Hydrologic Processes Affected by Harvesting

The Hydrologic Cycle

Water is one of the basic necessities of life. It is needed by all organisms in all ecosystems. Quality and quantity can be affected by processes occurring in watersheds. Many of these watersheds are located in forest lands. The hydrologic cycle (at right) depicts the flow of water through an undisturbed forested watershed. Water flow is affected by climatic factors and other environmental factors and will vary from site to site (Neary 2002).

There are three major outputs of water from watersheds including streamflow, evaporation, and transpiration or evapotranspiration (ET). Streamflow refers to water located in and moving through the watershed in stream channels. Evapotranspiration is the water lost from soil, leaves, and stems of plants, while direct evaporation is from water bodies in the area. Inputs of water are generally from precipitation and include rain, fog, snow, and sleet.

Partitioning of Precipitation

Water movement is influenced by the various pathways precipitation follows en route to streams, such as those affected by plants, including interception, streamflow, and throughfall; those affected by soils including infiltration, surface runoff, interflow, baseflow, and by the intensity and duration of storms (Neary 2002). Interception refers to precipitation that is intercepted by leaves, branches, and stems of the forest canopy as it falls to the ground. In undisturbed forests, 91% of precipitation passes into the soil. Only 1% of watershed precipitation does not enter the soil and runs off as overland flow. The figure to the left shows how rainfall is partitioned as it falls into a forested watershed.

The forestlands in an area can affect the amount of precipitation that enters the water supply. For example, the forest canopy can influence the amount of water that is intercepted and water lost through evaporation and evapotranspiration. The condition of the forest floor can influence the amount of water that infiltrates the soil surface and it can influence the amount of water that is lost due to surface runoff. Healthier soils will allow for more infiltration, while more compacted and eroded soils will favor runoff and result in sediment transfers to surface water in streams and lakes. These factors are all influenced by forest management practices, so it is important to understand that soils, hydrologic processes, and forest management are all related.