Fire Effects on Water

September 27, 2006

The Watershed Concept

• What is a watershed?• Area of land that drains into a common

outlet • Watershed condition- health or status of a

watershed– Hydrologic function- receive and process

precipitation– Support sustained plant growth

Characteristics of a healthy watershed?

• Precipitation infiltrates and percolates into the soil.

• Limited overland flow that causes severe erosion.

• Streamflow response to precipitation is relatively slow.

Fire effects on watershed condition

• Fire can be a determinant of watershed condition through vegetative change

• Effects of fire on watersheds depends on:– Severity, i.e. plant succession– Extent, Season

• Persistence of watershed conditions depends on the rate of revegetation

Why is the watershed concept useful when considering the

effects of fire on water?

Fire effects on the hydrologic cycle

Removal of vegetation and litter effects:• Plant interception • Infiltration rates• Snow accumulation and melting rate• Rates of evapotranspiration• Soil moisture storage• Overland flow (surface runoff)• Streamflow discharge

Fire Effects on Interception

• Vegetation and SOM on the soil surface intercept the fall of precipitation.

• Fire reduces vegetation canopy and OM accumulation on soil surface.

• Fire decreases interception and increases amount of precipitation reaching the soil surface

Effects on Snow Accumulation and Melt Patterns

• Snow in higher elevations often a primary source of water to downstream locations.

• Snow accumulates in open spaces with less interception and wind deposition

• Melting rate of snow is more rapid in open spaces than under vegetative canopies.

How does fire influence infiltration capacities of the soil ?

• Soil organic matter loss• Change in soil structure and reduction in

soil porosity• Direct impact from raindrops• Ash and charcoal residues clog pores

Evapotranspiration

• ET represents water losses from:– Evaporation from soil, plant, and water surfaces– Transpiration- absorption, movement, and loss of

water through plants.

• ET determines the proportion of precipitation that enters into:– Soil and groundwater– Surface and subsurface runoff

Effects of Fire on ET• Reduction in vegetation leads to a reduction in

ET losses

• Increases runoff component

• Quantity of overland flow and surface erosion depend on:– Steepness of slope– Soil characteristics– Rainfall severity

Soil water storage

• Loss of vegetation leads to change in ET losses and more water in subsoil– Surface runoff and stream flow increase

• Soil water over the long term depends on vegetation response.

Cumulative Effects: Streamflow Response

• Why is there an increase in stream discharge?

• Reduction in interception, infiltration, and ET

• Increase in surface and subsurface runoff

Impact of fire on water quality

• Changes in physical properties.– Sedimentation and turbidity– Increase in water temperature

• Changes in chemical properties– Nutrient loss in solution vs sedimentation– Organic matter export– Generally short term consequences

Changes in aquatic habitats

• Increased turbidity effects light penetration and photosynthesis

• Increased temperature alters habitat suitability

• Stream discharge alters stream habitat structure