Connecticut Streamflow Overview
Know Your Flow!
Thu June 25, 2015 Summary: As expected, regular rainfall over the past couple weeks has 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). Some are flowing well above average, only one on the map is still below average.
Does this mean the recent drought is done? Streamflow between storms depends on groundwater levels. The groundwater graphs for CT do show improvement in response to recent rains but are still a little below average for most of them with only one recovered to above average. One well is still at levels for today's date lower than any for the past six years. Poweful thunderstorms do 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. The slow melt of the large snowpack this past winter kept streamflow from falling very fast, but that water is long gone.
The U.S. Drought Monitor weekly map is based on conditions every Tuesday and comes out on Thursday. This week's map labels 62% of the state as "D0-Abnormally Dry" and the rest of CT in "D1-Moderate Drought". This is an improvement from the week before. It is likely that next week's map (if they do one over the holiday), will show more improvement if we get more rain, but we can expect some of the state to still be ranked abnormally dry.
According to the Advanced Hydrological Prediction Service of the National Weather Service, Connecticut rainfall during the last 30 days for the southeast coastal parts of the state has been around 75% what we normally have at this time of year, even with the recent heavy rain. The rest of state has received near or above average rainfall. One small area of northeastern CT got 2 to 3 times what is normal for the last 30 days.
Connecticut rainfall during the last 60 days, 90 days, even 180 days also show closer to average precipitation than previously, but most of the still still has not caught up to the rainfall deficit. Only that area in northeast CT is labeled average. It is the long-term rainfall totals that control groundwater levels, which in turn keep streams flowing between storms.
If we do not get the regular rain, flows can quickly drop (why? see below). Luckily, the long-term forecast shows that the good rainfall pattern will continue. The NWS Climate Prediction Center predicts that we have a 40% chance of above normal rainfall for July. Of course, this also means a 30% chance for normal and a 30% chance for below normal rainfall.
Whenever there are thunderstorms however, 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.
In May, our public water supply reservoirs averaged 96% full, a little lower than in April. Empoundments of 30 (29 in April) water companies were about where they usually are in May. 2 companies (4 in April) were more than 5% higher than average. The Middletown Water Department, New London Dept. of Public Utilities (Veolia NA), and the Bristol Water Company reservoirs were more than 5% under average (1 more than in April). However, groundwater provides for about half the state's drinking water, and monthly data for public water supply wells is hard to come by.
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."
Also, 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.
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