The Impact of Forest Management on Water Supply and Quality

Forest management can have either detrimental or positive effects on watershed supply and quality. The type of effect depends on the type of operation(eg. harvesting vs. reforestation), the way it is conducted, and the view point of the land manager(eg. more or less water yield).

Timber harvesting activities tend to increase the water yields of a watershed. Evapotranspiration is decreased by the removal of vegetation. This decrease in ET causes the amount of soil water that is moving into the channel system to increase. Lower ET rates also increase the amount of water available for deep percolation that provides baseflow (Satterlund and Adams, 1992). The decrease in ET may be short lived. For example, when a stand is thinned, the remaining trees quickly capture the increase of resources that is made available through the thinning. Understory vegetation also increases growth after removal of the overstory. Therefore, timber harvesting should be viewed as a temporary reduction in ET rates.

Another way harvesting increases the water yield of a watershed is by increasing runoff. When an area is totally or partially harvested, temporary or permanent logging roads are usually constructed. These logging roads can capture subsurface flow and divert it to ditches associated with the road. These roads can divert surface flow that would ordinarily by dispersed and infiltrate (Satterlund and Adams, 1992). Because of the surface characteristics of roads, precipitation that falls on them has a reduced capacity to infiltrate. The effects of logging roads can be temporary or permanent. Temporary logging roads eventually revegetate, and may even be planted with herbaceous plants after logging. Permanent roads are usually kept clear of vegetation and may have permanent drainage ditches associated with them

Site preparation techniques may also temporarily increase the water yields. Burning, discing, piling of debris, and other techniques are used to make the replanting of a harvested area easier. These techniques smooth the land surface, causing greater runoff velocity and less infiltration. In addition, when these techniques are employed, a greater amount of vegetation is removed, greater decreases in ET than with natural regeneration(no site prep/tree planting).

Intermediate stand management is done during a forests life, prior to final harvesting. Burning, thinning of overstory, mowing, herbicide application are all done to improve the wood quality or increase the growth of the desired trees. These techniques can alter ET and runoff rates. Because the entire overstory is not removed, these effects are short-lived.

Just as timber harvesting increases the water yield of a watershed, reforestation can reduce the watershed's yields. Watersheds that include agricultural areas that are seasonally void of vegetation, abandoned fields, mine spoils, etc., can have water yields manipulated by the re-introduction of forested lands. These reforested areas will reduce the amount of runoff and increase evapotranspiration. Thus, reforestation can be a technique for reducing the yield of a watershed. Water Quality

There is a direct relationship between the amount of disturbed land in a watershed and the amount of erosion. If sediment is allowed to enter a