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Summary: as of Oct 16, overall streamflow is still at very high levels, but peak streamflows during storms has been decreasing since the heavy rains of Oct 1-3 brought flooding and other problems to many communities in Connecticut. The low point between storms is also dropping, and groundwater levels, though high, are generally steady or falling slightly. These trends suggest we will not soon see widespread flooding unless weather patterns change. A computer model is predicting little precipitation until a nor'easter in the last days of the month.
Trends: the graph below is an example of Connecticut streamflow patterns since the beginning of August when periods of unusually heavy rainfall were separated by 2 dry weeks. The first three weeks of August saw high streamflow during storms as expected, but in most cases streamflow did not drop back to normal before the next storm hit. Then, during the last week of August and the first week of September, storms were diverted around us by a persistent high pressure system, causing flows to fall steadily until most stream gages were reporting near normal flow rates for the time of year.
Since September 7, storms have consistently tracked over Connecticut, resulting in streamflow peaking at higher and higher levels and coming often enough that the flow rates between storms also became progressively higher. 11 of the 77 USGS gaging stations in or near Connecticut reported flow rates that exceeded the two-year recurrence level (a level not expected statistically to occur more than once every two years) for those stations. 6 of those also exceeded their ten-year occurrence level. The result of new rainfall on saturated soil running off to streams still high from previous storms was that more flooding occurred with every storm. From Sep 12 to Oct 2 there were 64 reports of flooding to the National Weather Service and many news stories. There is some general information about flooding below. Remember: Turn Around, Don't Drown!
Click the graph below to see all CT stream gage graphs for Aug 1 - Oct 15 (very large download, takes a while to load).
On Oct 8 (between storms), 40 of 63 USGS stream gages in CT were reporting very high streamflow, 22 gages were at high flow. 10 of those gages were still at flow rates higher than any average daily flow recorded for the date, even several days after the most recent rain. We define high flow as at or over their 75th% percentile for that date, very high are 90% or higher, from the data file at https://waterwatch.usgs.gov/webservices/realtime?region=ct&format=xml when rivers are near their low points between storms.
Why do we count? The flow between storms is an indicator of long-term stream conditions. Although expansion of a river or stream channel onto its floodplain is a normal, natural occurrence, human interference can cause higher than normal and more frequent flooding. The higher the water level remaining in streams and rivers before a storm, the more likely there will be dangerous conditions, streambed scouring, and other detrimental effects of high water.
When water levels are low, portions of the streambed become too dry to support the aquatic life that usually colonizes those areas. Groundwater seeping from the stream banks can help organisms survive for short times; therefore, well measurements can also be used to identify areas where aquatic life might be in jeopardy.
A count of the number of USGS stream gages reporting low flow at their lowest point between storms is valuable as an indicator of conditions statewide. If the number of streams that reach low levels between storms is increasing over time, it raises concern for the recovery of stream ecosystems from dry conditions of previous years. If the count decreases, we can look for recovery. If most of the low-flow gages are in the same area of the state, it could indicate a regional problem. We also note any record low flows for the date (see Sep 14, 2017 discussion).
Brief high water, even flooding, caused by sporadic heavy rain does little to alleviate long-term effects of dry conditions, because it takes time for ecosystems to recolonize streambeds after a drought has killed off aquatic organisms. Direct observation of any particular segment of stream is necessary to determine actual streambed conditions, because there are only 62 stream gages in Connecticut and the conditions they report may not represent the conditions in the hundreds of streams with no gages. A discussion of normal streamflow patterns can be found here.
In Connecticut, the USGS has 10 real-time groundwater monitoring wells. All of them show large rises in the water table over the past months to well above normal for the time of year. The rise began in August for two them; the rest showed more modest increases until September. Since the end of September, the water level in those wells has shown small increases and decreases as storms have added groundwater, but overall their levels have remained pretty steady or has fallen slightly.
On Oct 16, of the six realtime well reports that compare the current depth to water table with the average for the date over multiple years of record:
1 well was near normal,
The USGS Groundwater Watch website displays data from 60 other wells that are measured once a month, comparing the most recent measurements to each well's measurements taken in that month in previous years. At the end of September:
6 of the 60 wells were near normal,
Monitoring the water table can provide clues to future conditions. There was an apparent pattern from 2013 to 2016 in which normal groundwater levels in winter and spring alternated with increasingly well-below-average levels in summer and fall. Although 2017 groundwater levels did get as far below average as in 2014 or 2015, but they did not drop as far as they did in 2016. This year began to look like 2017 but the water tables began rising again in mid-July to mid-August so that this year's lowest water table levels were higher than any low point in at least 5 years. Click here to see all the CT USGS real-time groundwater measurements that compare levels from the last four years to the median level for each day of the year (lots of data, so it takes a long time to load).
Groundwater levels influence streamflow. 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 (click here for details from a previous Know Your Flow! webpage). 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 contrast, public water supply systems that depend on reservoirs benefit from heavy rains because the reservoirs collect the runoff. The runoff, however, can wash pollutants into the reservoirs, lowering water quality.
High water tables keep streamflow from dropping very far or very fast between storms, so when the next storm hits, the runoff pushes streams to higher levels with more flooding.
Go to the National Weather Service (NWS) Advanced Hydrologic Prediction Service at this link for updated, detailed maps and data about precipitation amounts for Connecticut at various time frames. Normal October rainfall across Connecticut ranges from 3 to 6 inches for the month. Weekly normal rainfall therefore is from 2/3 of an inch to 1 1/3 of an inch. On Oct 16, one-week forecasts were predicting a half to one inch of rain for CT. A long-range (15-day) model was predicting below normal rainfall after that until a possible nor'easter Oct 29 or so.
Click here for a prediction of total precipitation for the next 15-days.
For more information about past broad-scale weather patterns, see NOAA's National Centers for Environmental Information State of the Climate Synoptic Discussions. September 2018 information should be available the second week of October.
Your water utility and town are the best sources of information on the status of your water supply. A few systems post their current capacities regularly on their websites.
Aquarion maintains on its website a weekly updated water usage report for their systems in southwest CT. The report includes water demand graphs, system transfers, system capacities, days usable storage remaining, and precipitation. As of Oct 8, all those systems were well above where each system usually is at this time year based on a 20-year average. As of Oct 1, MDC reservoirs were at "99% of capacity with over 39 billion gallons in storage representing 657 days of supply assuming typical water use and no precipitation." The South Central Regional Water Authority reported that on Sep 30, its reservoirs ranged from 82% to 100% of capacity, averaging 86% of capacity, which is 18% higher than usual for the time of year..
The most recent CT Department of Public Health's Monthly Reservoir Status Summary was dated July 13. At a Water Planning Council meeting, a DPH representative reported that a new, more comprehensive and timely report system should be online in January.