Changes of karez showed the trend of first increasing and then declining over the last 60 years.The reasons for the dramatic changes of karez might be attributed to followings:(1)Oasis was expanded gradually in the Turpan basin.The changes of karez were consistent with arable land in 1949-1960

namely both increased

and however

the number and water flow of karez declined with increased arable land since the middle of 1960's.(2)Water supply for karez declined.In the beginning of 1960's

surface water began to exploite and many reservoirs and antiseepage canals were constructed.Eight large and middle reservoirs with a capacity of 0.8 × l08 m3 were constructed in 1993

the total length of antiseepage canals reached 4 774 km

and the seepage ratio was 70%

which caused the decline of water supply for karez.(3)Pumped wells were blindly established around the karez irrigation districts and groundwater was exploited in a plunder manner.There were 127 electric pumped wells at the end of 1966 and the number reached 5309 in 2009.Correspondingly

the annual discharge increased from 1.10 × l08 to 7.04 × l08 m3.Moreover

the layout of pumped wells was unreasonable and lots of them were established in the areas where karez was mainly distributed.The groundwater decline and karez drying-up had seriously negative influences on the Turpan basin

which presented in following aspects:Daily life of residents was influenced and many villages were migrated;The areas irrigated by karezs declined from 2.17 × 104hm2 in 1957 to 0.88×104hm2 in 2003 and meanwhile

the disaster area caused by drought and windstorm increased;The vegetation area was 12.30 × 103 hm2 in 1975 and it declined to 6.72 × 103 hm2 in 2003

reduced by 45.37%.