Assessment of wildland fire impacts on watershed annual water yield: Analytical framework and case studies in the United States

Abstract

More than 50% of water supplies in the conterminous United States originate on forestland or rangeland and are potentially under increasing stress as a result of larger and more severe wildfires. Little is known, however, about the long-term impacts of fire on annual water yield and the role of climate variability within this context. We here propose a framework for evaluating wildland fire impacts on streamflow that combines double-mass analysis with new methods (change point analysis, climate elasticity modeling, and process-based modeling) to distinguish between multiyear fire and climate impacts. The framework captures a wide range of fire types, watersheds characteristics, and climate conditions using streamflow data, as opposed to other approaches requiring paired watersheds. The process is illustrated with three case studies. A watershed in Arizona experienced a +266% increase in annual water yield in the 5 years after a wildfire, where +219% was attributed to wildfire and +24% to precipitation trends. In contrast, a California watershed had a lower (−64%) post-fire net water yield, comprised of enhanced flow (+38%) attributed to wildfire offset (−102%) by lower precipitation in the post-fire period.Changes in streamflow within a watershed in South Carolina had no apparent link to periods of prescribed burning but matched a very wet winter and reports of storm damage. The presented framework is unique in its ability to detect and quantify fire or other disturbances, even if the date or nature of the disturbance event is uncertain, and regardless of precipitation trends.

Regional patterns of postwildfire streamflow response in the Western United States: The importance of scale‐specific connectivity

“…We are not aware of any evidence for prescribed fire‐induced floods within the humid subtropical or other climate regions of the Eastern United States; however, an 8% increase in the 5‐year annual water yield of a watershed in South Carolina was likely not caused by prescribed fire but by a combination of exceptionally high winter precipitation, beaver activity, and storm damage (Hallema, Sun, et al . , ).…”

“…Annual water yields in the Del Puerto Creek in Northern California declined by 64% in the 5 years following a wildfire, but after filtering out the impact of drought using a climate elasticity model, an increasing trend emerged that attributes a 38% higher annual water yield to the wildfire (Hallema, Sun, et al . , ).…”

“…Notwithstanding, this area has also known some of the most extreme hydrological responses to severe wildfire (Hallema, Sun, et al . , ).…”

“…Postwildfire floods with severe erosion have been observed in semi‐arid Arizona (Wagenbrenner, ) and New Mexico (Moody & Martin, ), and to a lesser degree in Mediterranean California and other parts of the West (Hallema, Sun, et al . , ; Jung et al, ), but such occurrences are not common in the humid subtropical Southeast (Hallema, Sun, et al . , ).…”

“…, ; Jung et al, ), but such occurrences are not common in the humid subtropical Southeast (Hallema, Sun, et al . , ). Regional variability in fire trends results in a patchwork of areas with a potential to either exclude or promote fire (Parks et al .…”