Nagog Brook Had a Warm Year in 2021

by Kim Kastens and Song Leav

As Green Acton reported in November 2021, Nagog Brook in western Acton experienced unusually high stream flow in the summer of 2021. This post reports that during that same time interval, Nagog Brook’s water temperature was also unusually high.

A sensor installed by the Green Acton Water Committee has been collecting water temperature data in Nagog Brook every 15 minutes since August of 2017. The 2021 data differs from all previous years of our data collection in one important way:  The summer water temperature is substantially warmer than in 2017, 2018, 2019 or 2020.

To illustrate this point, the graphs below compare 2020 and 2021 Nagog water temperature, shown on the same scale. In 2020, the water temperature almost never exceeded 20°C (68°F), marked by the red line on the graphs. In 2021, the brook’s water temperature exceeded that threshold on 107 days.

A reader who has been following the news might ask whether we are seeing an impact of global climate change. Almost certainly not: the change is too rapid, and it shows up only in the water temperature, not in local air temperature data.

As Green Acton has previously reported, in the summers of 2017-2020 Nagog Brook’s water temperature stayed cooler than the air temperature as recorded at nearby Hanscom Field.  But in 2021, the Nagog summer water temperature was approximately the same as the air temperature. This shows up on the pair of graphs below, which compare 2020 and 2021 on the same scale. In the hottest months of both years, average air temperature (red dots) bounced around in the 20-30°C band. In 2020, the water temperature (blue line) stayed solidly below the air temperature on most summer days. In 2021, in contrast, the summer water temperature was in the same range as the air temperature.  

We attribute this difference to the rainy summer of 2021.  In drier years, much of the summer flow at Nagog Brook would have come from base flow, which is groundwater coming up through the stream bed. Groundwater stays at a near-constant temperature year round. In Massachusetts, that temperature is about 10°C or 50°F.  Input of that constant-temperature baseflow is why Nagog Brook resists freezing even during the coldest days of winter and stays cooler than air in a typical summer.

During the rainy summer of 2021, Nagog Brook’s streamflow stayed much higher than in the other summers in Green Acton’s dataset. In 2017-2020, Nagog Brook’s July and August streamflow rarely even covered the stream monitor’s sneakers (around 0.1 feet). In July and August of 2021, in contrast, the streamflow was often at or above the top of her tall rubber boots (more than 1.0 feet). The sneaker-high summer flow of earlier years was mostly base flow. The boot-high summer flow of 2021 must have had a substantial contribution of water that ran off the landscape and into the brook. In addition, there may have been occasional overflow across the spillway from Nagog Pond. Runoff and spillway overflow would have been in the same range as air temperature, and substantially warmer than groundwater temperature.

Our conclusion, then, is that in 2021 a large amount of near-air-temperature runoff diluted a small amount of cold base flow, resulting in the warmest summer water temperatures Green Acton has yet recorded at Nagog Brook.