Connecticut Streamflow and Groundwater Overview
Know Your Flow!
April 21, 2016: recent normal flows may be drying up. Since our mid-March Know Your Flow! update, streamflow in Connecticut has begun to drop from the dramatic improvement seen in the first three months of 2016. According to the National Weather Service Advanced Hydrologic Prediction Service at this link (choose CT from the state drop-down list to see current data), precipitation over the first 90 days of the year was at or close to average over the whole state. Since the middle of March however, total precipitation has dropped below 75% of normal over the western half of CT. Groundwater levels are still near average but are lower than in mid-March. No extreme preciptation events were recorded by CT weather stations since mid March.
The 50 stream gages at the website
Water Data for Connecticut normally show pulses of
above average flow when it rains, then below average between storms. Dry
conditions that cause flow to be below average more than it is above will
be affect stream life if it continues for too long. For more information on the
relationship between streamflow and precipitation, see below. When we put their maps from three months ago next to the March 17 map, we can see the
distinct improvement by the color of the dots, but the more recent map at the
right is not so normal :
Groundwater: The water level in streams and rivers between storms depends on the flow of water out of the ground into the stream channel. The higher the water table, the more water is available to keep streams flowing. During a drought, smaller upland streams dry up faster than the larger rivers (see the graph below). Groundwater levels recovered during the first three months of the year, but now may be dropping again.
Real-time data on groundwater in Connecticut is available from seven monitoring wells run by the US Geological Service (USGS). Five wells report levels below average for the date although only two of those are below 75% of their records. This continues the pattern of the last four years in which normal water levels in winter and spring have alternated with below-average in summer and fall. What is worrisome, however, is that the below-normal levels got worse each of those four years. Four years is not long enough to make any statistically valid conclusions about climate, however, and those station records only go back seven to 14 years. Hopefully, the El Nino affecting us right now has broken the pattern (if there is a pattern).
The dire situation last year (see below) was caused not only by less total rainfall than normal over a long period of time but also by the intensity of the rain we did get. An occasional brief, powerful rainstorm does not do much to help groundwater levels rise. The rain falls faster than the ground can absorb, and thus much of it becomes stormwater runoff. In that situation, public water supply systems that depend on reservoirs are affected less than those with wells because the runoff is collected by the reservoir.
Since the beginning of this year only three weather stations in CT have reported extreme precipitation events to the Northeast Regional Climate Center (NRCC), all one-year recurrence events. This means that station can expect rainfall to exceed that rate on average about once per year. Over the same amount of time at the end of 2015, five CT weather stations reported 1 one-year events, 3 reported two-year events (50% chance of occurring in any year), and 1 ten-year precipitation event (10% chance of occurring).
The amount of rainfall that has come during extreme precipitation events has risen faster in the northeastern United States than in any other region of the nation. Whenever there is a powerful storm, watch for flooding! Flash flooding can easily take people by surprise. Remember, if water is across the road, Turn Around, Don't Drown! See below for general flooding information.
Many public water supply companies asked for voluntary or mandatory restrictions on water use last year. The most recent monthly report (February 2016) from the CT Department of Public Health, however, reported no restrictions, with state reservoirs at 71% to 100% of their capacity.
Still a drought? The U.S. Drought Monitor weekly map for April 19 labels 53% of Connecticut as "Abnormally Dry". The portion of CT labled that way has increased since March 15, which labeled only 30% of the state as "Abnormally Dry," the same as the previous week. That was the lowest percentage of the state since last April. Every week between then and March 8, 2016, more than half the state was labeled as "Moderate Drought," and more of the state was in that condition than any other Northeast state. During some weeks, we were the driest state east of the Rockies.
Will the healthy rain storms continue? The NWS Climate Prediction Center's three-month prognostication for April, May, and June gives equal chances for normal, or above average, or below average precipitation. These forecasts said nearly the same thing for most of the dry period last year, yet we consistently got less than normal precipitation.
For a good analysis of last year's stream problems across the state, see this press release from Pomperaug River Watershed Coalition (PRWC). Here is a key quote:
The September and October low river flows demonstrate that rivers were not supporting the critical flows needed for a healthy river habitat. In fact, sections of the Weekeepeemee River had dry river beds and the Pomperaug River was flowing only at a rate of 15% of the critical flow requirement. DEEP officials confirmed that they would expect fish mortality to increase significantly during such conditions...
Meanwhile, parts of Coppermine Brook in Bristol were drawn down to rocks and dirt,
as shown in the photo below taken Dec. 11 in Bristol. The sandy ditch in the
foreground is the stream channel. Bristol Water Company and New Britain Water
Company have eight registered diversions near Coppermine or its tributaries with a
combined registered water withdrawal of 36.6 million gallons per
(source: CT DEEP). Company representatives report
they do not withraw anywhere near that much water. Bristol Water says
they stopped pumping from their well near this photo for a day but with no
apparent effect on the stream. This is a perennial stream
with a state Trout Management Area below where this picture was taken. Click on
the photo for a full-screen version in a new tab.
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."
The Connecticut Department of Health website has a good guide called:
There are places in Connecticut where the storm drains and the sewage pipes are combined into one system. With high rainfall, many of these combined pipes are designed to overflow into rivers and streams so the wastewater treatment plants are not overwhelmed. You really do not want to be downstream when raw untreated human sewage is entering the water. The DEEP has a map of Combined Sewer Overflows that shows the six urban areas where these can occur. Zoom in to any of them to see exactly where the combined flow may enter streams and rivers. Not every rain event is enough to cause these overflows, but it's a good idea to avoid contact with the water downstream from them after significant rain.
Unless you are an expert paddler, do not attempt to canoe or kayak on floodwaters; there are usually one or two fatalities per year. Our Connecticut Water Trails website and the webpages of the many paddler groups in the state all have good safety procedures. Here is a good article.
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.
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.
Photos by Joan Smith GOSA taken 9/20/2015 at The Merritt Family Forest in
Groton. The stone slab bridge lies over Eccleston Brook, which had only a few
puddles despite last week's downpour. The muddy bottom is also part of EB,
further downstream. The photo with the two upright stone slabs is Cowslip Brook,
a tributary to EB, and the wooden bridge traverses another small tributary to
EB, flowing from a vernal pool. Used with permission.
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