Three Decades of Precipitable Water Vapour over Poland from GNSS Observations and ERA5 Reanalysis: Trends, Variability and Long-Term Consistency (1996–2026)

Abstract. Atmospheric water vapour is a key greenhouse gas and an important indicator of changes in the hydrological cycle. A three-decade record of GNSS-derived precipitable water vapour (PWV) from five Polish stations of the EUREF Permanent GNSS Network was analysed for the period 1996–2026. The GNSS-derived PWV series were homogenised for documented antenna-configuration changes using ERA5 as a reference and were analysed together with ERA5 PWV. Long-term trends were estimated using harmonic regression with annual and semi-annual components and complemented by non-parametric trend analysis; changes in seasonal amplitude, monthly anomalies, annual PWV maxima, and GNSS–ERA5 consistency were also assessed. Statistically significant positive trends were obtained for the homogenised GNSS series at all five stations, ranging from 0.396 to 0.623 mm decade⁻¹, compared with 0.418–0.598 mm decade⁻¹ for ERA5. At one station, homogenisation changed the estimated GNSS trend from a non-significant 0.200 to a significant 0.424 mm decade⁻¹. Over a nominal 30-year interval, the fitted trends corresponded to PWV increases of 1.189–1.868 mm for GNSS and 1.253–1.795 mm for ERA5. Annual-cycle amplitudes increased at all stations, while trends in annual maxima of daily mean PWV exceeded the corresponding annual-mean trends. Monthly GNSS–ERA5 correlations ranged from 0.9991 to 0.9994, with biases of −1.491 to −0.178 mm and RMSE values of 0.292–1.512 mm. No statistically significant linear trend in the monthly GNSS−ERA5 differences was detected at any station. Positive PWV trends were therefore consistently observed across the analysed stations, while the change in the WROC trend after homogenisation showed the sensitivity of long-term GNSS trend estimates to instrumental discontinuities. The multi-year mean monthly MAE peaked in August (1.54 mm), whereas the strongest negative MBE occurred in September (−1.05 mm), indicating a seasonal dependence of GNSS–ERA5 consistency.