Connecticut Streamflow Overview
Know Your Flow!
Tue Oct 7, 2014 Summary: So far, October has been very good for Connecticut's streams and rivers. According to The Advanced Hydrological Prediction Service of the National Weather Service, Connecticut rainfall during the last 30 days (as of Oct 7) was back to average for the south-central section of the state, above 50% of average for over half the state, with only the northwest and southeast corners below half of normal rainfall. This is a great improvement from previous weeks when the entire state was more than 75% below average.
Connecticut rainfall during the last 60 days shows that the rainfall deficit, though more evenly distributed, is still below average for almost the entire state. It is the long term rainfall totals that control groundwater levels that in turn keep streams flowing between storms.
The USDA Drought Monitor, based on data from one week ago before the most recent significant rain, shows 45% rated "D1 - Moderate Drought", 45% at "D0 - Abnormally Dry", with only the northwest 17% of the state is anywhere near normal. They also reported: "Population Affected by Drought: 1,111,537". Their report for this week will probably show much improvement.
The most recent numbers available for CT's reservoir status are over a month old from August and show that of the 34 public water supply reservoirs monitored, 17 were below where they normally were in August. Though their levels do not seem too bad as a percentage, the numbers indicate that half of our water supply empoundments average only 75% of their capacity (median 78%). The meaning of these numbers is bit obscured however because the state and water utilities insist on keeping secret the amount of water available to Connecticut residents. It is difficult to know how much water is really being referred to when the Department of Public Health publishes their percentages.
Remember that groundwater provides for about half the state's drinking water as well as streamflow between storms. Groundwater levels are still very low, but have leveled off or are starting to rise. All the USGS measuring stations in CT are still recording levels lower than average for this time of year. Click here to see the groundwater graphs. Even if we do get a thunderstorm or two in the near future, it will not change the groundwater levels very much because soil has a maximum rate it can absorb water. If more rain falls in any given time than can be absorbed, the extra runs off the land and does not contribute to groundwater. Longer, less intense rainfall is what recharges our aquifers.
The USGS streamflow state map shows fewer of the red symbols for flow rates under the 10th percentile which means less than 20% of the average for this time of year. Many of the streams with green symbols that represents flows within 25% of average are actually lower than the numerical mean.
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 they have had decent rainfall, or on streams 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. Watersheds with lots of impervious roads, roofs, parking lot, patios, and compacted soil in lawns typically cause a lot of the rain to flow over the surface or through storm drains directly into the streams, leading to flooding problems far more often than natural surfaced watersheds. Their streamflow then drops quickly after the storm ends and the flow levels off far lower than streams with more natural surfaces in their waterheds.
Even though its dry now, Watch for Flooding when it rains!
When floodwaters return to use that 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 back yard. It is very important that any well that was flooded be pumped and flushed out thoroughly and the system sanitized or "shocked". The Connecticut State Department of Health website has a good guide called: Flooding: Information for Homeowners About Private Wells, Sewage and Clean-Up
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 its a good idea to avoid contact with the water downstream from them after significant rain.
Please note that 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 are traditional terms that 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 each and every year there is a one-percent 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 maybe the design criteria for managing runoff should be updated if we are building our landscape to control the flow from a 100 year storm.