RESEARCH ON FLOOD DISASTERS OF JINGHE RIVER VALLEY IN TANG DYNASTY

Through collection and analysis of historical materials,changes,ranks and origins of flood disas- ters in the Tang Dynasty in Jinghe River valley were studied in this paper.The results show that 82 flood disasters occurred altogether in the Tang Dynasty,and averagely occurred once every 3.52 years.Flood disaster in the Jinghe River valley in the Tang Dynasty mainly concentrated in the middle stage(703-840 AD)of the Tang Dynasty,51 flood disasters occurred altogether in this stage,averagely once every 2.69 years,and accounts for 62.2%.In the early(618-702 AD)and late stages(841-907 AD)of the Tang Dy- nasty,flood disasters were fewer,and there were 15 and 16 flood disasters respectively,which occurred once every 5.6 and 4.13 years and accounts for 18.3% and 19.52% respectively.The first rank of flood disasters in the Tang Dynasty in this valley takes 45.1%,the second rank is 41.5%,the third and fourth ranks are 9.6% and 6.1% respectively.Of the 82 flood disasters that occurred in Tang Dynasty in this val- ley,moderate and mild disasters are dominant,big flood disasters come second,and catastrophic flood dis- asters are much less.The flood disaster rank and frequency indicate that the climate can be divided into three stages in Tang Dynasty in the valley,the early and late stages had small precipitations,and the mid- dle stage had more precipitation.Main reasons for flood disaster occurrence were climate fluctuation,con- centrated rainfall and rainstorm.The big and catastrophic flood disasters were caused by obviously in- creased mean annual precipitation which indicates short moist climate,and the moderate and mild disasters were caused by concentrated rainfall and rainstorm.According to the duration and annual precipitation in recent Weihe River valley,the mean annual precipitation was mostly 900～1 000 mm during the period of catastrophic disaster in the Tang Dynasty and the big flood represented a significant increase in precipitati- on.