Connecticut Streamflow Overview
Know Your Flow!
Thu Sep 4, 2015 Summary: worsened stream conditions despite T-storms.
Streamflow in Connecticut is generally low, continuing the pattern described here last week, earlier this month, at the end of last month. Although flows obviously rise during and after thunderstorms, they drop rapidly afterwards if groundwater levels are low. Recent rains have not provided even average precipitation and so do not make up for the rain deficit this year, and they barely affect the groundwater that supplies streamflow between storms (see the groundwater discussion below). The long-term forecast predicts normal rainfall for the next two weeks but the more reliable short term forecasts predict almost none, and if the rain falls as heavy downpours much of it will not contribute to groundwater supplies. Fortunately for water companies, heavy rain does get captured by reservoirs and this is why reservoir capacity is not dropping as fast as the flow in more free flowing streams. Unfortunately, even during low flows, local flooding can occur (see below) when a heavy rain hits unusually dry ground.
Can we call this a drought? The U.S. Drought Monitor weekly map for Sep 1 again labeled 23% of CT as "D1-Moderate Drought" that affects almost eight-hundred thousand CT residents. 72% of the state is labeled as "D0-Abnormally Dry", the same as it has been for two weeks, but a major jump from 39% three weeks ago. Even general news media are starting to notice.
Groundwater: Streamflow between storms depends on groundwater levels. Live well data is available from 7 wells from the US Geological Service (USGS), but these records only go back six to twelve years. Falling water tables are normal for Summer, but all 7 wells show current levels below and falling faster than their daily averages for this time of year. 5 of those 7 groundwater graphs for CT show groundwater levels lower than 75% of the daily records for those wells. The Durham well is at a level for late August lower than any for the past six years. The other two wells are below average but not down to their 75th percentile (yet). The Southbury well is near average, but it has only 9 years of record and a couple of those years were so unusually low that what is called average for that area is questionable. An occasional poweful thunderstorm does not help groundwater levels very much. This is because heavy rainfall, above the maximum infiltration rate of the surface, runs off into the surface waters.
On Sep 4, 39 of the 51 ranked streams and rivers on the USGS streamflow map of CT were flowing at rates lower than 75% of all the records for that date. That number was 27 a week earlier. Of those, 26 (up from 11) were lower than 90%, and 6 of those, Coginchaug River at Middlefield (Middletown), Sasco Brook near Southport, Pawcatuck River at Wood River Junction RI, the Salmon River near East Hampton CT, and the Little River near Hanover, CT were at or lower than ANY daily average for the date. Interestingly, the dot representing the Pawcatuck River at Westerly RI, has been alternating red (lower than all records) with blue (greater than 75% of records) probably due to hydroelectric dam power generation.
Also, the Aspetuck River at Aspetuck (Easton), Byram River at Pemberwick (Greenwich), Coginchaug River at Middlefield (Middletown), Latimer Brook near Flanders, Pootatuck River at Berkshire (Newtown), Ridgefield Brook at Shields Lane near Ridgefield, Still River at Route 7 at Brookfield Center, Sasco Brook near Southport Weekeepeemee River at Hotchkissville (Woodbury), and the Willimantic River at Merrow Rd (Mansfield) were all lower than their lowest daily averages, but those stations have short record time spans and so do not show up as a bright red dot on the state map. Only three of these were in this list 10 days ago.
Here is the list of all stream gages were recently showing flow rates that were close to their lowest recorded for late August - early September. Names in bold are those added since last week.
Aspetuck River at Aspetuck (Easton)
The NWS Climate Prediction Center predicted in mid-August that we have equal chances of above, or normal, or below rainfall for September, so no clues to the future there.
In July, our public water supply reservoirs averaged 90% full, down from 96% in June. Empoundments of 21 (29 in June) water companies were about where they usually are in July. 6 companies (5 in June) were more than 5% higher than average. Aquarion Water Co of CT-Stamford, the Regional Water Authority, the Southington Water Department, the New London Dept. of Public Utilities (Veol) reservoirs were more than 5% under average (only 1 in June). However, groundwater provides for about half the state's drinking water, and monthly data for public water supply wells is hard to come by.
More about our rainfall: Whenever there is a thunderstorm, watch for flooding! Flash flooding can easily take people by surprise after dry periods. Remember, if water is across the road, Turn Around, Don't Drown! See below for general flooding information.
At the beginning of July, regular rainfall brought most of our rivers and streams back to the normal pattern of varying above and below the average flows measured for any particular date (see an explanation for this below). However, below-normal rainfall has caused a steady decrease in steamflow punctuated by occasional spikes in the flow rates from thunderstorms. According to the Advanced Hydrological Prediction Service of the National Weather Service, rainfall during the last 30 days has been near or a little above average only along the paths of recent storm cells. Most the state has received between 25% to 75% what we normally have at this time of year. The last 60 days of precipitation records shows about half of the state receiving under 75% of normal even though June was rainy.
Our total rainfall has been below normal for a long time. The last 90 days, 180 days, even back to last October shows why we are still experiencing long-term near-drought conditions. It is the long-term rainfall totals that control groundwater levels, which in turn keep streams flowing between storms.
Some General Flood Information: When floodwaters fill the part of the stream channel that is called a floodplain, the water may find that someone has built a building or two in the channel. The water then saturates and fills any leach fields, often flushing untreated sewage out into the flow.
The floodwaters also find these really nice holes in the ground called wells to flow down into. All kinds of interesting things can be delivered to the bottom of the well, such as the aforementioned sewage, and soil, bugs, leaves, pesticides such as weed killer and insect poison, even the dog droppings from the backyard.
Very important: Any well that was flooded should be pumped and flushed out thoroughly and the system sanitized or "shocked."
As the graphic to the right illustrates, where a storm moves across the state, the streams show flows that peak above their averages for this time of year but then quickly drop. The streams that maintain healthy flows between storms are those draining the parts of the state where there has been decent rainfall or those controlled by managed dams.
How quickly the flow drops after each rainstorm is unique for each stream, because it depends on how much water soaked in to increase the groundwater baseflow. In watersheds with lots of impervious roads, roofs, parking lots, patios, and compacted soil in lawns, a lot of the rain typically flows over the surface or through storm drains directly into the streams, leading to flooding problems far more often than in watersheds with mostly natural surfaces. Their streamflow then drops quickly after the storm ends, and the flow levels off far lower than in streams whose watersheds have more natural surfaces.
What IS Normal?
The National Oceanic and Admospheric Administration (NOAA) uses a rolling 30-year average of modern rainfall data to calculate its "normals." A discussion of the 1981 to 2010 precipitation calculations can be found at http://prism.oregonstate.edu/normals. To see Connecticut's average annual precipitation, go to http://prism.oregonstate.edu/gallery/view.php?state=CT_RI.
Recently there has been much discussion of what constitutes a "100-year-storm" or a "50-year storm." These traditional terms can be misleading unless you keep in mind they are an old-fashioned way of describing probablities. A "100-year" rainfall event means that in any year there is a 1% chance of that rate of rainfall.
Here is a map showing that in Connecticut we have a 1% chance of getting 7 to a little over 8 inches of rain in a 24-hour period, depending on where you are in the state. (Image source: http://precip.eas.cornell.edu/)
Comparing this to Table 7.2 (copy below) of the CT Stormwater Manual, we note that the design criteria for managing runoff possibly should be updated if we are building our landscape to control the flow from a 100-year-storm.
State of CT - Drought (Search)
Water Conservation tips
Water Conservation is not just for droughts; it is important because:
What Can I Do?
Model Water Use Restriction Ordinance (PDF, 28KB)
Connecticut's Draft Drought Management Plan is being updated by the CT Water Planning Council Advisory Group Drought Plan Work Group.
Rivers Alliance of Connecticut