The Atmospheric and Environmental Conditions Responsible for High Stream Flow at the Hubbard Brook Experimental Forest in New Hampshire

In light of the increase in destructive flooding events in the mountainous terrain of northern New England (e.g., remnants of Katrina 2005, Tropical Storm Irene 2011), more research into atmospheric and environmental conditions that lead to these flooding events is needed. The goal of this project is to understand the atmospheric and environmental conditions responsible for high stream flow from a small-scale watershed (42 ha) at Hubbard Brook Experimental Forest located in the White Mountain National Forest of New Hampshire. Synoptic scale atmospheric conditions for the top 30 high stream flow events during a 56 year record of continuous monitoring were assessed using gridded atmospheric data (sea level pressure, lower tropospheric temperature and wind, precipitable water, precipitation rates, and 500 hPa geopotential height) from the 20th Century Reanalysis dataset. Storm tracks, moisture sources, moisture pathways, and seasonality of these atmospheric conditions were categorized for each high stream flow event. Daily precipitation amount and weekly snow depth measured in the watershed were examined for their relative contributions to high stream flow. Results showed that atmospheric rivers were present during all of the top 30 events. Extratropical cyclones were associated with 87% of the events and the other 13% were associated with tropical or post-tropical cyclones. A combination of moderate rainfall and snow melt contribute to high stream flow events during winter and early spring. High precipitation amounts from multi-day rain events or tropical systems were the dominant contributors of water for late summer and autumn high stream flow events.