Spatiotemporal variation of Gaoping River plume observed by Formosat-2 high resolution imagery

Abstract

River plumes play an important role in river–ocean boundary systems, where suspended and dissolved materials are exchanged between the land and ocean. Optical images acquired from space-borne platforms have been widely employed to study the spatiotemporal variations of major river plumes around the world. Gaoping River (GPR), which flows in a small basin with steep slopes, has a high sediment load with dynamic spatiotemporal variations. This research employed a total of 41 images of the GPR estuary acquired by Formosat-2 from 2005 to 2008 to study the spatiotemporal variations of the related river plume, as well as the influences of tidal, precipitation, river discharge, wind, current, and extreme weather events. All images were processed to be geo-referenced and radiometric-calibrated images using the Formosat-2 Auto Image Processing System. Three different water types, including plume core, plume/shelf water and offshore water, were classified by the supervised maximum likelihood classification method. The results show that the size of the plume core is directly correlated with the amount of river discharge and precipitation, while the direction of the plume on- and off-shore is controlled by the flood and ebb tides, respectively. A notable feature of the frontal patterns in the region of plume/shelf waters in GPR mouth region is that the angle between the sea surface wind direction and sea surface current direction is less than 90°. According to the observations made after four major typhoon and storm events, GPR plume expanded to as large as four times its normal size, and this condition lasted for up to 18days. These results may be applied to the other regions that are similar to GPR.