Connecticut Streamflow Overview
Know Your Flow!
Wed Nov 18, 2015: Long-term low flows continue, in spite of brief heavy rain.
Since our Know Your Flow! update last month, streamflow (between storms) has continued to be well below normal for most of our rivers and streams. On Nov 18, 84% of them were flowing at rates lower than 75% of all records for that date. Of those, most were actually lower than 90% of the averages for the date. Only 8 of the 52 USGS stream gages on their CT map were reporting flows anywhere near normal, and even those were below average. Click the map to the right for the current conditions.
Much of what rain we have received lately has fallen as heavy downpours. Much of this runs off instead of soaking into soil to become the groundwater that keeps streams flowing between storms. 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). 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), most of the state is still getting roughly half of normal precipitation, whether measured in one-month or many-month time scales.
The long-term forecast predicts little rain after tomorrow's inch or so. 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.
Many public water systems have asked for 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 are 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.
Is this a true drought? The U.S. Drought Monitor weekly map for Nov 10 labeled almost all of CT as "D1-Moderate Drought" that affects 3.4 million CT residents. This is the highest percentage in this category of any northeast state. Rhode Island is the only other northeast state other than CT that is 100% drought-affected, but they are only "Abnormally-Dry". Most of CT has seen these moderate drought conditions for the last two months. Most or all of the state has been abnormally dry for six months. Even general news media are starting to notice.
The NWS Climate Prediction Center's three-month prognostication issued mid-October gives equal chances for normal or above average or below average precipitation. These forecasts have said nearly the same thing for most of the past six months yet we actually consistently got less than normal.
Groundwater: 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). 3 of the 7 are at levels lower than any for the past five or ten years. The other four are all lower than 75% of their records over the past 6 or 11 years. The data are even more striking because the records cover only the last 6 to 12 years, when prolonged dry spells were already common.
Public Water Supplies At the end of August, the CT Department of Health (DPH) issued a letter to public water systems in which they cautioned 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. According to the October 2015 - Monthly Reservoir Status Summary from the CT Department of Public Health, the public water supply reservoirs of the largest companies averaged 70% full. That report states that Bristol Water is down to 60% usable storage and has issued mandatory restrictions on water use. Aquarion Water Company is asking its customers to voluntarily conserve water and stop non-essential outdoor water use. Southington reported 30% usable storage, the lowest in the list, but its website says it has withdrawn their request for voluntary water restrictions. Several other companies are lower than usual: New London Dept. of Public Utilities reported 35% (no restrictions), Second Taxing District City of Norwalk 43% (no restrictions), CTWC - Shoreline Region-Guilford System 47% (no restrictions), New Britain Water Department 52% (no restrictions), Middletown Water Department 54% (no restrictions), Norwalk First Taxing District 56% (voluntary water conservation), Waterbury Water Department 58% (no restrictions), Regional Water Authority 59% (no restrictions).
UCONN's water department has issued mandatory conservation measures. The Manchester Water Department website reported 66% capacity as of Nov 9 and the same page says they would issue an Alert at 80% and an Advisory at 70%. 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 town's website, your health district, and news outlets.
On Sept. 21, 2015, 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 were lower than 90% of all their records, 8 were 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 storms. 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