Water exchange plays a vital role in controlling water quality and the assessment of environmental carrying capacity for marine spatial planning. Water exchange in water bodies is determined by residence time. A comparative study of the different residence times in the coastal waters of Vietnam by radioactivity, numerical dynamic modeling, and land–ocean interactions in the coastal zone (LOICZ) approaches is conducted. The residence time calculated using the radioactivity method was higher than that obtained through other approaches. The results of radioactivity and numerical dynamic modeling presented full images of water exchange in the study areas, whereas the LOICZ approach indicated water exchanges that occurred in the entire water body. Thus, a satisfactory agreement between calculated and measured water exchange was obtained for Cam Ranh Bay by integrating the radioactivity method within numerical modeling to improve the calculation of carrying capacity for marine spatial planning.

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