Connecticut Streamflow Overview
Know Your Flow!
Wednesday Sep 30, 2015: heavy rain will rapidly change stream conditions; may not help long term low flows.
Whenever there is a powerful storm, 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.
Although streamflow up to this week in Connecticut has been dangerously low (continuing the pattern described here on Labor Day), rivers and streams are now rising rapidly and many may hit flood stage. Before this week, rains have not provided even average precipitation over most of the state. We may even get the FOUR inches of rain by the end of the week that would bring 60 day precipitation up to normal, THEN get hit with a tropical storm! But when rain falls as heavy downpours much of it does not contribute to groundwater supplies. The long-term forecast predicts little, if any rain after this week. Many public water systems have asked for voluntary restrictions on water use (see below). Fortunately for water companies, heavy rain does get captured by reservoirs and this is why reservoir capacity did not drop as fast as the flow in more free flowing streams, but even they were beginning to see the effects of the long dry spell (discussion below). Unfortunately, even during low flows, local flooding can occur (also below) when a heavy rain hits unusually dry ground. Although flows obviously rise during and after storms, they drop rapidly when groundwater levels are low (see groundwater discussion below). Smaller upland streams dry up faster than the larger rivers (see the graph below).
Is this a true drought? The U.S. Drought Monitor weekly map for Sep 15 labeled 64% of CT as "D1-Moderate Drought", up from 37% the week before. This is the highest percentage in this category of any northeast state. The rest of the state is labeled as "D0-Abnormally Dry". No other northeast state is 100% drought-affected. Even general news media are starting to notice.
Conditions may get worse. The NWS Climate Prediction Center's October forecast gives us a 40% to 50% chance of below normal precipitation, a 33% chance of normal precipitation, and only 17% to 27% chance of the above normal precipitation we would need to keep our streams healthy. This is ominous. Usually these forecasts give us more hope for rain.
Streamflow between storms depends on groundwater levels. The water under the
ground (aquifer water) flows downward and comes back to the surface in streams
and wetlands. A low-flow stream drains water from the ground in most cases, but
if the ground is exceptionally dry the flow may reverse. 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.
Real-time data on groundwater in Connecticut is available from 7 monitoring
wells run by the US Geological Service (USGS). Falling water tables are
normal for summer. But before the heavy rain Sep 30, all 7 USGS wells are at levels below (in 4 cases
well below) their daily averages for this time of year. Moreover, the levels
have been dropping unusually quickly. The data are even more striking because
the records cover only the last 6 to 12 years, when prolonged dry spells were
The Durham well, the
Public Water Supplies: Several towns in CT have issued water restrictions. At the end of August, the CT Department of Health (DPH) issued a letter to public water systems in which they caution them to:
... keep a watch over the groundwater levels and surface water levels, assess the current capacity to maintain adequate supply & pressure, and as necessary institute water conservation plan and/or measures in order to effectively reduce water consumption. You may want to consider enacting voluntary water conservation measures while our state continues to be within the present weather situation.
Reservoirs provide about 2/3 of our water supply. In August, the public water supply reservoirs of the largest companies averaged 80% full, down from 90% in July. However, that July report did not include data from MDC, Middletown Water Company or Bristol Water Company. The August report states that Bristol Water has asked for voluntary water restrictions; their reserve was down to 70%. Southington reported 30% usable storage in August (down from 60% in July) but has withdrawn its voluntary water restrictions. UCONN's water department has issued mandatory conservation measures. The Manchester Water Department website reported 68.3% capacity as of Sep 21 and the same page says they would issue an Alert at 80% and an Advisory at 70%. Overall, of the 34 water companies listed by DPH, 7 reported below 90% of normal August capacity. Aquarion Water Company is asking its customers to voluntarily conserve water and stop non-essential outdoor water use. We have listed websites for many of the hundreds of water systems below if you want to check for water restrictions. Other water restriction sources include your towns website, your health district, and news outlets.
On Sep 21, 40 out of 53 (74%) of our ranked rivers and streams had less water flowing in them than 75% of their recorded flows for the date. 25 of those are lower than 90% of all their records, 8 are lower than all records for that date.
More about our rainfall: 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 of 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.
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.
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.
*On the USGS Stream Gage map, streams lower than their
10th Percentile and have records longer than 30 years are indicated with a
maroon dot. We choose rivers for our low-flow list by looking at all rivers and
streams Most Recent Instantaneous Value compared with their Minimum Average
Daily flow and their 25th Percentile rate. Those with a flow rate closer to
their Minimum than to their 25th Percentile get added to the list.
Rivers Alliance of Connecticut