Water Quantity and Quality
Water quantity and water quality are major criteria for measuring the effects of forest management for bioenergy and bio-based products on water resources. Water quantity refers to the timing and total yield of water from a watershed, while water quality refers to the suitability of the water coming from ground and surface water supplies for drinking water, recreational uses, and as habitat for aquatic organisms and other wildlife (Neary 2002). Water quality is measured using chemicals, biological, and physical (i.e. temperature, color, clarity) indicators.
Water quantity is measured in two ways: total yield and peak flow, both of which can be affected by forest management practices including harvesting and residue removal. Total yield is the total amount of available water that flows out of a watershed area. In general, water flow is increased in the first year after harvesting. Removing vegetation and litter from the watershed allows more water to fall and infiltrate the ground and streams. The actual increase in water yield resulting from forest management will depend upon the amount of precipitation, evapotranspiration, percent of cover removed, and other environmental factors including soil conditions (Neary 2002).
Peak flows, or flood peak flows, can also be affected by forest management. However, it is not a simple matter to predict how management will affect watershed responses to storms. For example, some areas will have declines in peak flows, while peak flows will increase in other areas. The effect of management on peak flows will be determined by site specific characteristics which affect the hydrological cycle directly, such as plant leaf area, soil water storage capacity, time of year, and slope and aspect. Sites should be evaluated by qualified professionals prior to any harvesting and production decisions so that watershed responses to management can be predicted reliably. While intensive management can create a risk for an increase in peak flows, severe wildfires produce a greater increase in peak flows than do harvesting operations. Therefore, in order to minimize the risk of adverse impacts on water resources due to active versus passive management, landowners and professional foresters must undertake risk assessment procedures which compare alternative management operations including harvesting biomass in order to reduce fuel loads and wildfire hazards. This is a good example of the ways in which biomass removal can provide a renewable energy source, reduce wildfire risk, and potentially improve water resources. In such situations, doing nothing results in loss of multiple values gained by active management.
A second factor to consider when discussing water and biomass production is the effect on water quality. Water quality is measured by the amount of sediment, water temperature, the concentration of pollutants, and the level of nitrogen and other nutrients and chemicals in the water supply. The nitrate-nitrogen concentration in water is one indicator of water quality. High concentrations of nitrate-nitrogen in water can be harmful to human health. However, no long-lasting, harmful increases in stream water nitrate-nitrogen have been observed following harvesting operations (Neary 2002). Herbicide usage to suppress vegetation after fire, the use of chemical fertilizers during regeneration, and atmospheric deposition generally have a greater effect on stream nitrate levels than do harvesting operations.
Sediment yield from forested watersheds is variable and site specific, but generally negligible when compared with many other land uses. Much of the sediment movement is caused by disturbance of the soil during site preparation operations. Proper site preparation techniques, proper road construction, and other best management practices can lessen the potential for sediment movement and adverse effects of forest management on water quality. Readers interested in knowing more about the physical, ecological, and socio-economic effects of forest roads in the mountainous Southeastern U.S. should visit the section on Forest Roads in the Encyclopedia of Southern Appalachian Forest Ecosystems.
Forest canopies protect streams from solar radiation, thus reducing the variability in water temperatures. Therefore, harvesting operations can adversely affect stream temperatures. The effect of stream temperatures will depend upon the aquatic species located within the forest streams. Some species are able to survive post-harvest temperature changes better than others. Management practices designed to maintain effective buffer strips along streams and prevent adverse affects of harvesting on water quality have been developed for the major Southern physiographic regions, as for example the Southern Appalachians as a result of research conducted at Coweeta Hydrologic Laboratory (see Forest Roads; Stickney and others 1994), the Alto Watersheds in East Texas, and elsewhere. Managers typically incorporate recommendations for managing Streamside Management Zones (SMZs) into forest management plans, as described under Adaptive Forest Management. Streamside management zones (above right) take into account the area directly around the stream as well as the effects of any management practices that may affect the stream quality.
Recommendations for maintaining water quality in managed forests have been published for all states in the Southern United States. Many of these publications are available through forestry extension programs in each state.
- Alabama
- Arkansas
- Florida
- Georgia
- Kentucky
- Louisiana
- Mississippi
- North Carolina
- Oklahoma
- South Carolina
- Tennessee
- Texas
- Virginia
- Forestry BMP
A recent review by Shepard (2006) indicates that Best Management Practices developed to protect water quality in forests managed for a conventional mix of forest products should be applicable to bioenergy production systems.
Encyclopedia ID: p1283
