Fire Effects on Water

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.

Why is the watershed concept useful when considering effects of fire on water?

• Fire can be a determinant of watershed functions through effects on vegetation and soils

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

• Persistence of post-fire watershed condition depends on the rate of revegetation

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

Fire 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.

Fire effects on soils: infiltration capacity

• Soil organic matter loss

• Change in soil structure and reduction in soil porosity

• Direct impact from raindrops

• Ash and charcoal residues clog pores

Evapotranspiration: What is it and why is it important

for the H-cycle?• 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

Effects of fire on 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: Effects of Fire on Streamflow Response

• Often fire results in an increase in stream discharge – why?

• Reduction in interception, infiltration, and ET

• Increase in surface and subsurface runoff

Impact of fire on water quality

• Changes in physical properties.

Sedimentation and turbidityIncrease in water temperature

• Changes in chemical properties

Nutrient loss in solution vs sedimentationOrganic matter exportGenerally 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