A new calculation method for background temperature of thermal discharge from coastal power plants: Offshore Thermal Gradient Exponential Fitting Method (OTGEM)

To address the need for background temperature calculation in studies on the environmental impacts of thermal discharge from coastal power plants and the limitations of traditional methods in characterizing regional temperature distribution, this study proposes a novel method for calculating background temperature, namely the Offshore Thermal Gradient Exponential Fitting Method (OTGEM). Based on 47 remote sensing images (2015–2024), OTGEM establishes 30 m-interval SST gradient strips to fit offshore water temperature natural gradient curves, linking with in-situ SST via characteristic parameters and reference points for precise calculation. Applied to Fujian Hongshan Thermal Power Plant and Ningde Nuclear Power Plant’s distinct coastlines, results show obvious offshore temperature gradients (inner-outer differences ∼ 0.9°C and 1.5°C, respectively) and tidal-phase-related variations. Error analysis indicated maximum average error ≤ 1°C within 4 km offshore: errors > 0.5°C are confined to 0.24–0.5 km nearshore sea area, ≤ 0.4°C in 0.5–1 km, and <0.2°C beyond 1 km. In conclusion, OTGEM dynamically reflects flood and ebb tides’ real-time impact, delivering regional characteristics, highly accurate results with low data demands and simple operation, suitable for diverse coastal scenarios like open coast areas and semi-enclosed bays.