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September 2017 Water Conditions Summary

In September 2017, 36 percent of groundwater levels and 82 percent of monthly mean streamflows were in the normal range at sites used to monitor the response of water resources to changes in weather conditions in Maryland, Delaware, and the District of Columbia. Since August, groundwater levels decreased at 25 wells and increased at 3 wells. Monthly mean streamflows decreased at 31 streamgages and increased at 2 streamgages.

Groundwater levels at 10 of 28 USGS observation wells were in the normal range (25th-75th percentiles) at the end of September. Groundwater levels were above normal in nine observation wells, with two wells above the 90th percentile, and below normal in nine wells, with three wells below the 10th percentile.

Monthly mean streamflows were in the normal range at 27 of 33 USGS streamgages. Streamflow was above normal at six streamgages, with two streamgages above the 90th percentile.

September 2017 freshwater flows to the Chesapeake Bay were in the normal range. Precipitation was below the long-term average at the five Mid-Atlantic National Weather Service (NWS) weather stations. Hydrologic and weather data have not been reviewed, and are therefore provisional and subject to revision.

Graph showing percentiles for monthly water conditions

*A percentile is a value on a scale from 0 to 100 that indicates the percent of a distribution that is equal to or below it. A percentile between 25 and 75 is considered normal. For example, a groundwater level in the 90th percentile is equal to or greater than 90 percent of the values recorded for that month.

To view more detailed information about Monthly Water Conditions in Maryland, Delaware, and the District of Columbia, use the navigation menu on the left.

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Why is it important for the USGS to collect and analyze water resources data?

USGS water data are valuable to the public, researchers, water managers, planners, and agricultural users, especially during extreme conditions like floods and droughts. The USGS is known for its consistent measurement techniques and the most extensive set of historical groundwater and streamflow data available to the public. Since these long-term data were collected during wet and dry periods, they can be used to assess how water resources respond to changes in temperature and precipitation, and to evaluate how current data compare to the historical data. The uniformity of the dataset enables multi-state comparisons and other comparative statistical analyses that can better inform policy makers of possible water-resources conditions they might encounter in the future.

The sites used in this water summary were carefully selected to include long-term datasets, and show the response of streamflow and groundwater levelsto weather conditions, rather than the effects of human influences. Of the USGS sites presented in this summary, 13 wells and 29 streamgages have more than 50 years of data. The current streamflow and groundwater data are ranked in comparison to the historical record and summarized. In addition to groundwater and streamflow data, this summary includes precipitation and temperature data, reservoir levels, and freshwater streamflow to the Chesapeake Bay to give a more complete picture of the region’s water resources.

 


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