| Being the core issue of agriculture,the sustainability of crop planting is crucial for food security.It is fundamental for the sustainable development of farming in a region,particularly the one like the northwestern China where food supply has yet been self-sufficient,to rapidly and accurately obtain changes in croplands,crop planting manner,and planting area etc.In this study,the land cover types and changes in the NDVI time series were firstly retrieved from the 250-m MODIS NDVI(Moderate-resolution Imaging Spectroradiometer,Normalized Difference Vegetation Index)produced based on the 16-day maximum value.Along with the local phenological character,they were then employed to retrieve croplands,cropping manner and planting information for the main food crops,systematically investigate the spatial pattern of crop index,wheat and maize and their changing trends,and explore the impact of urbanization and transformation of agricultural economy on the production of food crops.The results in the doctoral project provide a useful reference to the planning of regional agricultural production and agricultural supply side reform.They are also meaningful for the application of remote sensing technology to the investigation and monitoring of agricultural production in western China.The main results are shown below:(1)During 2000-2014,the average area of croplands in Guanzhong area were 1828.98 ×103 hm2,with the area in major cities following the sequence of Weinan(673.07 × 103 hm2)>Xianyang(394.25 × 103 hm2)>Xian(338.51 × 103 hm2)>Baoji(311.63 × 103 hm2)>Tongchuan(111.52 × 103 hm2).Croplands were mainly distributed in terraces on the river bank of the middle reach of the Wei River,with a scatterd distribution in northern Loess Plateau and a small amount of them being in valleys of the Qinling mountainous area.In terms of area and the proportion in total land area,crop lands appeared to be concentrated in the middle rather than the south or north,with most of them lying in the eastern part of Guanzhong area.Over the study period,total area of croplands in Guangzhong area experienced a decreasing trend,which was mainly attributed to the decline in 2014.Cropland circulation mainly occurred in the cities and towns along the Wei River,while the implementation of the new urbanization planning accelerated the cropland circulation in the rural-urban fringe zone,such as the conversion of croplands to orchards in the north(e.g.Longxian and Qianyang)and middle(e.g.Liquan),and the development of tourism in the north of Qinling Moutain drove the southern border of croplands to move northward.(2)There were one and two plantings per year in Guanzhong area,and the average crop index between 2000 and 2014 was 142.7%.Two plantings per year was mainly adopted in the terraces of Wei River with low elevations,while areas with one planting per year was mainly found in the loess terraces and Qinling mountain with some of them detected in the vicinity area of Dali,which is located in the east,including Heyang,Poheng and the plain area of Linwei district.County crop index appeared to decrease from Xian to the surrounding areas.Temporally,the crop index decreased from 156.4%in 2000 to 140.1 in 2014.The decreasing rate was lower than 2%before 2005 and as high as 9%after 2005.The decrease in crop index was mainly driven by the conversion of the planting system from one planting per year to two plantings per year,with the decrease being slow in food-producing counties and fast in counties with more lands used as orchards.Besides,there was an apparent decline in the crop index in the central area,which is close to the major cities and underwent a relative quick urbanization.(3)The area of wheat and maize was found to be 928.47 × 103 hm2 and 935.07 × 103 hm2 based on the difference in the NDVI time series of wheat,maize and other crops.A combination of winter wheat and summer maize was mainly found in the central area of Guanzhong area,and the area decreased.One-planting-per-year winter wheat and spring maize tended to be in the northwest and northeast,and the area increased during the study period.Overall,the area of winter wheat declined during 2000-2014 and the average decreasing rate was 1.94%,while the area of maize increased first and then decreased with an overall average decreasing rate of 1.83%.(4)Based on the statistical and overlap analysis of the area and spatial pattern of winter wheat and maize in 2000,2007 and 2014,the changes in area of the major crops and transition manner of crop types was achieved.Results are as follow:① During 2000-2007,the area of one-plant-per-year spring maize increased by 405.91 ×103 hm2,and the increasing rate was 176.19%.The area of winter wheat-summer maize and one-plant-per-year winter wheat was found to decrease by 292.89 × 103 hm2 and 3.04 × 103 hm2,with the decreasing rate being 39.78%and 0.8%respectively.The expansion of one-plant-per-year spring maize was a result of the exchange with other plants,with 229.51×103 hm2 of other plants and 55.95 × 103 hm2 of one-plant-per-year spring maize converted to one another.The area of one-plant-per-year winter wheat remained relative stable mainly as a result of considerable transfers between other plants and one-plant-per-year spring maize,for which the conversion areas were 117.89 x 103 hm2 and 110.8 x 103 hm2 respectively.Winter wheat-summer maize was mainly converted from one-plant-per-year winter wheat and to other plants,with the areas being 59.79 × 103 hm2 and 159.96 × 103 hm2.② During 2007-2014,the area of one-plant-per-year winter wheat slightly increased by 6.19 × 103 hm2,and the increasing rate was 1.65%.The area of one-plant-per-year spring maize and winter wheat-summer maize decreased by 334.11 × 103 hm2 and 37.46 × 103 hm2,with the decreasing rates being 54.52%and 8.45%respectively.One-plant-per-year winter wheat was subject to strong dynamics,mainly converted from winter wheat-summer maize(132.78 × 103 hm2)and to other plants(114.87 × 103 hm2).One-plant-per-year spring maize was mainly converted to other plants,which was also the major one converting to one-plant-per-year spring maize.The conversion areas for one-plant-per-year spring maize and other plants were 226.97× 103 hm2 and 63.11 × 103 hm2.Winter wheat-summer maize was mainly converted to other plants(103.39 × 103 hm2)and from one-plant-per-year winter wheat(132.78 × 103 hm2).③ During 2001-2014,the area of winter wheat decreased,and was mainly transferred into other plants in the northeast of Guanzhong area,northern plateau of Xianyang and the boundary of Lantian and Lintong.There decline was rapid between 2000 and 2007 and relatively slow after 2007.The area of maize experienced a decreasing trend resulting from being transferred into other plants in the vicinity area of Dali,Fengxiang.Chengcang and Changan etc.But it should be noted that the area of maize increased between 2000 and 2007 while decreased after 2007.The area of two-plantings-per-year winter wheat-summer maize significantly decreased,dominating the shrinkage of winter wheat and maize. |
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