Potential evapotranspiration (ETp) measures the ability of the atmosphere to remove water from the surface by evaporation and transpiration. Because the reference evapotranspiration is often used to infer this ability, ETp is usually considered only influenced by meteorological conditions. Due to the close linkages within the soil-vegetation-atmosphere system, ETp is likely influenced also by surface conditions like soil water content and vegetation cover. Therefore, this study is aimed at investigating the relationships between ETp and the associated environmental variables at different time scales. The results show that ETp has increased significantly by ~2.4 mm yr-1 during 1982-2015, alongside significant increase in vegetation index (NDVI), wind speed (Ws), temperatures and significant decrease in relative humidity (RH). Linear trends varied across seasons but similarities were found between spring and winter and between summer and autumn. Summer saw the greatest changes in ETp per unit change in environmental variables, which implies a likelihood of greater water demand with a warmer summer. Solar radiation, RH and precipitation exerted overall stronger influence on ETp (R2>0.50) than other factors, and NDVI and SWC was found positively and negatively affecting ETp at all time scales (p>0.05 only for ETp-NDVI at annual scale). Furthermore, partial correlation analysis showed significant effects of NDVI and SWC on ETp at the monthly scale and SWC also influenced ETp in summers (p<0.05). This study proves that ETp is related to surface conditions in addition to meteorology, and shows the major factors effectively explaining the changes in ETp across different time scales.