Hydrologic and Erosion Responses of Burned Watersheds

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The enhanced probability of catastrophic wildfires has increased our need to understand the risk of floods, erosion, and debris and contaminant transport in burned watersheds. This project investigates the relation between rainfall intensity and peak discharge; erosion and deposition processes; and water-quality impacts to minimize the loss of life and property resulting from post-wildfire floods.

SCIENTIFIC QUESTIONS

The enhanced probability of catastrophic wildfires in the western United States and elsewhere in the world has increased the need to understand the flooding risk and the erosion and depositional responses of burned watersheds. In addition, surface water flowing from burned areas may carry increased levels of sediment, organic debris, and chemicals that may contribute to significant degradation of municipal water supplies and aquatic habitats. Our project has three main thrusts: (1) we are investigating the relation between rainfall intensity and peak water discharge from burned watersheds, a relation that depends on the size of the rainstorm, the size of the burned area and burn severity, and the changes in infiltration capacity of the soil; (2) we are investigating the hillslope and channel erosion and deposition processes after wildfire with a focus on predicting these processes on a watershed or landscape scale rather than on a single hillslope plot or channel cross-section scale; and (3) we are examining the water quality impacts of wildfire and are synthesizing post-fire water-quality sampling protocols.

 

RESEARCH GOAL

An extensive body of literature exists on the effects of wildfire on watersheds. Wildfires have burned across the landscape of the western United States for centuries, but the magnitude of the geomorphic effect on the landscape is unknown. By understanding the magnitude of the runoff response and the erosion and deposition responses of recent wildfires, we can provide data for landscape evolution models in areas prone to wildfire. In addition, an understanding of the runoff response will contribute to better methods of predicting post-fire flooding to minimize the loss of life and property. Watershed-scale predictions of erosion and deposition from these natural disasters can be used by land managers to prioritize forest treatments based on erosion potential before and after wildfires. Moreover, we hope to contribute to an understanding of wildfire as an element of an ecosystem’s disturbance regime.

An alluvial fan at the mouth of a tributary to Buffalo Creek, CO.

The Buffalo Creek Fire in May 1996 burned 4,690 hectares in the mountains southwest of Denver, Colorado.  This wildfire lowered the erosion threshold of the watershed.  As a consequence of this wildfire, a 100-year rainstorm in July 1996 caused erosion upstream and deposition of this alluvial fan at the mouth of a tributary to Buffalo Creek. Buffalo Creek is flowing to the right at the bottom of the photograph. (Credit: R. H. Meade, USGS.)

Flash flooding at Spring Creek, CO, 29 July 1997.

A consequence of wildfire is the increased probability of flash floods. This flash flood occurred in Spring Creek on July 29, 1997, within the area burned by the Buffalo Creek Fire. The view is upstream and the discharge is about 5.0 cubic meters per second from a maximum 30-minute rainfall intensity of about 19 millimeters per hour. Rainfall-runoff relations suggest a rainfall threshold at about 10 millimeters per hour above which much larger flash floods occur. (Credit: John A. Moody, USGS.)

Incised erosion channel after the Cerro Grande Fire near Los Alamos, NM.

Another consequence of wildfires and subsequent rainfall is erosion. This erosion of a drainage created an incised channel after the Cerro Grande Fire near Los Alamos, NM. The view is upstream and the blue backpack is about 1-meter tall. The maximum 30-minute rainfall intensity was about 20 millimeters per hour. The incision seen in this photo was after the wildfire and rain storm; prior to the storm this drainage had no definite banks.  (Credit: John A. Moody, USGS.)

Debris deposition related to the Cerro Grande Fire near Los Alamos, NM.

Channels draining burned areas have zones of erosion and zones of deposition. This deposition was downstream from an erosion zone shown in the previous photo. The peeled bark indicates the highest level of water and debris during a flash flood. Sediment is coarse sand and gravel. The view is downstream and the blue backpack is about 1-meter tall.  (Credit: John A. Moody , USGS.)

Water release from Strontia Springs Dam.

Water being released from Strontia Springs Dam after wildfire-related flooding.

Organic debris in Strontia Springs Reservoir, CO.

Organic debris and sediment were deposited in Strontia Springs Reservoir, which supplies drinking water to the cities of Denver and Aurora. This debris came from two watersheds (Buffalo Creek and Spring Creek) burned by the 1996 Buffalo Creek Fire. Associated with this debris was an increase in manganese, which increased the chlorine demand of water treated for municipal usage. (Credit: John A. Moody, USGS.)

Rill erosion on a burned hillslope after the Buffalo Creek Fire

Rill erosion on a burned hillslope after the Buffalo Creek Fire. (Credit: John A. Moody, USGS.)