The Evaporation Variation and Its Influence Factors in Xi'ning of Qinghai Province

The change trend of evaporation in Xi'ning and its influcing factors were analyzed by using the ground meteorological observation data from 1954 to 2005,and according to Penman formula the regression analysis was made to evaporation.The results show that the decadal,inter-annual,seasonal and monthly evaporation declined significantly in recent 52 years.From the 1960s to the 1990s,the annual evaporation decreased by 377.98 mm and about 27.2 mm per year on average,and in the 1980s the evaporation decreased most.The evaporation in spring and summer decreased obviously and accounted for about 42% of the total annual decrease amount,and in June evaporation declined fastest.The yearly change trend accorded with the Giddings function fitting curve distribution.The decadal variation of interannual distribution showed that there was a higher concentration degree in the 1960s and 1970s,and after the 1970s it tended to be even.There was a close linear relation between evaporation and sunshine hours,saturated water vapor pressure according to regression analysis based on Penman formula.The evaporation variation are mainly caused by temperature,sunshine hours and relative humidity change.The downword temperature in summer and reduction of sunshine hours in summer and autumn resulted in decrease of evaporation,and increase of the total cloud covers caused the reduction of sunshine hours.