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July 2014 - Record high groundwater levels were observed in Baltimore County, Maryland for the third consecutive month. More than 85% of groundwater levels and monthly mean streamflows were normal or above normal in July.

Precipitation | Groundwater | Streamflow | Chesapeake Bay | Reservoirs | Archive of WC Reports

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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 floods and droughts. These data can be used to assess how water resources respond to changes in climate. Scientists at the USGS have measured streamflow and groundwater levels in wells to assess water resources for over 125 years.

In addition to providing the most extensive set of historical streamflow and groundwater data available to the public, the USGS collects water data and quality-assures the data by employing standardized techniques across the country. The uniformity of the dataset allows for multi-state comparisons and other comparative statistical analyses that better inform policy makers of the possible water resource conditions they might encounter in the future.

The sites used in this water summary were carefully selected to show the response of streamflow and groundwater levels to precipitation. Ideally, these sites will show minimal effects from human influences. The streamflow and groundwater data are ranked in comparison to the historical record and summarized. Precipitation and reservoir data are also presented to give a more complete picture of the region’s water resources.


USGS July 2014 Water Conditions Summary

Fifty-eight percent of the groundwater levels and 67 percent of the monthly mean streamflow values at sites used to monitor the response of water resources to changes in climatic conditions in Maryland, Delaware, and the District of Columbia were normal (between the 25th and 75th percentile) in July.

Water levels in eight of 26 wells were above normal in July. The groundwater level at an observation well in Baltimore County was at a record July high. In Howard County, the groundwater level was above the 90th percentile in one observation well.
There were two groundwater levels below normal (between the 10th and 24th percentiles) and one below the 10th percentile.

July monthly mean streamflows were below normal at 4 streamgages, normal at 22 streamgages, and above normal at the remaining 7 streamgages in Maryland and Delaware.


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.

For example, a groundwater level in the 90th percentile is equal to or greater than 90 percent of the values recorded for that month.


June 2014 Precipitation and Weather

Precipitation in July was variable, which is typical of summer weather with localized storms instead of large, broad regional weather patterns. The long-term monthly average precipitation for July ranged from 2.80 inches at the National Weather Service (NWS) station in Baltimore, Maryland, which was 1.27 inches below normal, to 7.02 inches at the NWS station in Georgetown, Delaware, which is 2.83 inches above normal.

Rainfall was above normal at four of the NWS Mid-Atlantic weather stations: Ronald Reagan Washington National Airport in Arlington, Virginia, Hagerstown, Maryland, and Georgetown and Wilmington in Delaware. The map below shows the departure from average at each of the five NWS weather stations.

*The NWS normal (long-term average) period used for determining records is from 19812010.

The NWS Middle Atlantic River Forecast Center’s 365-day precipitation data showed that all counties in Maryland, Delaware, and the District of Columbia were classified as average to above average. Two counties in Maryland were more than 10 inches over the 365-day average from July 2013 to July 2014. See the links below to view the NWS data.

July air temperatures were below normal at all five NWS Mid-Atlantic weather stations and ranged from 0.8 degrees Fahrenheit below the long-term average in Wilmington, Delaware to 1.4 degrees Fahrenheit below the long-term average in Baltimore and Hagerstown, Maryland. In Baltimore, five daily record low temperature records were set, and it was the 17th coolest July on record.

*Note from the National Weather Service: September 2011 was the first month to incorporate the new 1981-2010 climate normals that were calculated by the National Climatic Data Center. The new normals replaced the 1971-2000 normals.

National Weather Service MD and DC
National Weather Service DE
Middle Atlantic River Forecast Center (MARFC)


The USGS monitors groundwater levels in unconfined aquifers, providing observations that can be compared to both short-term and long-term changes in climatic conditions. Twenty-six groundwater wells were selected based on the following criteria:


July 2014 Groundwater Levels

The groundwater level at one of the USGS observation wells in Baltimore County, Maryland set a July record high. In Howard County, a USGS observation well was above the 90th percentile.

Groundwater levels were normal (between the 25th and 75th percentiles) in 15 of the 26 wells used to monitor climatic conditions in Maryland and Delaware in July. Groundwater levels were normal to above normal at 23 of 26 wells. Water levels were below normal at only 3 wells – at one well in Charles County, Maryland and at two wells on the southern Delmarva Peninsula.

Groundwater levels in Delaware were normal at two observation wells and above normal at an observation well in Kent County.


Click here to access the clickable groundwater map.


The groundwater level in observation well BA Dc 444 in Baltimore County, Maryland, set a new record high for July at 34.27 feet below land surface. The previous record was 35.55 feet below land surface in 1998. Data collection began at this site in 1988. This is the third consecutive month of record high groundwater levels at this observation well.

Click here to view five-year groundwater hydrographs for other wells.


These 5-year hydrographs show groundwater levels as a dark blue line, the minimum and maximum monthly values, and the normal range (between the 25th and 75th percentiles) as a white band based on the period of record. The maximum water level is at the top of the upper blue section and the minimum water level is at the bottom of the lower blue section in the graph. Each monthly measurement is colored according to the percentile rank in which it falls for the month.


Streamflow data are used for many purposes. A few of the most obvious uses are to assess water supply and the risk of droughts and floods. Streamflow data are also used to calculate loads of chemical constituents and assess how biological communities are affected by hydrologic conditions. The USGS operates the most extensive network of streamflow gages in the region.

The streamflow locations chosen for the monthly water summary were selected based on the following criteria:

July 2014 Streamflow

Monthly mean streamflows were normal at 22 of the 33 USGS streamgages used to monitor climatic response in Maryland, Delaware, and the District of Columbia in July. Normal is considered to be between the 25th and 75th percentiles.

Monthly mean July streamflow was above the 75th percentile at seven streamgages, including two USGS streamgages that were above the 90th percentile--one in Carroll County, Maryland and one in Frederick County, Maryland.

Monthly mean streamflow was below normal at four streamgages in Dorchester, Garrett, and Worcester Counties in Maryland.

In Delaware, all monthly mean streamflows were in the normal range in July.


Click here to access the clickable streamflow map.

Real-time streamflow on Nassawango Creek in July was below normal for most of the month, except during the middle of the month, when it climbed above normal for 2 days, then continued to be below normal for the remainder of the month.

Precipitation at the nearest NWS station in Georgetown, Delaware was 2.83 inches above the long-term average for July. Nassawango Creek does not appear to have been affected by the 7 inches of rainfall in Georgetown, Delaware. Most likely explanation is the Nassawango Creek drainage area did not receive the same amount of rainfall as the weather station.

The monthly mean streamflow at Nassawango Creek in Worcester County, Maryland, dropped from normal to below normal in July. Monthly mean streamflow at this streamgage had been at normal to above normal levels since September 2012.

The monthly mean streamflow at Nassawango Creek in Worcester County, Maryland, dropped from normal to below normal in July. Monthly mean streamflow at this streamgage had been at normal to above normal levels since September 2012.


The dark line in the 5-year hydrograph represents the monthly mean streamflow for this period and the white band shows the normal range (25th to 75th percentiles) based on the period of record. The maximum monthly mean streamflow is at the top of the blue shaded section, and the lowest monthly mean streamflow is at the top of the dark orange area. Each monthly mean measurement is colored according to the percentile rank in which it falls for the month.

Click here to view five-year streamflow hydrographs for other stations.


Estimated Streamflow to the Chesapeake Bay

The estimated monthly mean freshwater streamflow to Chesapeake Bay was normal in July 2014 at 35,800 cubic feet per second (ft3/s; provisional, and subject to revision). The average (mean) monthly streamflow for July is 38,200 ft3/s. The normal range for average (mean) monthly streamflow for July is between 24,800 ft3/s and 43,800 ft3/s, the 25th and 75th percentiles of all July values. These provisional statistics are based on a 77-year period of record.

Data and more information on the freshwater flow to the Chesapeake Bay can be found here: Chesapeake Bay Inflow.

Graphs and data are available on the “Estimated Streamflow Entering Chesapeake Bay” website.

Reservoir Levels

Available reservoir storage at the end of July in the Baltimore reservoirs (Loch Raven, Liberty, and Prettyboy) remained at 100 percent of available storage capacity, or a total of 75.67 billion gallons of water. The Baltimore reservoirs have been full since December 2013.

Total normal storage in the Triadelphia and Duckett Reservoirs, which serve parts of Howard, Montgomery, and Prince George’s Counties in suburban areas around the District of Columbia, dropped 3 percent to 95 percent of normal storage capacity in July, with 10.07 billion gallons of water. Not all of the water in the Patuxent Reservoirs is usable; for operational purposes, percent of normal storage capacity is used, but this value can exceed 100 percent of the usable storage.

July 2014

Percent available/
normal storage

Volume (billion gallons)


Baltimore Reservoirs

Baltimore City - Environmental Services Division

Loch Raven

Patuxent Reservoirs

Washington Suburban Sanitary Commission (WSSC)



More Information

Additional Archives
Water Conditions Reports: 2000-present

Maryland-Delaware-District of Columbia
USGS Drought Watch
USGS Water Summary
Chesapeake Bay National
USGS Streamflow and Groundwater levels
U.S. Drought Monitor from the National Drought Mitigation Center

Compiled by:

Wendy S. McPherson, USGS Hydrologist



U.S. Department of the Interior
U.S. Geological Survey

Wendy S. McPherson (

* If you want to receive notification of updates to the water conditions or receive the file, send an email to Wendy S. McPherson at wsmcpher@usgs with the following message: "Send monthly water conditions report."

Maryland-Delaware-D.C. Water Science Center
5522 Research Park Drive
Baltimore MD, 21228


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