Background The arid and alkaline land in the coastal plain around the Bohai Sea is a special land type with unique geographical position and ecological environment. The characteristics of evaporation and its influencing factors are closely related to soil and water conservation and management. However, the long-term change characteristics and influencing factors of evaporation in this region are not yet clear, it is necessary to conduct relevant research.

Methods This work used the trend coefficient, climate tendency rate and cumulative distance equality and ratio methods, analyzed the long-term change characteristics and influencing factors of evaporation in Cangzhou region using the pan evaporation observation data from 1957 to 2018 at coastal and inland observation stations. The observation station, the coastal station Huanghua, and the inland station Botou were selected for comparative analysis, and the differences in the characteristics of evaporation changes and influencing factors between coastal and inland areas were analyzed.

Results 1) The variation trend of pan evaporation from 1957 to 2018 showed that this trend was contrary to the expectation that global warming will be accompanied by an increase in terrestrial evaporation, known as the pan evaporation paradox. The pan evaporation climate tendency rates of Botou Station and Huanghua Station were −107 mm/10a and −43 mm/10a, respectively, with the evaporation paradox showing a more accentuated manifestation at inland stations. 2) The pan evaporation anomaly at Botou Station demonstrated a transition from positive to negative anomalies. Notably, since 2002, the pan evaporation anomaly in Bohai Bay rapidly shifted to negative values, with the cumulative anomaly displaying a downward trend. This shift signified an evaporation mutation, which is attributed to the sudden increase in total cloud cover and the subsequent rapid decrease in sunshine duration. Following the evaporation mutation, the evaporation rate in the coastal area increased at a rate of 111.5 mm/10 a, indicating a clear upward trend. 3) A statistically significant negative correlation was identified between inland pan evaporation and key meteorological variables, including total cloud cover, water vapor pressure, and relative humidity, with correlation coefficients of −0.614, −0.521, and −0.450, respectively. Conversely, a robust positive correlation was observed between pan evaporation and average wind speed and sunshine hour, yielding correlation coefficients of 0.667 and 0.591, respectively. In coastal regions, an additional significant negative correlation was found between pan evaporation changes and relative humidity, with a correlation coefficient of −0.407.

Conclusions The "evaporation paradox" phenomenon observed along the west coast of Bohai Bay is primarily attributed to an increase in total cloud cover, a subsequent reduction in insolation hours, a decrease in sunshine duration and surface wind speed, as well as fluctuations in water vapor pressure. Notably, following an abrupt change in 2002, pan evaporation in the region has showed a consistent upward trend, which is likely to exacerbate soil drought, alkalinity, and erosion processes. These findings provide a valuable theoretical foundation for the management and conservation of soil and water resources, offering insights into the intricate relationships between evaporation dynamics and environmental factors in coastal ecosystems.