西宁市蒸发量变化特征及影响因素

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

利用西宁市1954～2005年地面气象观测数据蒸发量资料，分析西宁市蒸发量的变化趋势及其影响因子，并根据彭曼公式（Penman Formula）对蒸发量进行了回归分析。结果表明，西宁52a蒸发量的年际变化、年代际变化、不同季节和月份的年代际变化均呈显著下降趋势。1960～1990年代，年蒸发量减少了377．98mm，平均每年减少约27．2mm，其中，1980年代减少的幅度最大，春季和夏季蒸发量显著减少，占年蒸发量减少总量的42％左右，6月的蒸发量下降最快。年际变化趋势符合格丁斯函数（Giddingsfunction）拟合曲线。年内分配形式的年代际变化表明，在1960、1970年代年内分配较不均匀，集中程度较高，1970年代以后年内分配不均匀性减小。依据彭曼公式建立了二元回归方程，经过显著性检验，蒸发量与日照时数、饱和水汽压差的线性关系密切。影响蒸发量变化的因子主要有平均气温、日照时数、相对湿度等，夏季的降温和夏季、秋季日照时数的减少是蒸发量减少的主要原因．而日照时数的减少主要是总云量增加造成的。

The change trend of evaporation in Xiiaing and its influcing factors were analyzed by using the ground meteorological observa- tion 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.