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

Increases in destructive flooding events (e.g., remnants of Katrina 2005, Tropical Storm Irene 2011) in the complex terrain of New England motivate the need for more research into the atmospheric and environmental processes that lead to these flooding events. The goal of this project is to understand the atmospheric and environmental processes responsible for high stream flow in a small catchment (42 ha) and assess trends in stream flow on a first order stream at Hubbard Brook Experimental Forest located in the White Mountain National Forest of New Hampshire. Synoptic scale atmospheric conditions for the top 80 high stream flow events (99.6th percentile) during 1957-2012 were assessed using gridded atmospheric data (sea level pressure, lower tropospheric wind, precipitable water 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.

Hydrograph analysis was completed to calculate the total volume of stream outflow for each event. Daily precipitation amounts and weekly snow depth measured in the watershed were examined for their relative contributions to overall stream output. Antecedent soil conditions were also examined using both a calculated antecedent precipitation index (Kohler and Linsley 1951) and modeled storage outputs from the HBV hydrology model.

Results showed that atmospheric rivers were present for 77 of the top 80 events. Extratropical cyclones were associated with 74 (92.5%) of the events, 4 (5%) were associated with tropical cyclones or post-tropical cyclones and 2 (2.5%) were associated with mesoscale systems. Extratropical cyclones were most frequent in the month of October and least frequent in the months of July and September. Tropical and post tropical cyclones were most frequent in August with 3 of the 4 occurring in that month. High precipitation accounts for the most impactful contributor to high flow events in winter, summer and fall. Antecedent soil conditions show the highest impact in springtime peak flow events. Extreme streamflow events were most frequent in the months of April and October and least frequent in the months of July and September. An overall increasing trend was observed in the annual frequency of extreme streamflow events. Understanding all of these factors is crucial in developing a water budget and hydrometeorological model to predict when storms will lead to high stream flow.