March 2017 Water Conditions Summary
In March 2017, at sites used to monitor the response of water resources to changes in weather conditions in Maryland, Delaware, and the District of Columbia, 68 percent of groundwater levels and 58 percent of monthly mean steamflows were below normal. Rain on the last day of the month helped to increase groundwater levels the last day of the month which may affect their ranking. The groundwater level at an observation well in Montgomery County, Maryland was at a record monthly low for the second consecutive month. In March, groundwater levels decreased at 7 wells, increased at 20 wells, and remained unchanged at 1 well, whereas monthly mean streamflows increased at all 33 streamgages.
In March, groundwater levels at 8 of 28 USGS observation wells were in the normal range (25th-75th percentiles). Groundwater levels were below normal at 19 wells, with 7 wells below the 10th percentile and one well at a record March low. One well had above normal groundwater levels.
Monthly mean streamflows were in the normal range at 14 of 33 selected USGS streamgages. Streamflow was between the 10th and 24th percentiles at 11 streamgages, and below the 10th percentile at 8 streamgages.
Freshwater flows to the Chesapeake Bay were in the below normal range. 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 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 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.