Long-term pan evaporation trend and preliminary investigation of evaporation complementarity in taiwan

Under global warming, the evaporation is expected to increase as the temperature increases. However, pan evaporation data over the past 50 years show a decreasing trend in many places. This has recently been discussed, and is termed the “pan evaporation paradox.” Pan observations during recent two decades in Taiwan also exhibit a decreasing trend. Although long-term pan evaporation data are available and the recording periods of some stations are more than 100 years, no long-term trend analysis of pan evaporation data has been studied in Taiwan. This is because that long-term pan evaporation data in Taiwan are not homogeneous in data properties. This inhomogeneity in long-term data is due to the use of different evaporation pans in the early time (20-cm pan) and the modern time (Class A pan). The observations from the 20-cm pan are usually greater than those from the Class A pan in the magnitude of as large as 50 mm/month in Taiwan. This study developed a data reconstruction model by using support vector machines to homogenize the long-term evaporation data from the two evaporation pans. Mann-Whitney-Pettitt method was used to detect the trend of long-term pan evaporation data. Analytical results reveal that, except for one station, the long-term pan evaporation trend in Taiwan exhibits an increasing trend in the north and a decreasing trend in the south. Preliminary analysis of the evaporation complementary relationship in Taiwan was also discussion. Using the precipitation as a variable to explain the environmental wetness, the evaporation complementary relationship can be demonstrated in Taiwan.