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 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