Remote sensing of algal blooms using a turbidity-free function for near-infrared and red signals

This article presents a method for realtime mapping of algal blooms in turbid coastal waters using the remote sensing reflectance of red band (Channel 1, 580-680 nm) and near-infrared band (Channel 2, 720-1100 nm) of the AVHRR sensor on the NOAA series satellites. A turbidity-free function for near-infrared and red signals, α0 = (bb/bb)(a/ a) based on the first order bb/(a+bb) model deducing equation Rrs = α0 Rrs −+ g− (1−α0), were selected as a chlorophyll-a related index for detecting algal blooms, and the algal blooms with chlorophyll-a concentration of 64-256 mg/L could be defined by window of 1.6 < α0 < 5.2 and 0.01< Rrs/g < 0.2. Such turbidity-free two-band method is supported by both sea-truth data and remote sensing experiment for an algal blooms event on the near-shore water off the Minjiang estuary of southeastern China during early June of 2003. Comparisons of this algorithm with other published algorithms, one-band method (i.e. method of bright water) or two-band methods (i.e. method of ratio, method of NDVI, and method of subtracting) have suggested that the turbidity-free function method could be regarded as a standard algorithm in capabilities of AVHRR imagery or other high resolution but wide near-infrared and red band imagery for detecting algal blooms events in coastal waters.