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Fire effects on water supply, floods, and sedimentation

Author(s): H. W. Anderson
Year Published: 1976

From the text ... 'Many forest types owe their origin, perpetuation, and distinctive characteristics to fire. What may be called the normal hydrologic behavior of many forested watersheds already incorporates some effect of fire -- both natural and man-induced. Fire may have a greater effect than harvesting on peak flows, erosion, and water quality. The protective influence of the forest may be destroyed, slope instability augmented, and channel banks subjected to scour. How we choose to use fire as a tool and how we manage our forests for fire prevention and control depends in part on the consequences of fire to water supply, floods, and sedimentation. Burning the forest can increase both water yield and streamflow discharge. The amount of increase will depend on the intensity, severity, and frequency of fire occurrence and on the proportion of the watershed burned. Where much of the foliage is destroyed, interception and evapotranspiration will be reduced. And where the organic layers of the forest floor are consumed and mineral soil exposed, infiltration and soil water storage capacities may be reduced. Soil-water storage capacity is reduced by fire if the humus layers and organic material in the mineral layers are burned, or if exposure of the soil accelerates oxidation of soil organic matter. In the upper 2 inches of soil, severe surface fires can reduce the capacity by about 1/4 inch (Dyrness et al. 1957). If the overlying 2-inch layer of humus is destroyed, the reduction would total about 1 inch. Crown fires, obviously, would drastically reduce evapotranspiration and the opportunity for soil-water storage; the result would be similar to clear-cutting the forest. Soil water storage may be reduced by fire-induced repellancy in the surface soil (DeBano, 1968; DeBano and Rice, 1971). At high elevations, forest fires may destroy natural windbreaks, resulting in different snowdrift patterns and changes to water yield (Billings, 1969). The duration of fire effects ranges from a single season to many decades, depending on the extent of the fire itself and the rate of recovery as influenced by natural conditions, post-fire land use, and remedial measures applied by man. In the East, recovery from fires, usually surface fires, may be rapid, in high elevation areas of the Northwest, regrowth after severe burning can be very slow. This paper summarizes the effects of fire on water supply and water control in the western timber types, and considers the role of post-fire management.' From the Conclusions ... 'The effects of fire on water in the forest are variable. Light or spot burning, away from channels, have little impact. Wildfires that kill the trees or consume the forest floor and other vegetation over a large area have a major impact on storms, erosion, sedimentation, and quantity of streamflow. The duration of effects is strongly influenced by the rate of revegetation. On the whole, the current level of fire protection has greatly reduced the hydrologic importance of fire as compared to conditions some decades ago. Where severe, widespread wildfire still occurs, as in some areas of the Pacific Northwest, it continues to be a serious threat to water supply and to water and erosion control.'

Citation: Anderson, H. W. 1976. Fire effects on water supply, floods, and sedimentation, Proceedings Annual [15th] Tall Timbers Fire Ecology Conference: Pacific Northwest. Portland, OR. Tall Timbers Research, Inc.,Tallahassee, FL. p. 249-260,
Topic(s): Fire Effects, Ecological - Second Order, Soils, Water
Ecosystem(s): None
Document Type: Conference Proceedings
NRFSN number: 18472
FRAMES RCS number: 36574
TTRS (Tall Timbers Research Station) Number: 10962
Record updated: Nov 20, 2018