At 1pm on 26 June 2011 a tree fell onto a power line in the Santa Fe National Forest in New Mexico, starting one of the largest wildfires the state has ever seen. The flames swept across 16,000 acres of land, burning nearly half of the vegetation in the watershed and threatening the town of Los Alamos and the Los Alamos National Laboratory. By 3 August the fire was contained, but the impact of this massive burn continued to be felt long afterwards. In particular the region was plagued by flash floods in subsequent years, as bare soils failed to absorb heavy rains.

This relationship between wildfire and streamflow is well known. What isn't clear is whether there are also wider regional effects on water supplies. In order to find out, Michael Wine and Daniel Cadol from the New Mexico Institute of Mining and Technology, US, analyzed the impact of wildfires on three New Mexico watersheds – Jemez (1223 sq. km), Mogollon (191 sq. km) and Gila (4807 sq. km). Using satellite data and United States Geological Survey records going back to 1982, the researchers studied the spatial patterns of burning from more than 100 wildfires. By comparing this with stream gauge data they discovered the effect that these various wildfires had on water flow.

After controlling for climatic and snowpack variability, Wine and Cadol showed that annual streamflow increased by as much as one fifth in the Gila watershed in the three to five years following any major wildfire. "Soils under evergreen trees are often resistant to water, and if the fire has removed the soil organic horizon then high-intensity precipitation may run-off instead of infiltrating," said Wine. In addition, the reduction in vegetation reduces the amount of transpiration, so water that would have been stored in trees ends up on the ground instead.

Increases in streamflow were also seen in the Mogollon watershed, where more than 90% of the watershed was burned during one fire, but no significant increases in water discharge were observed following sizeable wildfires in the Jemez watershed.

Wine and Cadol believe that this difference is due to the wildfire regimes in the regions, with the wildfires in Jemez tending to be smaller – they never exceeded 20% of the watershed area. "It seems that the wildfire burn area has to exceed a certain threshold for the disturbance to cause a measurable impact on water yields," said Wine, who published the findings in Environmental Research Letters (ERL).

The results show that when wildfires occur over a large enough area they have far-reaching consequences for regional water supplies, increasing the risk of floods, mudslides and rock-flows over a large area. There is also evidence that wildfire burn scars can influence local weather, with the decreased albedo from the darkened soil encouraging convection and the formation of thunderstorms.

Understanding how wildfires and streamflow interact could help to minimize the risks of flash flooding in the future. Better wildfire management of forested areas, such as allowing small controlled burns, could help to prevent the very large burns associated with increased streamflow. And if a large burn does occur, local land-managers can ensure that adequate preparations are made to protect people and property from potential flash-floods.

Related links

Related stories