Effects of Wildfires and Fuel Treatment Strategies on Watershed Water Quantity across the Contiguous United States

Abstract:

Wildfires can negatively affect watershed functions by altering forest ecosystem structure, soil properties, flow pathways, evapotranspiration rates, and particulate matter emission and deposition. The recent Wildfire Cohesive Strategies recommends fuel treatments as a forest management tool to reduce the occurrence and likely impacts of catastrophic fires in populated areas. However, little is known about how these treatments affect watershed peakflows and water yield in different physiographic regions in the continuous United States (CONUS). There is an immediate need to identify high priority watersheds for integrated management. We hypothesize that water yield responses to wildfires vary by regions due to differences in climate, watershed characteristics, and magnitude of fire disturbance regimes. The monthly WaSSI water balance model was applied to 88,000 HUC-12 watersheds in the CONUS and results were scaled to 18 Water Resource Region (WRR) to examine the sensitivity of hydrologic response to changes in vegetation structure (leaf area index) and soil porosity. We found that, by WRR, a reduction of LAI by 50% can have substantial (>17-93 mm/yr, or 7-21%) influence on water yield in wet regions (annual precipitation > 800 mm) and smaller impacts on water yield in terms of absolute values (3-32 mm/yr) in drier regions (annual precipitation 300-800 mm). However, the relative change in the drier region can be equally large (10-20%). In contrast, water yield in the dry regions is more responsive to soil disturbances than in the wet regions. Water yield was estimated to increase by 9-24 mm/yr or 1-14% if soil infiltration capacity (as simulated by soil field capacity) was reduced by half. In the drier regions, the increase in water yield would be 3-24 mm/yr or 5-40% as a result of soil disturbance. Reduction of both LAI and soil field capacity by half could elevate water yield by 50% at the WRR scale, although larger changes in water yield may occur at smaller scales in individual HUC12 watersheds. Additional research is needed to assess fire effects on surface energy balances and potential evapotranspiration (PET). Also, realistic scenarios should be developed to represent the true effects of fires on vegetation and soil parameters at the watershed scale.