*Result*: Parallelization of the estuarine saltwater intrusion numerical forecast model UFDECOM-i using Fortran DO CONCURRENT.

*High-resolution simulations of estuarine saltwater intrusion are computationally demanding and require efficient execution on heterogeneous computing platforms. In this study, the use of standard Fortran parallelization—DO CONCURRENT—to accelerate the unstructured quadrilateral grid finite-differencing estuarine and coastal ocean model (UFDECOM-i) within a unified codebase for both multicore CPUs and GPUs was investigated. Using the NVFORTRAN compiler, three versions were implemented: MC-UFDECOM-i on multicore CPUs, GPU-UFDECOM-i using automatic data migration, and GPUA-UFDECOM-i using lightweight OpenACC directives for explicit data management. The results show that DO CONCURRENT enables scalable shared-memory parallelism on CPUs, with speedups of up to 16.32 × , and provides functional portability to GPUs without code modification. However, optimal GPU performance requires explicit data management, with GPUA-UFDECOM-i reaching a maximum speedup of 21.48 ×. These results demonstrate that DO CONCURRENT ensures portability and maintainability, whereas explicit data control remains essential for high GPU efficiency. • A unified CPU/GPU parallel framework for UFDECOM-i was constructed using DO CONCURRENT. • DO CONCURRENT with OpenACC was used to optimize the data transfer and execution efficiency. • GPUA-UFDECOM-i achieved a speedup of 21.48 × and a runtime reduction of 95.3%. • Standard Fortran parallelization enables flexible and portable model deployment. [ABSTRACT FROM AUTHOR]

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