Variation of evaporation capacity and influence factors in Hebei Province between 1963-2003

The variation of evaporation and its causes are analized with the observational data set of pan evaporation from 1963 to 2003 in Hebei Province. The results show that in the latest 40 years the averaged seasonal and annual evaporation of Hebei Province are all above normal in 1960s and 1970s,while below normal in 1980s and 1990s, and the turning points are all between the late 1970s and the early 1980s. The significant decline trend of spring and summer have been detected at the rate of -31.3 mm and -27.4 mm per decade, which is the main cause of the annual evaporation decrease. The spatial analysis indicates that the annual evaporation of 94% stations has decline trend, and 70% has significant decline trend. The eastern part of south central Hebei is characterized by the largest decline rate of annual evaporation ( at - 100 mm per decade) . The correlation analysis between evaporation and other elements shows that evaporation has significant correlation with the conditions of sunshine duration, low cloud and daily temperature range, therefore, solar radiation is the key factor of the variation of evaporation since the three elements mentioned - above are all closely related to it. Meanwhile the decline of wind speed is another key contributor to the decrease of evaporation.