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Jane Hodgins

Forest Data Show How a Fading Tropical Storm Irene Triggered Historic Flooding

DURHAM, NH, October 5, 2011 - In the days after Tropical Storm Irene, scientists at the U.S. Forest Service’s Hubbard Brook Experimental Forestused data from monitoring points throughout the forest to demonstrate not only how the fading storm triggered record-breaking flooding, but what flood waters carried with them.

Part of the Forest Service’s Northern Research Station, Hubbard Brook Experimental Forest has been important to studies of forest hydrology, precipitation, and ecological studies for more than half a century.

“Experimental forests are famous for the decades of data they provide, and that insight is truly priceless,” according to Lindsey Rustad, a research ecologist with the Northern Research Station. “This is an example of how experimental forests give us more than history; they also provide essential data that help us understand events as immediate and large as a tropical storm.”

Water samples taken by Hubbard Brook Experimental Forest scientists show that by the time storm waters had rushed past the Plymouth gauging station on the main branch of the Pemigewasset River, the flood had carried over 59 million tons of sediment, 23,000 lbs of phosphorus, and 82,000 lbs of aluminum. That is equivalent to 454 freight train cars filled to the top with sediment, 43,500 50-pound bags of 5-10-5 fertilizer, and 2.3 million empty 12-ounce aluminum soda cans in just one storm event.

Data collected on the experimental forest shows that two storms earlier in August set the stage for the flooding triggered by Tropical Storm Irene. The standard measurement of soil’s capacity for holding water is called “soil water deficits.” At the beginning of August, an automated soil water sensor maintained by the Natural Resources Conservation Service showed soil water deficits of 1.3 to 4.7 inches, indicating that the soil could hold a substantial amount of water.

And then it started raining. In the two weeks before Tropical Storm Irene, storms on August 14-16 and again on August 21-22 reduced the soil deficits to just a quarter of an inch or less, meaning that any additional rain over a quarter of an inch would become stream flow and flood very quickly. Measurements varied by location, but Tropical Storm Irene dropped an average of 5.9 inches of rain on Hubbard Brook Experimental Forest.

Waters drained out of the Hubbard Brook Valley quickly, joining water from other nearby catchments and converging on the main branch of the Pemigewasset River. U.S. Geological Survey gauging stations started to pick up rising waters in the early morning on Sunday, August 28. The Pemigewasset River peaked at close to 1:00 p.m. on August 28 in Woodstock and at 2:00 a.m. on August 29 downstream in Plymouth. Total discharge from these gauging stations were 3.0, 4.6, and 3.3 inches respectively in Lincoln, Woodstock, and Plymouth, resulting in the largest August flood on record and ninth largest flood overall in Plymouth.  

The Hubbard Brook Experimental Forest is a 7,200-acre, bowl-shaped valley located in the southern part of the White Mountains, New Hampshire. One of 80 experimental forests within the U.S. Forest Service’s Research and Development arm, Hubbard Brook Experimental Forest has served as an outdoor laboratory for ecological study since 1955. Forest Service scientists as well as scientists from agencies and universities throughout the world have studied the quantity and chemistry of water going into the forest in precipitation and out of the forest in stream water at Hubbard Brook Experimental Forest.

The mission of the U.S. Forest Service is to sustain the health, diversity, and productivity of the nation’s forests and grasslands to meet the needs of present and future generations. The agency manages 193 million acres of public land, provides assistance to state and private landowners, and maintains the largest forestry research organization in the world. The mission of the Forest Service’s Northern Research Station is to improve people’s lives and help sustain the natural resources in the Northeast and Midwest through leading-edge science and effective information delivery.