A geographical region, usually a topographical basin, that tends to have uniform air quality. The air quality within an airshed is influenced predominantly by emission activities native to that airshed, since the elevated topography around the basin constrains horizontal air movement. Pollutants move from one part of an airshed to other parts fairly quickly, but are not readily transferred to adjacent airsheds. An airshed tends to have a relatively uniform climate and relatively uniform meteorological features at any given point in time.