The Impact of Climate Change on Past and Future Streamflow at the Hubbard Brook Experimental Forest, New Hampshire, USA

Long-term data from the Hubbard Brook Experimental Forest show that air temperature has increased by 1-1.5 °C over the last half century. While more variable, annual precipitation has also increased by 19-26% during the same period. These changes in climate influence streamflow, which provides an integrated climate signal that incorporates physical (snowpack, evaporation) and biological (evapotranspiration) responses. Unlike the western United States, water is generally abundant in the Northeast. However, changes in flow could nevertheless affect stream ecosystem services in the region, such as drinking water, irrigation, recreation, wastewater assimilation, and hydropower. We analyzed long-term data at the Hubbard Brook Experimental Forest to determine if past changes in climate have affected the distribution and quantity of streamflow. We also analyzed future changes in streamflow using the forest ecosystem model, PnET-BGC driven by climate input scenarios generated using downscaled AOGCM output. Past streamflow data indicate that the timing of streamflow has changed at Hubbard Brook. The winter/spring center of streamwater volume is occurring 0.18-0.25 days earlier each year, and streamflow increases during snowmelt have become less extreme over the 50 year record. Despite declines in snowmelt runoff, the number of high flow days per year has increased, due to increases in precipitation. Similarly, greater precipitation amounts have resulted in fewer low flow days. The overall amount of annual streamflow has increased significantly over the last 50 years, consistent with an increase in precipitation and no change in evapotranspiration. Future climate projections for Hubbard Brook show that air temperature and precipitation will continue to increase during the 21st century. Unlike historical data, preliminary PnET-BGC results indicate that projected increases in evapotranspiration will balance increases in precipitation, resulting in no significant change in streamflow. A more thorough modeling analysis is warranted because future changes in the hydrologic regime of headwater streams at Hubbard Brook have important implications for downstream water supply.