Fires, floods, and the future of water supplies in the southwestern U.S.

About half of the water supply in the southwestern U.S. is derived from forested land, which generally yields higher quality water than any other land use (such as agriculture or urbanized land). However, forests are vulnerable to wildfire: in the southwestern states of Colorado, New Mexico, Arizona, and Utah, more than 12 million acres of land (nearly 5% of total area), including important forested water-supply watersheds, have burned in the past 30 years. Wildfires increase the susceptibility of watersheds to both flooding and erosion, which can lead to short- and long-term impairment of water supplies.

Scientists funded by the USGS Climate Research & Development Program, the USGS Water Mission Area, and the Boulder Creek Critical Zone Observatory collected streamflow and water-quality data for three years after the 2010 Fourmile Canyon Fire in Colorado, in a geographic setting typical of the American Southwest. They then compared the results with data from a high-density rain gage network to assess the role of precipitation type, intensity, and spatial distribution on concentrations and yields of stream constituents that are often elevated after wildfire: total suspended solids, nitrate, dissolved organic carbon, and manganese.

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The researchers observed that for three years after the wildfire, water quality downstream of the burned area substantially deteriorated in response to commonly occurring local thunderstorms. Suspended sediment, dissolved organic carbon, nitrate, and manganese concentrations were 10-156 times higher downstream of the burned area than upstream, reaching concentrations that could impair the ability of water-treatment plants to effectively treat water for human consumption.

Results from this study quantitatively demonstrate that wildfire can affect downstream water quality for several years, even in a watershed that was only 23% burned. Climate models forecast the potential for future increases in wildfire frequency and size in the southwestern U.S., as well as more frequent and intense storms. These could result in more common wildfire impacts on water quality, intensifying water supply and quality problems related to projected decreases in runoff and continued population growth. The study suggests several potential adaptation strategies to minimize the introduction of problematic constituents into water-treatment facilities or reservoirs after wildfire. One such strategy involves the development of real-time monitoring networks to provide advanced warning to water treatment facilities of high-intensity rainfall and flooding events that have the potential to impair water quality. Water-treatment plants could then close intakes during the event and/or use alternate sources.

The paper, "The Role of Precipitation Type, Intensity, and Spatial Distribution in Source Water Quality after Wildfire," was published in Environmental Research Letters.

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