Change characteristics of humid index and dry/wet climate boundary in Northeast China

Using basic daily data of 124 weather stations from 1961～2007,based on Penman-Monteith method,reference evapotranspiration(RE) and HI are calculated as well as theirs trends and changes of borderline in the wet and dry climate zone are studied.The results show that RE ranges from 500～1 000 mm and is distributed with large value in the southwestern part and small value in the northeastern part of Northeast China.Annual RE which is decreasing especially from 1960s～1990s and increasing significantly after 2000,demonstrates uptrend in 1/3 zone and downtrend in 2/3 zone in terms of spatial distribution.Humid climatic region where HI is larger than 1.0 is basically consistent with where precipitation is greater than 700 mm.The area of region where HI is smaller than 0.5 is larger than where precipitation is less than 400 mm.The coefficient of variation of HI with range from 25%～36% is larger in semi-arid climatic region and is smaller in semi-humid and humid climatic region.With decreasing in the mass HI,semi-arid,semi-humid and the great mass humid climatic regions where positive amplitude of variation of climate tendency of HI is larger than the negative are running dry.The boundary between semi-arid/semi-humid and humid/semi-humid runns from northeast to southwest swings wildly and shows a trend of monolithic translation.At the basic line of 1960s' boundary,the 1980s boundary move westward and the boundary after 2000 move evidently southeastward,which leads to semi-arid region expanding and humid region contracting,in turn,Northeast China is drying gradually and drying areas are enlarging.