基于两种潜在蒸散发算法的SPEI对中国干湿变化的分析

Analysis of Dryness/Wetness over China Using Standardized Precipitation Evapotranspiration Index Based on Two Evapotranspiration Algorithms

利用美国普林斯顿大学高分辨率的全球陆面同化数据集和美国国家环境预测中心的辐射再分析数据,根据Thornthwaite和Penman-Monteith公式分别计算了1948～2008年中国区域潜在蒸散发量;而后,使用降水和两套潜在蒸散发数据分别计算得到标准化降水蒸散发指数SPEI（Standardized Precipitation Evapotranspiration Index）,并以此研究了1949～2008年中国区域干湿变化时空特征以及两种SPEI结果之间的差异;最后,给出了两种SPEI在中国的适用区域.结果表明：两种SPEI均显示中国地区整体上存在变干趋势,季节上以春季的变干趋势最为显著;空间上表现为以长江为界的南涝北旱,显著变干的区域有内蒙古中部、华北、东北以及四川东部地区,显著变湿的地区主要位于新疆北部和西部.同时,各种不同等级干旱也呈增加趋势,其中以中等干旱增加最为显著.1990年代中后期以来是中等和极端干旱发生最多的时期,空间上与SPEI显著减小的区域相对应.两种SPEI在冬、春季差异最大,这主要是由于期间两种潜在蒸散发的计算结果之间存在很大差异.在Penman-Monteith公式中,由于空气动力项对冬、春季北方潜在蒸散发的贡献显著增加,基于该公式的SPEI相对而言能更合理地描述干湿变化特征.

On the basis of a high-resolution global meteorological dataset from Princeton University and monthly radiation data from the National Centers for Environmental Prediction （NCEP）,two types of Standardized Precipitation Evapotranspiration Index （SPEI） were established using precipitation in addition to two sets of potential evapotranspiration （PET） data,in which PET was calculated by Thornthwaite and Penman-Monteith formulas,respectively.On the basis of these results,the dryness/wetness trends over China during 1949-2008 measured by two SPEIs and their differences are analyzed.In addition,the applicable regions of the two SPEIs are evaluated.It is determined that a dryness trend for the entire country was indicated by the two SPEIs.Seasonally,the most significant dryness trend occurred in spring.Geographically,drought clearly increased in northern China but decreased in southern China,with a dividing line approximately along with the Yangtze River valley.A significant dryness trend was apparent over the areas of western Inner Mongolia,North China,northeastern China,and eastern Sichuan,whereas the opposite was indicated for northern and western Xinjiang.Moreover,different grades of drought events increased across the entire country.Among them,the moderate drought event showed the most significant increasing trend,and the occurrences of moderate and extreme drought events were the most frequent after the late 1990s for the entire study period.Spatially,the regions with an obvious upward trend of drought events corresponded to the regions with a downward trend of SPEI.It is noted that the difference between the two types of SPEI was largest in winter and spring because of differences in PET.In northern China,the contribution of an aerodynamic term to PET in the Penman-Monteith formula was determined to be important in winter and spring and hence gave more reasonable results.