Mean and great diurnal tide-range trends had no clear difference at six US gauges in 1975-2025 cycle fits

We tested whether two monthly tide-range measures changed at different rates at six selected US gauges in 1975-2025. We used fixed NOAA records and matched dates for both measures. Great diurnal range is mean higher high water minus mean lower low water. Mean range is mean high water minus mean low water. We fit their difference with a linear trend, calendar-month terms, and a fixed 18.613-year cycle. No station passed the Holm correction for six trend tests. Difference estimates ranged from -0.092 to +0.118 mm/year. The smallest adjusted p-value was 0.243. Annual complete-year checks also left all six differences uncertain. The result does not prove equal rates. It is conditional on the linear model, a fixed cycle, and the error model. Data gaps, inferred values, source changes, and selected harbor gauges limit inference. The package has source hashes, monthly evidence, code, figures, sensitivity checks, and repeat instructions.

tide range · NOAA CO-OPS · lunar nodal cycle · paired trends · null result · monthly water levels