On July 21, 2022, the Massachusetts Drought Management Task Force declared that the Northeast Region of Massachusetts, including Acton, had moved into Level 3 Critical Drought status. The Drought Management Task Force (DMTF) uses six different types of information in deciding when to declare a drought. This post takes a look at how these six indicators have been doing in Acton and surrounding regions in this drought-y summer.
People tend to assume that dry weather becomes a “drought” when rainfall drops below some threshold amount. But in fact, “drought” is a more-nuanced concept than a simple “how much precipitation has fallen recently.” No single metric, or even combination of metrics, captures the full concept. Instead, the DMTF considers seven indicators: Precipitation, Groundwater, Streamflow, Lakes and Impoundments, Evapotranspiration, and Fire Danger (see Note 1).
As spelled out in the 2019 Massachusetts Drought Management Plan, the state is divided into seven drought regions, based on historical precipitation patterns. Acton sits in the Northeast Region, which spans Middlesex, Essex, and Suffolk counties. When conditions are dry or drying, the DMTF convenes, reviews data on each of the indices for each region, and deliberates until members agree on a consensus drought level for each region. This strategy may seem rather qualitative, but it is an accepted approach when a decision needs to be made that requires comparing multiple and dissimilar types of information; an analogous deliberative approach is used to determine both earthquake risk and the point at which the economy has entered into a recession. Presentation slides from the most recent meeting (Aug 8, 2022) of the task force are here.
Categories used by the DMTF are Drought Level 0 (Normal / No Drought), Level 1 (Mild Drought), Level 2 (Significant Drought), Level 3 (Critical Drought), or Level 4 (Emergency Drought). Much of Massachusetts, including Acton, is now in Level 3: Critical Drought. The last time the Northeast Region was at this level was from July 2016 through January 2017 (see Note 2).
Now let’s take a look at how these indices have been faring in Acton this summer.
Precipitation
To get a handle on how recent and not-so-recent rainfall has impacted drought conditions, the DMTF looks at an index called the Standardized Precipitation Index (SPI). This index compares the amount of precipitation over the past several months with the multi-year average amount of precipitation for that place and that time of year. SPI can be expressed in several ways, but DMTF uses percentiles. SPI is calculated across multiple time spans: the DMTF looks at SPI across the previous one, two, three, six, 12, and 24 months, because drought conditions accumulate over time.
Concord and Littleton both have stations that are part of the Massachusetts Precipitation Monitoring Network. As of the end of July, the 1-month SPI for both of those sites was between the 10th and 20th percentile, as shown on the map below. The situation was even worse east of us.
Streamflow
The U.S. Geological Survey (USGS) maintains a stream monitoring station on Nashoba Brook off Wheeler Lane in Acton. The gage height of the water is automatically measured every 15 minutes; this measurement is then converted into discharge, measured in cubic feet per second. On August 4, 2022, the discharge of Nashoba Brook was 0.15 cubic feet per second (very close to one gallon per second). For comparison, the median discharge on this date, averaged over 58 years of data, is 2.7 cubic feet per second.
As shown in the graph below, Nashoba Brook discharge has, on average (yellow dots), been high in the winter/spring and low in the summer/fall. However, in the summer of 2020 the discharge was much lower than average. Then, in the summer of 2021, discharge was much higher than average. For 2022, discharge is again running much lower than average.
Groundwater
Groundwater is water found underground in the cracks and spaces in soil, sand, and rock. Since 1964, the U.S. Geological Survey has maintained a groundwater monitoring well in Acton, off of Wetherbee Street. The data record the distance (in feet) from the land surface to the the depth of the water table. In most years, the water table drops the farthest below the land surface during late summer and fall, and then rises again in winter and spring. The driest measurement (water table farthest below the land surface) was during the terrible drought of the mid-1960s, when the Wetherbee Street well recorded a 22 foot depth to water surface (the well is 33.8 feet deep). The wettest measurement (shallowest water table) of 13 feet was in the wet year of 2010.
On August 3, 2022, the depth to the water table was 19.09 feet. As shown in the graph below, the current water depth (red line) is almost a foot below the historical daily median (green line) for this time of year, and is dropping fast. But it is far from the driest ever observed at this time of year (the lower edge of the sky-blue band).
Lakes and Impoundments
Acton doesn’t have any lakes, but we do have at least six significant impoundments, which are bodies of water formed by, or enlarged by, a human-constructed dam (see Note 3). Most of Acton’s impoundments are created by old, extant mill dams built for hydropower. During times of medium or high streamflow, the water flows out of these impoundments over a “spillway,” or low section of the dam. The elevation of the spillway controls the water level in the impoundment, keeping the water level at a near-constant elevation.
A tour of Acton’s impoundments on August 5 revealed that water levels of most were below the level of their spillways. An exception was the dam controlling Ice House Pond, which had a tiny trickle of water still flowing over the spillway. The photos below show the dams of Nagog Pond and Robbin’s Mill Pond. In both impoundments, the top of the spillway was dry and the water level in the impoundment was many inches below the level of the spillway.
The Town of Concord draws some of its drinking water from Nagog Pond, and thus, they keep a careful record of water level. According to data provided by Concord Water & Sewer Superintendent Jeff Murawski, the level of Nagog Pond has been averaging about nine inches below the crest of the spillway so far this year. The last time our region reached this drought level, during the summer of 2016, the pond surface averaged about a foot below the level of the spillway crest. (See Note 4).
Evapotranspiration
Evapotranspiration refers to the combination of processes by which water moves from the Earth’s surface into the atmosphere. The term encompasses the physical process of evaporation, by which water moves directly to the air from from soil, water bodies, etc., and the biological process of transpiration, in which water enters the air via plants. The Drought Management Plan calls for the use of an index called the Crop Moisture Index (CMI). The CMI is intended to capture short-term status of agricultural drought, and is calculated from the weekly precipitation total, weekly mean temperature, and the previous week’s CMI value.
For the week ending August 6, the CMI for Acton’s region (Central Massachusetts) was -1.94, according to NOAA’s Climate Prediction Center. For the CMI, negative numbers indicate a deficiency of moisture, in other words, dry conditions. The range from -1.0 to -1.9 is characterized as “Abnormally Dry.” The same source provides “Precipitation Needed to End Drought,” which for that same week was 11.5 inches.
A walk around Acton confirms that crops and ornamental plants are struggling with the high temperatures of the recent heat wave and sparse rainfall, as shown in the photos below.
Fire Danger
For fire danger, the Drought Management Task Force uses the Keetch-Byram Drought Index (KBDI). This index is intended to estimate how much moisture there is in the soil available to vegetation, with more moisture equating to less danger of forest fire or brush fire. The KBDI for a given time and place takes into account mean annual precipitation for that location, plus amount of precipitation and maximum air temperature during the previous 24 hours. Note that air temperature does not enter into Massachusetts’s drought declaration process directly, but it does come into consideration indirectly, through both the KBDI (fire) and the CMI (crops). The KBDI scale runs from 0 (fully saturated soil with no moisture deficit) to 800 (maximum possible drought condition).
Acton falls in the North Middlesex Fire District. On August 6, the KBDI for our district was 602, according to Gregory Whittier of the Massachusetts Bureau of Forest Fire Control and Forestry. A KBDI between 600 and 800 is described as “often associated with more severe drought with increased wildfire occurrence. Intense, deep burning fires with significant downwind spotting can be expected. Live fuels can also be expected to burn actively at these levels.” By August 11, after a few small rains, the KBDI for eastern Massachusetts had dropped into the 500–600 KBDI range. Compared to most of California and parts of south Texas and Oklahoma, eastern Massachusetts is looking not so bad on the map below.
Despite the relatively high risk, Acton has not seen an uptick in fires. By email, Acton Fire Chief Robert Hart told Green Acton: “Acton is a fairly green community with plush lawns and well cared for landscaping surrounding most structures in town. This ‘green’ helps to reduce the risk of wildland fires, or brush fires, as we refer to them. We have not seen an increase of brush fires in Acton as a direct result of the drought. The responsible behavior of our residents reflects favorably on us not having an uptick in brush fires. We do worry about the impacts of having little rain and what the future will hold as a result of this. Water supply is always a concern as we rely immensely on this resource to suppress any fire that may occur in town.”
Local Consequences of Drought for Water Use
The Acton Water District (AWD) has Outdoor Watering Restrictions in place, both because of the current drought and to give the district more operational flexibility as it blends water sources to keep PFAS levels below the Massachusetts maximum contaminant level (MCL). Properties served by the AWD can water one day per week, before 7am and after 7pm, with the day specified on this map. Some properties along Great Road in Acton get their water from the Concord Water Department; they can water their landscaping one day per week, on Tuesdays. The 5% of Acton properties that get their water from private wells rather than from public water supplies are not currently required to follow drought-related outdoor watering restrictions. State Senator Jamie Eldridge sponsored a bill (S.530) that would have resulted in water conservation measures applying to all water users within a drought region. This bill did not pass in the just-completed legislative session; we hope for a better outcome in the next session.
For the Future
The Climate Change forecast for the Northeast U.S. anticipates that precipitation will become more volatile, with more-frequent droughts as well as more-frequent torrential downpours. Even if the average precipitation per year increases, the number of summer droughts is forecast to increase due to earlier snowmelt, increased evaporation, and more runoff — the latter caused by heavier summer rainfall. We are fortunate that Massachusetts has put in place a system to monitor drought status and issue timely warnings, and that water users in Massachusetts are becoming more attuned to the need to conserve water, especially during droughts. Acton’s own drought mitigation preparations are included in the Town of Acton Hazard Mitigation Plan (2018 update). The every-five-years update of this plan will begin soon, offering an opportunity to strengthen our resilience to both droughts and flooding.
Acknowledgments
For help in obtaining information in the Fire Danger section, thank you to Acton Fire Chief Robert Hart, Carlisle Fire Chief Bryan Sorrows, and Gregory Whittier of the Great Brook Farm State Park unit of the Massachusetts Department of Conservation & Recreation, Bureau of Forest Fire Control and Forestry. For providing summer water level data for Nagog Pond, thank you to Jeffrey Murawski, Water/Sewer Superintendent for the Town of Concord.
Notes
Note 1: In addition to the Massachusetts Drought Status, there is a separate U.S. Drought Monitor, which assigns five numerical levels to drought conditions. As of August 11, Acton was in D2 (Severe Drought), which is the middle of the five categories on that scale.
Note 2: During the drought of 2016–2017, the Massachusetts Drought Management Plan of 2001, which used categories of Normal, Advisory, Watch, Warning, and Emergency, was in effect. From Aug 2016 through Jan 2017, the Northeast Region was declared under “Drought Warning.” The current system of numerical levels (0 through 4) came into play with the revised Drought Management Plan of 2019. “Drought Warning” in the old system is comparable to Level 3 in the new system.
Note 3: Significant dams with impoundments in Acton include: (1) the High Street Dam on the Assabet River in the southernmost corner of Acton; (2) Nagog Pond on the border with Littleton; (3) the Robbins Mill Pond in the Nashoba Brook Conservation Area; (4) Ice House Pond by the East Acton Village Green; (5) Fort Pond Brook Reservoir behind the Erickson’s Mill Dam; and (6) the slated-for-removal dam at 53 River Street.
Note 4: The elevation of the crest of the spillway in the dam at Nagog Pond is 225.6 feet, according to a presentation to the Acton Board of Selectmen on September 16, 2016. Nagog Pond water level data (provided by Concord’s Water & Sewer Department) averages across the hot months of June, July, and August, and spans from 2015 to 2022. Not directly relevant to drought, but also of interest: during the wet summer of 2021, the pond’s average water level was slightly above the spillway crest. Data values (in feet of elevation relative to NAVD/North American Vertical Datum 1988): 2015, 224.80; 2016, 224.60; 2017, 225.58; 2018, 224.84; 2019, 225.36; 2020, 224.86; 2021, 225.74; and 2022 (so far), 224.82.