Streamflow responses to past and projected future changes in climate at the Hubbard Brook Experimental Forest, New Hampshire, United States

Climate change has the potential to alter streamflow regimes, having ecological, economic, and societal implications. In the northeastern United States, it is unclear how climate change may affect surface water supply, which is of critical importance in this densely populated region. The objective of this study was to evaluate the impact of climate change on the timing and quantity of streamflow at small watersheds at the Hubbard Brook Experimental Forest in New Hampshire. The site is ideal for this analysis because of the availability of long-term hydroclimatological records for analyzing past trends and ample data to parameterize and test hydrological models used to predict future trends. In this study, future streamflow projections were developed with the forest watershed model PnET-BGC, driven by climate change scenarios from statistically downscaled outputs of atmospheric-ocean general circulation models. Results indicated that earlier snowmelt and the diminishing snowpack is advancing the timing and reducing the magnitude of peak discharge associated with snowmelt. Past increases in precipitation have caused annual water yield to increase significantly, a trend that is expected to continue under future climate change. Significant declines in evapotranspiration have been observed over the long-term record, although the cause has not been identified. In the future, evapotranspiration is expected to increase in response to a warmer and wetter environment. These increases in evapotranspiration largely offset increases in precipitation, resulting in relatively little change in streamflow. Future work should aim to decrease uncertainty in the climate projections, particularly for precipitation, obtain a better understanding of the effect of CO2 on vegetation, determine if climate-induced changes in tree species composition will influence discharge, and assess the impacts of changing hydrology on downstream water supplies.