The survey of Karez in Qitai county,including the distribution,depth,length,direction,distance between wells,geology,water lever,the process of rise and decline,and analysis of social economic data,indicate that Karez in Qitai county were begun to dig from 1949,at 1960's the amount of Karez reach the most,at early 1980's,all the Karez were abandoned.This change is caused by excessively use groundwater and make groundwater level declined.While the basic reason of groundwater level decline is population increasing and agriculture development.The trend of quantity change of Karez is contrary to that of the population and ploughland.When population reached 90 000 and irrigable land was 50 000 ha,average groundwater level was about 5 m,Karez began to decline.The average groundwater level was about 8 m when all the Karez had been abandoned,and exploitable quantity of groundwater per person only was 1 300 m3,agricultural irrigation water was almost equal to the exploitable quantity of groundwater