Preliminary estimates show that approximately 455 million pounds of nitrogen reached the Bay during the 2011 water year (October 2010-September 2011). This is 162 million pounds more than the revised loads in 2010 and 116 million pounds more than the 339 million pound average load from 1990-2011.
Annual average river flow to the Bay during the 2011 water year was 73 billion gallons per day (BGD). This is 21 BGD more than 2010 and much higher than the 54 BGD average flow from 1990-2011.
Each day, billions of gallons of fresh water flow through thousands of streams and rivers that eventually empty into the Bay. That water also carries polluted runoff from throughout the watershed.
The amount of water flowing into the Bay from its tributaries has a direct impact on how much pollution is in the estuary:
Not all rain water runs off the land. Some water seeps into the soil, carrying nutrients into groundwater. The travel time of nutrients through the watershed ranges from weeks to centuries. This can result in a lag time between implementing management actions and improvements in water quality.
Scientists have estimated the threshold level of nitrogen that should not be exceeded for a healthy Bay. However, the nitrogen loads entering the Bay each year can not be directly related to the threshold. Work is ongoing to develop a total nitrogen load indicator that can be compared to the threshold and that can better track the progress of nutrient management programs within the watershed.
Nitrogen Load Trends
Note: Previously reported data for 2010 were considered provisional. The 2010 data have since been revised using and the entire dataset has undergone revision due to updates in the watershed model. Additionally, prior to 2010, the nitrogen indicator reflected loads from the watershed only and did not include loads to the Bay from atmospheric deposition of nitrogen to tidal waters. The historic data have been revised in order to account for those additional loads.
Long-term Trend (since start of data collection)
1990 was the first year where all necessary loads data were available. Rigorous statistical analyses to determine trends in loads have not been developed at this time.
The amount of nutrients delivered to the Bay from the watershed changes dramatically from year-to-year complicating efforts to determine trends through time. Between 1990 and 2011, nitrogen loads averaged 339 million pounds per year and ranged from 186 to 496 million pounds per year.
Short-term Trend (10-year Trend)
The last 10 years have highly variable nitrogen loads, largely related to yearly fluctuations in streamflow.
Change from Previous Year (2010-2011)
Nitrogen loads increased from 293 to 455 million pounds.
River Flow Trends
Long-term Trend (since 1990)
Between 1990 and 2011, annual mean river flow to the Bay has averaged 54 BGD and has ranged from 29 to 77 BGD.
U.S. Geological Survey analyses indicate annual average river flows mostly in the normal range prior to 1960, a dry period during the 1960's followed by wetter conditions in the 1970's. Since 1990, river flow has been extremely variable.
Short-term Trend (10-year Trend)
The last 10 years have had highly variable flow. Water years 2002 and 2009 were dry whereas 2003, 2004 and 2011 were wet. Water years 2005 through 2008 and 2010 were relatively normal. 2011 was one of the five wettest years on record due to a very wet spring followed by a hurricane and a tropical storm that hit the region in the late summer/early fall of 2011.
Change from Previous Year (2010-2011)
Annual mean river flow increased from 52 to 73 BGD.
To calculate the loads of nitrogen flowing to the Bay, scientists use a combination of:
Pollutant loads to the Bay in any given year are influenced by changes in land-use activities and management practices, as well as the amount of water flowing to the Bay (hydrology). As mentioned above, annual rain and snowfall influence the amount of water in rivers flowing to the Bay.
Katie Foreman
University of Maryland Center for Environmental Science (UMCES)
(410) 267-9837
