In forested catchments the hydrologic cycle, involving precipitation, interception, evapo-transpiration, overland flow, subsurface flow, groundwater flow, and stream flow is closely linked to water quality in that water movement through the forest ecosystem also transports sediment, and dissolved nutrients, as well as fertilizers, and pesticides if they are present. Understanding relationships between forested ecosystems and quality of surface and subsurface water associated with these systems is a key component of sustainable forest management because changes in water quality may result from forest management practices. These changes can reflect either positive or negative outcomes of forest practices. For example, logging road construction and harvesting of timber with improper consideration for erosion control can cause increased sedimentation of stream water and a degradation of water quality. In contrast, conversion from agricultural crop production to forestland can improve water quality by decreasing erosion rates and creating long-term storage pools (e.g., forest floor, woody biomass) for carbon and nutrient retention. This article provides a synthesis of our current thinking regarding (1) the concept of water quality, (2) the role of forested watersheds in providing water of relatively high quality, and (3) commonly evaluated water quality parameters and potential effects of forest practices on these parameters. The primary focus is on the relationship between water quality characteristics of streams draining forested water-sheds and forestry practices.
[...] Clearing of riparian vegetation is the primary forest management practice that can cause elevated stream temperature, particularly in small headwater catchments. This is the result of increased stream exposure to direct solar radiation. Temperature increases of as much as 151C have been observed in forest streams when riparian vegetation has been removed. However, the magnitude of the response is tempered by stream discharge, streambed characteristics, channel morphology, stream surface area, and degree of hyporheic exchange and groundwater influx along the stream length. [...]
[...] Nevertheless, excessive suspended sediment loads in streams are the major water quality concern for forest management because poorly planned forest management activities on hillslopes or in the vicinity of the stream channel that cause erosion can add to naturally derived levels of suspended sediment. Increases in suspended sediment levels resulting from erosion and soil mass movement (i.e., landslides) can degrade drinking water quality, detract from recreational values, decrease stream depth, fill pools in the stream channel, increase stream width, and cause sedimentation of gravel beds which lowers their permeability and degrades their habitat quality for spawning fish. [...]
[...] Finally, fire lines that are established by bulldozers to control the spread of fire can be potential sources of sediment in streams. If fire lines are established under emergency circumstances, concerns for proper planning, avoidance of sensitive areas (i.e., very steep or excessively wet), and erosion control are not always paramount and accelerated erosion and sedimentation may result. Temperature and Dissolved Oxygen Water temperature is a key water quality parameter because of its direct effect on chemical and biological processes and properties in the stream. It is also a determinant of the amount of dissolved oxygen available for aquatic fauna. [...]
[...] However, impacts of this type of conversion on water quality have received limited evaluation because there is limited documentation of comparisons between farmland and forestland on the same site. Net impacts on water quality depend on prior land use and crop management, current forest management practices, soil type, local hydrology, and climate. In general, conversions to forestland have the potential to reduce erosion and subsequent sedimentation, as well as reduce levels of dissolved nutrients and pesticides in surface runoff and groundwater. [...]
[...] There is general agreement in the literature that forest road networks and skid trails developed to extract timber are the greatest threat to water quality because they are frequently a source of erosion and sedimentation. Compacted surfaces of logging roads and skid trails reduce infiltration and often carry surface runoff and suspended sediments during storms. The amount of sediment delivered to streams is often proportional to the density of logging roads and skid trails within a watershed and inversely proportional to the time since road and skid trail construction. [...]
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