Fire can have numerous effects on a watershed and resulting water quantity and quality. It is important to understand that the effects depend on the size and severity of the fire, vegetation type, soil type, slope, proximity to a watercourse, and other factors.
The impacts of fire on water quality are the subject of ongoing studies that look at the relationships between forestry and vegetation management policies and their effect on watersheds. For example, see Evolving Attitudes Toward Fire in the Watershed: A Farewell to the 1900s (Watershed Management Council).
The percent of cover left on the ground is the most important factor controlling water quantity and quality. Ground cover helps break up raindrop impact (a significant cause of erosion), and can greatly increase the amount of water infiltration. A reduced infiltration rate often results in an increased flow of great turbidity and short duration shortly after the rain event. The various sediments that are carried in faster-moving surface flow may cause a change in water chemistry.
By removing vegetation and exposing mineral soil, fire impairs the ability of a watershed to hold soil in place and to trap sediment. As a result, increased amounts of sediment are delivered to streams, reducing both commodity and non-commodity beneficial uses. Fire-caused sedimentation can diminish reservoir capacity, costing $9 to $90 per acre burned in a large, intense fire. This risk is more imminent in reservoirs without large amounts of dead storage capacity, typically smaller reservoirs and reservoirs not originally designed to produce hydropower. Sediment removal after such a fire could cost $100 to as much as $1,000 per acre burned.
Increased sedimentation also causes additional wear and tear on hydroelectric generation equipment, harms fisheries, and has negative aesthetic impacts; none of those effects can be quantified easily. Fire and landslides triggered by lost vegetation are direct threats to water supply and hydro facilities, such as flumes borne on wooden trestles and canals on hillsides. Then there is the expense of watershed rehabilitation, such as reseeding or replanting vegetation or installing erosion controls.
Fire behavior is a significant factor to consider, since water-repellant layers can be formed when a fire penetrates heat into the soil. An indirect effect of fire is the possible addition of water to aquifers, reservoirs, and waterways caused by a reduction of plants, which depletes the underground stored water.
— Adapted from Introduction to the I-Zone (2001, PDF*, 2.1 MB)
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