Racing The Rain: A Post-Wildfire Case Study In Northern, Nm

In April 2022 a fire started in the Sangre de Cristo mountains of northern NM. By the end of July almost 342,000 acres had burned making this the largest wildfire in NM history. The wildfire, known as the Hermit’s Peak Fire, burned the upper watersheds associated with the Canadian and Pecos Rivers. A majority of these watersheds have perennial drainages that provide drinking water to smaller communities in northern NM such as Las Vegas and Mora, NM. But the wildfires were only the beginning of the challenges for these communities as convective storms in the summer, known in the southwest as the monsoons, can bring intense rainfall events. Concern over the potential damage caused by the floods and debris that these rainfall events could generate resulted in the charge to model 969 square miles of watershed to provide insight to emergency management crews on the expected increase in peak flows and the debris yields as a result of the fires. The goal was to provide this information before the expected start of the monsoon season at the beginning of July. Modeling was performed using the HEC-HMS software. Since time was of the essence, modeling built upon previous experience with the Cerro Grande Fire in 2000 that identified curve number changes as a result of burn severity. Four of the five watersheds modeled for the Hermit’s Peak fire had long-term monitoring of discharge measurements which facilitated the calibration of a pre-fire hydrology. The post-wildfire modeling resulted in estimates of the peak flow and debris yields. Post-wildfire peak flow estimates increased by a factor 1.6 to 11.7, with the largest increases predicted for the more frequent hydrologic events (less than the 1/10 AEP). Debris yields by subbasin were estimated and used to estimate a watershed bulking factor to provide a floodplain delineation more representative of the sediment laden flows. Estimated bulking factors for the modeled watersheds ranged from 1.2 to 2.0. The modeling effort, including the underlying calculations to identify long-term streamflow trends at the established USGS gages, proved useful in helping to design some advanced measures placed in the more populous areas to provide flooding and debris yield protection. With the onset of an early monsoon season in late June one of the completed models was used to provide a forecasted flood potential. While not quite beating the rain, models were developed, and potential event-based floodplains were delineated. The effort showed that high fidelity hydrologic modeling for post-wildfire conditions is possible. There were also limitations and research needs identified to improve the reliability of real-time hydrologic and debris yield modeling efforts in the future.