| As the largest underground water irrigation system in the world,karez in Turpan is hailed as the “the underground Great Wall”.To better understand the meaning of value of karez to ecodevelopment of Turpan oasis and the effect of land use changes in Turpan basin on karez,in this paper,the author tried to study karez and land use changes in Turpan basin by interpreting remote-sensing images of Turpan basin in the years of 1990,2000 and 2010 using GIS time-space analysis,literature review and field survey,etc.;it studied changes of land use in terms of space and time,relations between land use changes and karez,current status of karez,and factors that influence decline of karez,and discussed protection using the existing problems of karez.In the author's thoughts,current status of karez utilization and development forms the study basis,impact of land use changes on karez and the relations between the two are core,and to study factors that influence decline of karez is to make further discussion over future development trend of karez.Major conclusions from the study are as follows:(1)Processing of TM?ETM image data from years of 1990,2000 and 2010 using GIS supervised classification method and space-time correlation analysis method showed that,from the year of 1990 to 2010,the bare land,water area and saline-alkali land in Turpan basin manifested a declining trend in their areas by 2.01%,0.09%,and 0.70%,respectively;area of cultivated land and land for construction increased by 2.0% and 0.10%,respectively;grassland,increasing in the 10 years of 1990-2000 and declining in the 10 years of 2000-2010,showed an increase of 0.65% in sum.Cultivated land and bare land dominated the land utilization changes in Turpan from 1990 – 2010.(2)Time-space analysis to land utilization types of Turpan Depression discovered that,bare land was mostly transformed to cultivated land,grassland and land for construction;grassland mostly to bare land and cultivated land,with part of it to land for construction;cultivated land mostly to bare land and grassland;water area,land for construction and saline-alkali land didn't show obvious trend of transformation.In the first stage from the year of 1990 to 2000,saline-alkali land claimed the biggest change with a decline of 30.33% and bare land,with a decline of 1.48%,is the one with the minimum change;land types in declining order of change amplitude is saline-alkali land,grassland,land for construction,cultivated land,water area,and bare land in turn.In the second stage from the year of 2000 to 2010,the biggest and the smallest change was seen respectively in the land for construction which increased by 52.81% and the bare land which decreased by 0.72%;land types in declining order of change amplitude is land for construction,saline-alkali land,cultivated land,grassland,water area,and bare land.(3)Compared to the first stage from the year of 1990 to 2000,in the later stage of 2000-2010,whenever land use was experiencing relatively drastic changes,the number of karez would also decline rapidly.In the study area,the cultivated land and grassland increased and water area decreased in its proportion.Social and economic development resulted into more and larger area of cultivated land and a great amount of land reclamation.Ever increasing speed of urbanization made part cultivated land and land for construction replace the area where karezes are located,forcing them to disappear or be abandoned due to cutoff of streams.Increase in cultivated land and land for construction brought higher pressure to water area.Higher demand of agricultural water and domestic water required extraction of a great amount of surface water and ground water for which a great number of electro-mechanical wells and impermeable channels have to be built;some areas were so seriously over-extracted that water recharging local main lakes was dropping,leading to continuous decline of ground water table and shrinkage of water area.Within the total area,the number of karez declined from 725 in the year of 1990 to 103 in the year of 2010,a decrease of 622;their total water output dropped to 0.38×108m3;all these indicated that land use types in Turpan basin,especially changes in cultivated land,land for construction,and water area,play a direct or indirect role to dying of local karez.(4)Statistics of various types of data as well as field survey indicated that karez run dry mainly due to the following reasons:(1)continuous increase in cultivated land and land for construction in the oasis;(2)decline in water area and ground water table;construction of reservoirs and impermeable channels;(3)increase of electro-mechanical wells and their improper layout;(4)roaring increase in population;(5)born defects of karez. |
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