TABLE 3-1 General Principles of Forest Hydrology Describing the Direct Effects on Hydrologic Processes of Changes in Forest Structure, Changes in Water Flowpaths, and Application of Chemicals

Principles of Hydrologic Response to Changes in Forest Structure

1

Partial or complete removal of the forest canopy decreases interception and increases net precipitation arriving at the soil surface

2

Partial or complete removal of the forest canopy reduces transpiration

3

Reductions in interception and transpiration increase soil moisture, water availability to plants, and water yield

4

Increased soil moisture and loss of root strength reduce slope stability

5

Increases in water yield after forest harvesting are transitory and decrease over time as forests regrow

6

When young forests with higher annual transpiration losses replace older forests with lower transpiration losses, this change results in reduced water yield as the new forest grows to maturity

Changes in Water Flowpaths in Soils and Subsoils

7

Impervious surfaces (roads and trails) and altered hillslope contours (cutslopes and fillslopes) modify water flowpaths, increase overland flow, and deliver overland flow directly to stream channels

8

Impervious surfaces increase surface erosion.

9

Altered hillslope contours and modified water flowpaths along roads increase mass wasting

Hydrologic Response to Application of Chemicals

10

Forest chemicals can adversely affect aquatic ecosystems especially if they are applied directly to water bodies or wet soils

11

Forest chemicals (fertilizers, herbicides, insecticides, fire retardants) affect water quality based on the type of chemical, its toxicity, rates of movement, and persistence in soil and water

12

Chronic applications of chemicals through atmospheric deposition of nitrogen and sulfur acidify forest soils, deplete soil nutrients, adversely affect forest health, and degrade water quality, with potentially toxic effects on aquatic organisms

NOTE: These general principles are not predictions, so qualifying adjectives such as “may,” “usually,” etc., are omitted.

direct effects or first-order responses to changes in forest structure, changes in water flowpaths in soil and subsoil, and application of chemicals. These principles tie together the storage and movement of water in forests, how disturbance and management modify water storage and movement within forests, and how these internal changes are translated into changes in watershed outputs (Figure 3-1). These principles embody the state of knowledge of forest hydrology based on process, plot, and watershed studies conducted mostly in the second half of the twentieth century.

HYDROLOGIC RESPONSES WITHIN FORESTS

Forest disturbances and management affect the pathways of water within the forest system. Interception, evapotranspiration, infiltration, and overland (or surface) flow respond to forest disturbance and management (Figure 3-1). In turn, these changes affect watershed outputs.



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