Forests can have both a quantitative and qualitative effect on water resources. The amount of water interception, uptake, throughfall, and evaporation; and the quality of runoff and infiltration water are all influenced by the flora, the structure of the forest stand, the soil type, and the age of trees (Binns 1986).
Forests act as physical and biological filters for many nonpoint source pollutants, whereas cleared lands serve as conduits for eroding soils and contaminants that flow directly into streams and rivers, or indirectly, through groundwater (Academy of Natural Sciences 1996).
Forest removal or deforestation is associated with a decrease in transpiration, an increase in streamflow and an increase in loss of particulate material and dissolved nutrients. Soil compaction can occur by logging equipment and skidding of logs. Increased soil compaction decreases the soil's ability to absorb water, thereby resulting in increased runoff rates. Clearcutting of trees not only leads to greater nutrient losses, but also to greater losses of soil due to road building and timber extraction and to nutrient depletion when ample rotation is not provided to permit regeneration.
Numerous studies have addressed the effects of deforestation on runoff and water quality, specifically streamflow changes, sedimentation, nutrient budgets, and water chemistry. Drainage and cultivation can alter the response of a catchment to rainfall and can introduce suspended and dissolved solids into the runoff. (Binns 1986). Downstream effects of deforestation may include eutrophication of rivers and lakes due to increased nutrient loadings. Additionally, the acidity of drainage water may increase as organic material from the previous forest floor decomposes. This potential increase in acidity may be toxic to many invertebrates and fish, and also result in a release of metals from the sediment into the open water.
Many current research studies in deforestation are examining the global climate effects, which in turn, have local environmental consequences. Deforestation leads to increases in solar radiation received at the ground surface which may cause changes in surface temperature. These changes may have dramatic impacts on both terrestrial and aquatic species. Deforestation also affects albedo, interception by the canopy and surface roughness, as well as the radiation reaching the ground. All of these potential changes have major impacts on energy and water balances.
Reforestation, however, can lead to reduction in water flows, elimination of flooding and reduced soil erosion. Deep-rooted trees are prolific users of groundwater and soil moisture. The complex root systems of forest stands along streambanks also help stabilize the banks, thus decreasing erosion. Trees also play a role in reducing runoff by intercepting rainfall, and enhancing the evaporative process, either by direct evaporation or evapotranspiration (Mather 1986). Watershed research studies, including one conducted by Pereira in 1973, demonstrated forests' ability to absorb heavy rainfall and transmit water to the soil by infiltration through forest litter (Rogers 1994). The strength of this effect was found to be species dependent.
With good management practices, forest lands can be managed with little or no increase in soil erosion. Practices such as removing tress without disturbing ground cover, skidding logs across streams, and maintaing buffer zones of standing tress and undisturbed soils along streams, will result in minimal impact to water resources. Eleven potential resource impacts from silviculture activities (Ponce 1986) have been identified with regard to non-point source water pollution. These include:
aerial drift and application
of chemicals
bare
soil, due to the reduction of vegetative ground cover, rock and litter
channel
gradient changes caused by debris jams, mass wasting, and other introduced
materials
soil
compaction due to equipment and skidding of logs.
debris
in channel and obstructions in streams
excess
water due to increased channel flow
onsite
chemical nutrient balance changes
slope
configuration changes due to activities such as road building
stream
shading changes as a result of the removal of trees that provide protection
from direct solar radiation to streams and other surface waters
vegetative
change, including the removal of ground cover, canopy cover, or a change
in vegetation type
water
concentration which occurs when water is intercepted and allowed to converge
on the soil surface rather than infiltrating or spreading naturally.
With proper management, these inputs can be minimized or reduced.
Riparian buffer zones that maintain streamside forests have proven to be critical in helping control nonpoint source pollution. Forests contribute less nutrient and sediment run-off than most other land uses. Additionally, streamside forests absorb nutrients which would otherwise enter water courses. According to the United States Forest Service (USFS), a streamside forest can reduce the amount of nitrogen in runoff and shallow groundwater by as much as 80% (Academy of Natural Sciences 1996).
Forest cover can also assist in managing sediment runoff. Some sediment settles out as the speed of runoff flow is reduced by forest litter which contains leaves, twigs and other organic material. The forest floor also contrbutes a filtering effect, with its porous soil, vegetation, and organic debris.
