Long-term trends of pan evaporation and an analysis of its

Long- term trend of pan evaporation which was a key factor of hydrologic cycle and water resources management was investigated with the long-term variation of meteorological data: precipitation, air temperature, relative humidity, and wind speed. The causes of trends of pan evaporation were revealed from two points of view: complementary relationship and Penman's equation. The variations of pan evaporation showed the decreasing trends at the 5 stations and the increasing ones in the other 2 stations. The mechanistic causes for the decreasing trends were mainly the increases of the precipitation and the aerodynamic term in Penman's equation (1948). 1