| The large-scale development and unreasonable utilization of land and water resources in the arid area of Northwest China lead to vegetation degradation and land desertification,which affect the ecological security and sustainable development of the arid area.Tarim Basin,as an important area for the development of Western China and the development strategy of "one belt and one road" has been a hot area in the study of ecological environment in arid areas.The Keriya River is located in the south margin of Tarim Basin,which is one of the important rivers in Tarim Basin.In the past 20 years,driven by climate change and oasis development,the landscape pattern of the basin has changed dramatically,and the contradiction between the supply and demand of land and water resources has become increasingly prominent,which makes the study of basin sustainability more important.Based on the above reasons,this study did systematically research on the characteristics of land use/cover(LUCC)and landscape pattern changes in Keriya River Basin,which located in the southern Tarim Basin,in the past 20 years,and the response of vegetation growth to climate and water resources changes in the Yutian Oasis.The relationship between landscape pattern and water resources was also revealed in detail.The balance of water resource supply and demand in the past 20 years was analyzed.The stability of oasis in the river basin was evaluated based on hydrothermal balance model.The results can provide basis for rational exploitation and sustainable development of water and land resources in the Keriya River Basin.The main results of this research are as follows:1)Land use/land cover in the Keriya River Basin have obvious spatial and temporal characteristics from 1995 to 2015.Snow and ice area,lakes and low density grassland are the main types of LUCC for the upper reaches of the river.The middle reaches of the river is mainly covered by grassland(including high/middle/low density grassland)and cultivated land.Shrubs and grasslands are the main types for the lower reaches of the Keriya River Basin.Temporal changes of LUCC in the study area showed that the area of glacier and snow in the upper reaches decreased,and the area of natural vegetation such as grassland and woodland increased in the upper reaches of the river during the past 20 years.For the middle reaches of the river,cultivated land increased and a large area of low density grassland disappeared.The area of natural vegetation such as shrub and grassland in the lower reaches and the area of lakes and canals in the water area decreased.There are obvious spatial distribution characteristics of LUCC types along the two sides of the river channel.The nearer the river is,the larger the distribution area of low grassland,cultivated land,mid density grassland,shrub land,forest and woodland is.During the study period,low density grassland,the most far from the river,decreased,and the cultivated land in each buffer zone increased;2)There are obvious differences in landscape pattern changes among the upper,middle and lower reaches of the Keriya River.From 1995 to 2015,the glaciers in the upstream became more fragmented and landscape diversity increased.The vegetation pattern became continuously,and the landscape structure tended to be stable.In the middle reaches,the structure of cultivated land is becoming more and more stable,while the degree of grassland fragmentation increased,and cultivated land had become the dominant LUCC type gradually.In the lower reaches of the river basin,fragmentation of shrubs and grassland increased.The oasis landscape diversity decreased significantly,and its shape became simplified.There are obvious changes in the buffer zone centered on the river channel.At landscape level,fragmentation increased in the study area,and the landscape separation,landscape diversity decreased.Changes above were mainly observed along the river channel and the edge of oasis,which is far from the river.At class level,the fragmentation of cultivated land increased and separation decreased,mainly along the river.Grassland fragmentation increased mainly at the edge of oasis;3)Changes have taken place in the climate and runoff in the Keriya River Basin during 1960 and 2015.The average annual temperature in the Keriya River Basin was 11.8?,and its seasonal distribution was the highest in summer and the lowest in winter,in the past 56 years.The annual average temperature increased significantly(P<0.01),and the lowest temperature in winter contributed most to the annual average temperature.The annual average precipitation in the study area was 50.3 mm from 1960 to 2015,and the change trend of annual average precipitation was not significant.The annual average potential evapotranspiration(ET0)is 1323.24 mm,and it decreased significantly.Wind speed is the main climatic factor affecting ET0.The annual average runoff is 7.48×108 m3,and the summer runoff accounts for 66.4%of the annual runoff.During the study period,the annual average runoff increased(P<0.01)significantly.Runoff in 2000s increased by 35.88%compared with the runoff in 1990s.The linear regression and partial relation analysis showed that temperature was the main climatic factor affecting runoff;4)The spatial distribution and variation of NDVI in Keriya River Basin are quite different.NDVI is the highest in the middle reaches and the lowest in the downstream.NDVI in the upper(P<0.05)and middle(P<0.01)reaches of the river increased significantly,while the downstream NDVI showed a slightly decreasing trend.The distribution of NDVI along the river channel has obvious spatial gradient.The nearer the river is,the higher the NDVI is.There was a significant positive correlation between precipitation and NDVI in the upper reaches of the river(P<0.05),and the correlation between precipitation and NDVI in the middle and lower reaches of the river was not obvious.NDVI in the upper and middle reaches of the river increased with the increase of runoff,while NDVI in the lower reaches decreased;5)The spatial and temporal variation characteristics of annual average water requirement of vegetation in the Keriya River Basin are obvious.The variation of annual average vegetation water demand in the Keriya River Basin is as follows:the water demand in the lower reaches was the largest in 1995,2005 and 2000,and the demand in the upper reaches was the smallest.In the past 20 years,water demand in the upper,middle and lower reaches increased.In 2010 and 2015,the upstream water demand was the largest and the downstream water demand was the smallest,and the water demand of each region began to decrease.The growing season water requirement of vegetation accounted for more than 80%of annual vegetation water requirement,and the change was consistent with that of average annual water requirement of vegetation.Vegetation water requirement has obvious spatial gradient in the buffer zone:the closer to the river channel,the greater the vegetation water requirement was.Landscape index at different scales can well explain the change of ecological water demand of natural vegetation.At landscape level,the landscape indexes PD,LSI,PARA MN,IJI,SPLIT and COHESION can well explain the upstream vegetation water demand.COHESION and SPLIT are the good indicators of the vegetation water requirement in the middle reaches.Landscape index such as LSI,SPLIT,SHDI and IJI can better explain the change of vegetation water demand in the lower reaches of the river;6)The water demand of typical crops in the Keriya River Basin has an increasing trend.The typical crops in the Keriya River Basin are wheat,maize and cotton.During the study period,the sown area of wheat and maize increased,while the sown area of cotton decreased.The average water requirement of three typical crops in 1995-2015 was 754.94 mm,581.92 mm and 458.08 mm,respectively.The order of water requirement of three crops at different growth stages is:mid-growth stage>rapid development stage>mature stage>initial growth stage.The main factors affecting crop water demand are average temperature and planting area.In the future climate change scenario,crop water demand in the study area will increase by 0.13-7.28%;7)In the past 20 years,the water demand of various industries in the Keriya River Basin has been quite different,and the balance between supply and demand of water resources is not optimistic.In the past five periods,the ecological water demand in the Keriya River Basin accounted for the largest proportion,followed by agricultural water demand,and other water demand totaled less than 2%in 1995,2000 and 2015.While,the ecological water demand decreased during 1995 and 2015,and the agricultural water demand,domestic water demand and animal husbandry water demand increased.Agricultural water demand increased the most obviously and it became the dominant one after 2010.The utilization of water resources in the Keriya River Basin in 2010 is optimistic,and the water supply can meet the water demand of the middle and lower reaches of the basin at the same time.The water supply in other years can only meet the water demand in the middle reaches,while the downstream is in a state of water shortage;8)The calculation results of oasis stability based on the principle of'hydrothermal balance show that the study area has been in an unstable state for five periods in the past 20 years.When the stability of oasis is 0.75,the cultivated land area in 2005 and 2015 exceeds the suitable development scale.When the stability is 1,only the cultivated land area in 2010 is in the suitable development scale,and the cultivated land in other years is larger than the suitable development scale.According to the results of the suitable development scale of cultivated land in the middle reaches of the Keriya River in the future,the scale of cultivated land in 2025 and 2035 should be in 321.13-428.38 km2 and 340.79-454.62 km2 respectively. |
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