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January 2018 Water Conditions Summary

In January 2018, groundwater levels ranged from monthly record highs to monthly record lows. Monthly mean streamflows were normal to below normal. Thirty-six percent of groundwater levels and 61 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 December, groundwater levels decreased at 12 wells and increased at 16 wells. Monthly mean streamflows decreased at 1 streamgage and increased at 32 streamgages.

Groundwater levels at 10 of 28 USGS observation wells were in the normal range (25th-75th percentiles) at the end of January. Groundwater levels were above normal in three wells, including a record high in one well. Groundwater levels were below normal in 15 wells, including 10 wells in the 10th-24th percentiles, 3 wells below the 10th percentile, and 2 wells that set a record January low.

Monthly mean streamflows were in the normal range at 20 of 33 USGS streamgages. Streamflow was below normal at 13 streamgages, including 11 streamgages in the 10-24th percentiles, and 2 streamgages below the 10th percentile.

January 2018 freshwater flow to the Chesapeake Bay was normal. Precipitation was below the long-term average at the five Mid-Atlantic National Weather Service (NWS) 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.

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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 levels to 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 regionís water resources.


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