can be expressed as an absolute increase e. Water yield increases are highest after percent forest harvest, and are almost always highest in the first year after forest harvest, or the wettest year in the early post-harvest period, when changes in interception and transpiration have the greatest effect on the water balance Bosch and Hewlett, ; Sahin and Hall, The largest absolute water yield increases have occurred after cutting of forests in climates with relatively abundant precipitation 1, to 2, mm per year and relatively low evapotranspiration.
High water yield increases to mm per year have been measured in the Pacific North- west H. Mean elevation meters , mean annual precipitation millime- ters , and mean annual temperature degrees celsius are shown above the graph for each of these four sites. The trend line, shown for comparison, was taken from Figure 1 of by region were developed by Stednick , which represented 95 catchments spread across similar geographic regions as shown here.
Black lines on map are USFS ecoregions. Small water yield responses in the Southwest illustrate the limited potential for increasing water yields in dry forest types. In regions where precipitation is evenly distributed throughout the year Northeast, Southeast; Figure , water yield increases typically occur during the growing season Martin et al. In regions with dry summers and wet winters western forests; Figure , the largest water yield increases occur in the late fall and early winter due to a reduction in transpiration and resultant increase in soil moisture carryover Jones and Post, In snowmelt-dominated regions, most of the water yield increase occurs in spring because larger snowpacks ac- cumulate in cutover areas Harr et al.
Thus, in both eastern and western for- ests, water yield increases after forest harvest often occur during seasons when water is abundant, not scarce Harr, ; Troendle et al. Forest harvest experimental treatments have ranged from percent clear-cutting to partial cuts, overstory thinning, or selective harvest of a fraction of watershed area Figure In areas with more than mm of mean annual precipitation the Pacific Northwest, North- east, and Southeast , water yield increases are roughly proportional to the amount of forest area cut Hibbert, ; Bosch and Hewlett, Figure 3- 2.
Water yield increases are difficult to detect when less than 20 percent of the basin area has been harvested Stednick, Figure The spatial ar- rangement of cutting within a watershed also affects whether a water yield in- crease is detected; controlling for the amount of forest cut, there is less detect- able water yield increase for thinning or selective harvests than for patch cuts Site 4, Figure Perry, For- est harvest has different effects on water yield increase depending on whether the area is burned, herbicide is applied, or the treatment is conducted in stages compare vertical scatter of points from site 14 in Figure Year-to-year storage of water in deep soils or poorly drained areas may offset water yield increases in areas with deep compared to shallow soils compare site 20 versus 18, and 16 versus 14 in Figure Post-harvest changes in snow accumulation and melt rates also can affect water yield increases after harvest Verry et al.
Post-harvest water yields are higher when old-growth forests with high leaf area are harvested, compared to when younger forests with low leaf area are cut Swank and Crossley, ; Jones and Post, When forests of low interception or lower annual transpiration. As forests re- generate after harvest, water yield increases disappear. Water yield increases have persisted for as little as a decade in some areas, but for multiple decades in other areas, depending on the type and history of the forest, soils, climate, refor- estation methods, and harvest treatments Bosch and Hewlett, ; Troendle and King, ; Swank and Crossley, ; Hornbeck et al.
In some cases, water yields drop below pre-harvest levels after a couple of decades of forest regrowth Hornbeck et al. Many paired watershed experiments established to test forest management effects on water yield were terminated after the first years of post-treatment, so only a few paired watersheds are still providing information about the long-term consequences of past forest management for water yield.
In summary, water yield increases from forest harvest are highly variable. The highest increases in water yield occur from percent forest harvest in the first years after harvest and in areas where water is relatively abundant. Because of the inherent variability in water yield responses, the amount of forest harvest necessary to produce a water yield increase varies according to regional and site- specific characteristics e. There is little evidence that timber harvest can produce sustained increases in water yield over large areas.
Because of high evapotranspiration relative to precipitation, and dry summers, the potential for augmenting water yield on a sustainable basis in western forests and rangelands is very low Harr, ; Hibbert, ; Troendle et al.
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Water yield increases from the harvest of western forests occur in winter when water is relatively abundant, and these in- creases would have to be stored for up to six months to effectively augment wa- ter supplies when water is scarce in late summer Harr, Maintaining wa- ter yield increases requires continued forest harvest or conversion of forests to other land uses such as pastures, annual crops, and urban areas. Although the potential for augmenting water yield is higher in eastern than western forests, achieving this potential would require major changes in forest management ob- jectives and land use Douglass, 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.
Most studies show an initial increase in low flows immediately after forest harvest Harr et al. Observed water increases in low flows after harvest change are often short-lived, usually persisting for less than 10 years due to the relatively rapid recovery of leaf area, interception capacity, and transpiration rates. These short-term surpluses during the low-flow period change to deficits as forests regrow.
As in the case of annual water yields, the increase in low flows often is followed by a decrease in low flows to below pre-harvest levels Hicks et al. Be- cause relatively few studies have examined long-term trends in low flows, there is much uncertainty about this subject. Timber Management, Silviculture, and Peak Flows Decreases in transpiration and interception after forest harvest increase soil moisture, and higher initial soil moisture at the beginning of a storm increases storm runoff peak flow Box 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, ; Moore and Wondzell, ; Grant et al.
Much of this variation is explained by the differences in how peak flows were defined and analyzed, dominant hydrologic regimes e. Often, the percentage increase in peak flows after forest harvest decreases as the magnitude of the peak flow increases Harr, ; Beschta et al. However, in many cases the absolute increase in peak flows is larger with larger storms Box ; Verry, ; Jones and Grant, ; Jones, ; Moore and Wondzell, As storm magni- tude the total amount of rainfall or snowmelt increases, the proportion of pre- cipitation that can be stored by vegetation decreases.
Therefore, large peak flows often experience smaller relative increases than small peak flows. Nevertheless, small percentage increases in very large floods as a result of forest harvest Fig- ure may be quite large in absolute terms; a 10 percent increase in a typical year flood is the same amount of water as a 50 percent increase in a 1-yr flood.
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Small increases in extreme floods affect more people and may be of greater concern for managers than increases in small floods. The effect of forest management on flooding has been a recurrent scientific, social, and political theme Eisenbies et al. The notion that deforestation leads to wide- spread land degradation and exacerbates the risk of flooding dates to antiquity Hillel, , and the role of forest management on extreme floods is an important concern for policy makers and the public Figure There is little doubt that forests influence the storage and movement of water, particularly at annual and seasonal time scales.
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Under- standing the role of forest management in moderate and large floods requires a clear defi- nition of terms and careful consideration of the various processes by which forest man- agement can affect the size of peak flows. Floods are variously defined by scientists and affected populations. Floods are com- monly described as flows that exceed channel capacity and result in overbank inundation Brooks et al. Hydrologists typically define floods according to their probabilities of recur- rence or return period e. Public concerns about floods are commonly limited to the more extreme events that result in a loss of life or prop- erty.
The largest floods are associated with extreme storm events, such as tropical cy- clones. Some recent assessments have attempted to link the growth in flood damage in recent decades to development in flood-prone lands and to discount the role of deforesta- tion on large-scale extreme floods FAO, CIFOR, Nevertheless, considerable pub- lic and political pressure tends to follow extreme floods, and forest protection is often an important element of debates and policy formulation Eisenbies, Copyright by Associated Press.
In mountainous areas, snowmelt is naturally de- synchronized by the heterogeneity of terrain features such as slope, aspect, and elevation. The snowpack generally melts first at low elevations and last at high elevations, more quickly on south slopes than on east and west slopes, and most slowly on north slopes.
Differences in forest canopy type and density e. In the relatively flat terrain of the northern Lake States the effect of topography on energy exchange is minor in comparison to the influence of forest cover. Therefore, when timber harvesting e. In effect, forest harvest changes the snowmelt hydro- graph from one peak to two, with the first peak coming from the harvested openings and the second peak from the remaining mature forest.
As the proportion of the harvested area within a watershed increases e. Copyright by the Society of American Foresters. Storm events involving rain respond differently to forest harvest than those involving snow. 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.
Peak flow increases after forest harvest are proportionately larger in spring, summer, and fall compared to winter, because soil moisture levels are sensitive to transpiration by forest vegetation in spring, summer, and fall. However, peak flow increases in millimeters after forest harvest are abso- lutely see Figure larger in winter than in spring, summer, or fall, because peak flows are higher in winter, and reductions in transpiration in previous sea- sons carry over into winter Jones, ; MacDonald and Stednick, The larger the proportion of area har- vested, the greater is the increase in peak flows Jones, ; Moore and Wondzell, Peak flow increases have been detected after only 25 percent harvest of a small watershed Harr et al.
The effect of forest harvest on the energy balance, and resulting changes in snow accumulation and melt, vary with elevation and aspect. Forest harvest may increase peak flows during rain-on- snow events in the Pacific Northwest Harr, ; Harr, As forests regenerate, peak flows return to pre- harvest levels Troendle and King, ; Jones, Many studies of forest harvest effects on peak flows include some roads and skid trails, which can accentuate the effect of har- vest on peak flows Jones and Grant, Because roads and trails influence different components of the water balance, they are discussed separately below.
Timber Management, Silviculture, and Erosion, Mass Movement, and Sedimentation Many studies have shown that timber harvest practices greatly increase sur- face erosion summarized in Dunne and Leopold, ; Brooks et al.