Study On The Flood/drought Indeces In West-North Of Chine And Their Spatial Anomaly Features And Time Evolution Rule

Based on the climate data from 1961 to 2000 in Northwest Chine, a new flood/drought indeces-Palmer_Z index(ZNDX) is introduced . By analyzing and contrasting K-index, Z-index and ZNDX, demonstrated the rationality of ZNDX and again determined grades of flood/drought for ZNDX and regional flood/drought indeces. The modified ZNDX is recognized to be optimum for studing the flood or drought of Northwest Chine, acting to the result of comparing annual and season flood/drough grades with those of history . With EOF REOF, Merlet wavelet analysis, the spatial anomaly features and time evolution rule in summer of Northwest Chine by using the ZNDX grades data from 104 observational stations are studied . the result are as follows: (1) the calculation of ZNDX values is based around a supply and demand model of the soil moisture at a location. In addition to current precipitation (P), airtemperature(T), Available Water Holding Capacity(AWC), It thinks about some factors derive from Water Balance Equation such as Potential Evapotranspiration(PET), Potential Recharge(PR), Potential Runoff(PRO), Potential Loss(PL), ), Surface Soil Moisture Content(Ss) , Underlying Soil Moisture Content(Su) . Establishing of Every ZNDX value relys on factors of several pevious months . The correlativity between theoretical value and the actual value of Soil Moisture Content in 93.3 percent of observational stations is distinct.All this shows that calculational methods of ZNDX is more reason for studing the flood or drought of Northwest Chine.(2) Index_z and index_k can't objectively remark extent of drought and flood event.Both can't distinguish the difference of some years which have the same precipitation and have different temperature, AWC. The modified ZNDX in comparison with others is well indicative of the severity and extent of flood/drought event and of more objective classification of them , which is therefore quite applicable to a extended area.(3) The main anomaly structures of grades of drought/food in summer over Northwest China present pluvial pattern and dry pattern in all the areas , and spatial changes are opposite pattern from the noth to the south and the west to the east.The first four rotated loading vector fields represent four principal precipitation anomaly areas: the Weishui River basin, east of the Qinghai-Xizang Plateau. Hexi corridor and northern Xinjiang.The rotated principalcomponent and the data of grades of regional drought/flood in the first four regions show that in the recent 40 years the driest period is in the 1990's in Northwest China. From the 1980's to early 1990's, an increasing rainfall trend , more and less, appeared in every area .(4) On the interanuual time scaled in summer, the east of Qinghai-Xizang plateau and the Weishui River basin have 20 years and 8 years cycle.. The grades series of drought and flood have quasi 8-9a oscillation in the Northern Xinjiang, and In Hexi corrider not only have quasi 15a oscillation but also quasi 8~9a oscillation. It can been predicted by Morlet Wavelet transform that the 10 years after 2000 would appear an increasing rainfall trend in summer in Northwest China.(5) The drought and flood have obviously interannual oscillatory variabilities . The climate is moist from 1980's to early 1990's. It's the extremely dry period in the 40 years when the rainfall is plentiful while drought happened now and then after 1995. In the 1960's ,dry and rainfall happened in turns and the trend of drought/flood near normal trend. The climate is dry in 1970's ,but the driest period is in middle and later 1990's.(6) The exent of dry climate which is from the abnormity dry of September in autumer is most severity, the grades of drought/flood appeared slightly moist trend in summer which comes from increasing rainfall of June. Because of the obnormity dry climate of autumer causes the dry climate of Northwest China.