Regional complexity in trends of potential evapotranspiration and its driving factors in the Upper Mekong River Basin

The evaporative demand, a central process of the climate system, is expected to be altered along with the global change. The increase of surface temperature and decrease of pan evaporation is known as the “pan evaporation paradox”, requiring comprehensive and detailed attribution and sensitivity analysis to understand the changes in evaporative demand dynamics. In this paper, long-term records of meteorological and hydrological datasets from 1960 to 2005 in the Upper Mekong River Basin were used to calculate and analyze the trends of Penman-Monteith potential evapotranspiration (ET0). The results indicated no significant monotonic trend for the whole basin and the increasing and decreasing pattern of ET0 were blended spatially. To attribute ET0 changes to radiation, temperature, humidity and wind speed, differentiation equation methods were employed. The results demonstrated the sunshine duration hours (N) was the dominant factor contributing to ET0 changes in the Upper Mekong River Basin. Sensitivity analysis indicated ET0 varied greatly both temporally and spatially in relation to different climatic variables. A test of Bouchet's complementary relationship demonstrated that there were complementary relationships between actual evapotranspiration (ETa) and ET0 or pan evaporation (ETpan) though they did not strictly obey the classical hypothesis.