Reclamation Information Sharing Environment (RISE)

The effects of wildfire and ammonium-phosphate (AP) fire retardants on loads of nutrients (nitrogen, N, and phosphorus, P) in downstream waters were evaluated using a critical literature review and watershed modeling. The literature review suggests retardant application in an average first-order watershed (320 hectares, ha) would add about 0.5 kg-N ha-1 and 1.2 kg-P ha-1 per aerial drop, much less than the typical ranges of N and P yield following wildfire. No studies have demonstrated N and P transport from AP fire retardants into surface water via overland flow. The updated PFHydro water-quality module was applied to part of the Cache Creek watershed, California, which burned during summer 2015. Daily time-series loads of total N (TN) and total P (TP) fit calibrated daily TN and TP loads for WY 2015 (pre-fire) and WY 2017 (post-fire) at the watershed outlet well. Annual TN and TP loads during WY 2017 were 4.5–6 times those during WY 2015. Modeling results indicate that application of AP fire retardants would theoretically increase TN and TP loads less than 1%, even with 100 aerial drops of fire retardant applied, a rate much higher than what was documented during the 2015 fires. The input data to run the PFHydro water-quality module, the module code and results analyses were attached as a zipped folder.