Abstract

The area burned by wildfires is expected to increase in many watersheds of the world over the next century as a function of climate change. Increased sedimentation due to soil erosion in burned watersheds can negatively impact downstream aquatic ecosystems and the quality and supply of water. At least 65% of the water supply in the western USA originates in watersheds covered by trees, shrubs, and/or grasses that are prone to wildfire16. Understanding how changing fire frequency, extent, and location will affect watersheds, reservoirs, and the ecosystem services they supply to communities is therefore of great societal importance.

A primary threat to socio-ecological systems in this region from wildland fire, and subsequent post-fire sedimentation, is the decline or disruption of watershed-based ecosystem services, such as the maintenance of water quality and the quantity of water supplied. Projections also suggest a dramatic human population increase in Western states that will likely stress water resources with increased demand. However, the effect of expected post-fire sedimentation, any changes to water supply, and potential increases in water demand are all functions of unknown future climate and land use conditions.

To estimate the magnitude of these potential effects, we modeled expected post-fire sedimentation across the West at the subbasin level through the year 2050, using an ensemble of climate, fire, and sedimentation models. We show post-fire sedimentation may increase by 10% for ¾ of the watersheds modeled, while ¼ of watersheds are projected to increase by 100% or more. Looking at the range of population projections, the 2010 population of the eleven western states (70.7M) is expected to increase by 28-47% by 2050. Much of this population growth will occur in the Southwest around existing metropolitan areas where water resources are already strained. Northern areas, however, are more variable in their growth and change in water use, as some communities grow by displacing more water intensive agriculture, leading to a decline in demand.

Through merging both datasets, we explored watershed vulnerability to demonstrate where the effects of increased post-fire sedimentation and changing demand may lead to emergent areas of watershed-based ecosystem service disruption. Such areas deserve careful evaluation to determine if existing forest and watershed management plans are sufficient given the expected climate and population changes.