December 2016 Water Conditions Summary
Sixty-one percent of groundwater levels increased since November and 61 percent rank in the normal range. Monthly mean streamflows increased at 94 percent of streamgages since November and 64 percent of the gages rank in the normal range. Freshwater flow to the Chesapeake Bay was below average. Precipitation was below average at five Mid-Atlantic weather stations.
Although December precipitation was below normal for the second consecutive month, groundwater levels increased between November and December at 61 percent of the wells and streamflows increased at 94 percent of the streamgages used to monitor the response of water resources to changes in weather conditions in Maryland, Delaware, and the District of Columbia. The improvement between November and December may be attributed to the unusually low November water levels, including 6 wells below the 10th percentile, three of which were at record low November levels, and streamflow below the 10th percentile at 15 streamgages.
In December, 61 percent (17 of 28 USGS observation wells) of the groundwater levels were in the normal range (25th-75th percentiles). Groundwater levels were below normal at the remaining 11 wells, and 6 of these wells were below the 10th percentile.
Monthly mean streamflows were in the normal range at 64 percent (21 of 33 selected USGS streamgages). Streamflow was below normal at 12 streamgages, all of which were between the 10th and 24th percentiles.
Freshwater flows to the Chesapeake Bay were below average, and precipitation was below average at five Mid-Atlantic weather stations in December. 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 see how current data compare to the past. 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 have long-term data and show the response of streamflow and groundwater levelsto weather conditions rather than effects from 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.