Climatological Drivers of Pan Evaporation in the Riau Islands, Indonesia

Pan evaporation (Epan) is widely used as an indicator of atmospheric evaporative demand and plays an important role in understanding land-atmosphere interactions under climate variability. However, observed changes in Epan do not always follow the expected increase with rising temperature, a phenomenon known as the pan-evaporation paradox. The relative influence of climatological drivers on Epan remains particularly uncertain in humid equatorial regions, where high moisture availability may alter the controls on evaporation. This study examines pan evaporation and associated climatological variables in Riau Island, Indonesia. Temporal trends are assessed using the Trend-Free Pre-Whitening Mann–Kendall test, while Spearman correlation analysis is applied to evaluate the relationships between Epan and key climatic factors, including solar radiation duration, relative humidity, precipitation, wind speed, and air temperature. The results show that correlation analysis indicates that Epan is strongly and positively associated with solar radiation duration and negatively associated with relative humidity and precipitation. Wind speed shows a moderate positive relationship with Epan, while temperature variables exhibit weaker associations. Trend analysis further shows that minimum temperature exhibits a statistically significant increasing trend, whereas wind speed displays a statistically significant declining trend. In contrast, pan evaporation does not exhibit a statistically significant long-term trend. Overall, the findings suggest that pan evaporation variability in humid equatorial climates is primarily governed by radiative and moisture-related controls rather than temperature alone. The opposing effects of increasing temperature and declining wind speed likely contribute to the statistically insignificant long-term trend in pan evaporation observed, providing observational insight into evaporation dynamics under humid tropical conditions.