The New York Times: The “probable extent” of the oil slick is an estimate by the National Oceanic and Atmospheric Administration of where oil is mostly likely to go based on wind and ocean current forecasts, as well as analysis of aerial photography and satellite imagery. The “observed extent” show areas where oil was visible on the surface of the water during aerial surveys of the Gulf. The observed extents are not available every day. The extents may vary widely from day to day because of changes in wind patterns and ocean currents.
The loop currents are from NOAA and from Roffer’s Ocean Fishing Forecasting Service.
About the Estimates Used in the Chart
The totals for the amount of oil spilled are calculated beginning from the initial explosion at 10 p.m. on April 20. While both oil and gas are leaking from the well, the estimates here are only for the amount of oil. This was done so that all the estimates can be compared equally, since some of the methods have no way to account for the amount of gas. BP announced on May 21 that the fluid leaking was roughly half oil, half natural gas.
Totals are adjusted, beginning May 17, for oil diverted through a narrow tube that was inserted into the well’s damaged pipe. BP has made daily announcements of how much oil has been captured, which had been an average of 2,100 barrels per day, or a total of 8,400 barrels (352,000 gallons) through May 20.
The “NOAA” estimate is based on a figure released by the National Oceanic and Atmospheric Administration on April 28 when the government agency raised its estimate of the flow rate to 5,000 barrels (210,000 gallons) per day from its initial estimate of 1,000 barrels per day, over public objections by BP.
The “MacDonald minimum” is based on an analysis by Ian R. MacDonald, an oceanographer at Florida State University that was published by SkyTruth on May 1. By studying the amount of the oil visible in aerial imagery, Dr. MacDonald calculated the flow rate to be at least 26,500 barrels (1.1 million gallons) per day. He called this a “minimum estimate” since his calculations could only account for oil that was visible on the surface and did not include oil that had evaporated, mixed in with sea water, sunk to the bottom or been collected by response crews.