A COMPARISON BETWEEN RUNOFF TRENDS IN A HEADWATER BASIN AND MORE DEVELOPED WATERSHEDS: A CASE STUDY OF THE MERRIMACK WATERSHED, NH-MA

The Merrimack River Basin is a mid-size watershed of almost 13000 km² within NH and MA. Average annual precipitation and runoff is approximately 1000 and 500 mm, respectively. Merrimack is mainly has forested land cover, with only 10% of the total watershed area urbanized.

A recent and future climate impact study at Hubbard Brook Experimental Forest (HBEF), in the northern headwater portion of the basin revealed an increasing trend in total annual runoff, with a trend of the center flow volume of discharge occurring earlier in the year. There are 37 gauging stations within Merrimack Watershed with appropriate discharge records ranges 1903 to present.

We used the non-parametric Mann-Kendall test to evaluate annual, monthly, and seasonal runoff patterns in Merrimack. The results showed significant increase in total annual streamflow (α=0.1) for more than 50% of the stations. Increases in monthly flow data were also observed for all months with the strongest trends for the October through December period. Seasonal analysis showed significant increases in spring (March-May), fall (September-November), winter (December-February) and both increases and decreases in summer (June-August) runoff in the stations studied. Anomaly in summer flow patterns could be explained by summer operating rules of upstream dams. Changes in flow timing were also evident. The day of the year when 25, 50, and 75% of total annual flow passed occurred earlier compare to historical trends.

Although patterns in changes in streamflow in the Merrimack generally resembled the patterns at the HBEF, magnitude of changes in the Merrimack are not as strong as HBEF which is due to climate moderation, impoundments within the watershed and other human disturbances.