by Kim Kastens & Song Leav
The Green Acton Water Committee began monitoring the water temperature of Nagog Brook in the summer of 2017. After more than three years of data collection, we have established that the brook stays warm in winter relative to air temperature, and cool in summer, and rarely exceeds the temperature threshold for a cold water fishery resource.
Interest in the welfare of Nagog Brook emerged during the controversy over the Town of Concord’s plans to expand its water treatment plant on Nagog Pond. Water from the pond is discharged into Nagog Brook, which has been designated by the Massachusetts Department of Environmental Protection as a Cold Water Fish Resource. Amid worries that increasing the summertime withdrawal rate from Nagog Pond might harm the downstream ecosystem, the Water Committee began a volunteer program of stream monitoring. As detailed in a previous webpage, water temperature is measured automatically every 15 minutes and stream depth is measured weekly by a volunteer. This post presents findings from the water temperature measurements.
The graph below shows the 2020 data. The strongest feature in the data is the annual cycle. As any New England resident would expect, temperature was cold in January, rose markedly throughout the spring months, peaked during June–August, and then got colder again through the end of the year. Water temperature responds to changes in air temperature.
At first glance, it might look like the data in the graph above are really “noisy,” as though perhaps the instrument wasn’t very reliable. But an expansion of a portion of the data makes clear that what might have looked like noise is actually the daily temperature cycle. On most days, Nagog Brook is coldest in the morning and warmest in the afternoon.
The next graph again shows the 2020 data, but this time there are two horizontal lines superimposed on the graph. The upper (green) line (at 23.9°C) marks the recommended temperature limit for brown trout, and the lower (red) line (at 20.0°C) marks the recommended temperature limit for brook trout [OARS annual report]. Both of these species need relatively cool water to thrive. In 2020, Nagog Brook stayed cool enough for brown trout all summer, and exceeded the brook trout temperature threshold on eight summer days.
Twenty degrees Celsius is equal to 68°F. Every Acton resident knows that our air temperature gets much hotter than that in the summer. The next graph shows Nagog Brook water temperature as a blue line and air temperature as red dots. The air temperature data come from Hanscom Field in Bedford, chosen because it is relatively close by and has a long, continuous record of publicly available data. Each red dot is the average temperature for one day. The first thing to notice is that the air temperature is more volatile than the water temperature. In January 2020, for example, the air temperature daily averages reached up to +16°C and down to -9°C, while the water temperature stayed within a much narrower range. Water has a high heat capacity, which means it has to gain or lose a lot of energy to change its temperature even by one degree. It takes much less energy to change the temperature of a comparable volume of air. As weather systems pass across Acton bringing cold fronts and warm fronts, the average daily air temperature changes drastically, but the water temperature changes much less.
On the annual scale, there is another important difference between the air temperature and water temperature. Across June, July, and August of 2020, daily average air temperature was hotter than any of the water temperature measurements for nearly every day (all but three of the red dots for these months lie above the blue line). This part of the signal has a different explanation. Even after many days without rain, and even when the Nagog Pond spillway is dry, Nagog Brook keeps flowing. This indicates that a substantial fraction of the water in Nagog Brook comes from base flow, which is groundwater entering the stream from underneath the bed of the channel, rather than flowing overland in the stream channel. Groundwater in Massachusetts has a near-constant, year-round temperature of approximately 10°C (50°F). While hot summer air and abundant sunshine are pulling the temperature of Nagog Brook upward, the steady input of cold groundwater is pulling the temperature downward.
Base flow continues to deliver 10°C water to the brook during the winter, and in winter that 10°C water is usually warmer than the overlying air temperature. Although Acton winters have many days in which the average air temperature is below freezing (red dots below 0°C on the graph above), there have been only two days in our four winters of weekly observations when the volunteers have observed the brook to be frozen completely over at our observation site. Even after many sub-freezing days in a row, the brook is almost always found to be burbling along merrily, as documented by video from our first winter of observations. Even when the surface of the brook is frozen, the water underneath remains liquid and flowing; thus, the temperature at our temperature sensor has never gone below freezing.
Data from each of the older years look broadly similar to those in 2020. In every year, there is the prominent cold/warm/cold pattern of the annual cycle, with the high-frequency jitter of the day/night cycle superimposed. In any given year, there are also hot spells and cold spells, when the temperature is unseasonably warm or cool for days to weeks as weather systems pass over. The hottest summer in our Nagog Brook database was 2019, with 26 days above the brook trout temperature threshold and three days above the brown trout threshold. Nagog Brook wasn’t alone in this regard; NOAA reported that the entire Northern Hemisphere had its second hottest summer on record that year, and mapped our area with the designation “much warmer than average.”
The overall picture that has emerged from our observations is that the water temperature is a muted echo of the air temperature. Air temperature swings wildly up and down, but both the high heat capacity of water and the steady inflow of groundwater tend to dampen the temperature swings of Nagog Brook water temperature. When the air gets hotter, the water gets warmer — but generally not too warm for brown trout. When the air gets colder, the water gets cooler — but generally not cool enough to freeze.
As we enter a new season, Green Acton’s Nagog Brook monitoring team will continue with data collection. Watch this space for a web post about Nagog steam flow data.