January 2017 Water Conditions Summary
In Maryland, Delaware, and the District of Columbia, groundwater levels increased between December 2016 and January 2017 at 79 percent of the wells, and streamflows increased at 100 percent of the streamgages used to monitor the response of water resources to changes in weather conditions.
In January, 61 percent (17 of 28 USGS observation wells) of the groundwater levels were in the normal range (25th-75th percentiles). Groundwater levels were above normal at 14 percent of the wells, and below normal at 25 percent, with three wells below the 10th percentile and one of them at a record January low, breaking the record set during the drought of 2002.
Monthly mean streamflows were in the normal range at 61 percent (20 of 33 selected USGS streamgages). Streamflow was above normal at 15 percent of the streamgages, and below normal at 24 percent of the streamgages.
Freshwater flows to the Chesapeake Bay were average, and precipitation ranged from above to below average at five Mid-Atlantic weather stations in January. Hydrologic and weather data have not been reviewed and are therefore provisional and subject to revision.
*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 evaluate how current data compare to the historical data. The uniformity of the dataset enables multi-state comparisons and other comparative statistical analyses that 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 data sets, 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.