Fire Effects on Watershed Hydrology

Burning that removes vegetation from a site can potentially alter stream hydrology. As vegetation is removed, evapotranspiration in the watershed is reduced, thus providing greater quantities of water to streamflow generation. Water uptake from trees is species-specific. Conifers, which make up a large percentage of southern forests, transpire greater quantities of water than hardwoods. Dense foliage and longer growing seasons promote the high water uptake. Also, conifers have dense canopies that intercept rainfall and allow for greater evaporation losses.

Once a site has experienced vegetation loss and litter layer removal, water can cause serious erosion problems and result in higher stream discharges. Not only do fires consume the litter layer, but at high temperatures fires can also cause hydrophobic soil conditions, thus making the soils more susceptible to erosion (see Fire Effects on Soil Physical Properties).

In the South, no indication of these extreme conditions has been reported for burning alone. Swift et al. (1993) found that hydrophobic soil conditions were not apparent after burning and thus did not function to promote excessive overland flow. However, when burning was coupled with deadening of hardwoods using herbicides, about one-fourth of the stormflows were increased by significant amounts in upland hardwood stands in northern Mississippi (Ursic 1970). Ursic (1970) also reported increases in overland flow in watersheds that were burned and herbicided.

For management guidelines on reducing the impact of fire on water quality, see Management Guidelines for Reducing Fire Effects on Soil and Water. Special precautions should also be taken when burning near riparian zones.