| The middle reach of Heihe is one of the most important commodity grain and melon and fruit vegetable bases in Gansu province and also in Northwest China.And it has concentrated more than 90% of the cultivated land and population in the whole river basin.The agricultural water demand in this area is large,mainly irrigated by surface water in Heihe.Agricultural irrigation consumes huge amounts of surface water in Heihe,resulting in excessive utilization of surface water in the middle reaches of Heihe.The shortage of water resources has caused the deterioration of the ecological environment in the oasis area of the middle reaches of Heihe.It has caused great hidden danger to the development of Heihe oasis and agriculture.And the ecological environment deterioration in this area has a profound influence on the ecological environment and economic development of the whole basin and even in the northwest of China.Therefore,it is of great significance to the analysis of the spatial and temporal variation characteristics of oasis in the oasis area of the middle reaches of Heihe and its land use even the sown area of crops,which can provide strong support for the adjustment of agricultural planting structure,water resources protection and the improvement of ecological environment problems in this area.In order to study the long time sequence of Heihe oasis,firstly based on AVHRR/NOAA and MODIS satellite data,this paper extract the oasis area in the middle reaches of Heihe and calculate the area of oasis by formula in 1982-2016 years.Then,using Landsat satellite data to supervise and classify the oasis area in the middle reaches of Heihe in 2001,2006,2011 and 2016 years.Finally,combined with the phenological data of the main crops provided by the local stations,the NDVI value calculated from HJ1 A satellite data is filtered by Hants,this paper obtain Then this paper obtained smooth NDVI time-series of main crops.Then extract main crops distribution in oasis area in the middle reaches of Heihe by the CART algorithm.At the same time,the method of NDVI conversion is used to integrate Landsat satellite data into Hants filtering.After verifying all the results,we draw the following conclusions:(1)The oasis area extracted from this study is consistent with the field survey results.The range extraction accuracy is relatively high.The improved calculation formula of oasis scale has been greatly improved in AVHRR/NOAA and MODIS data.We analyzed correlationbetween oasis area and the area of oasis in the statistical yearbook in 1986,1995,2000 and2011.The correlation coefficient is 0.83(confidence level is 95%).The results are basically parallel to the standard line,which shows that the accuracy of the oasis area calculated by this formula is very good.The land classification method has been verified that the overall classification accuracy are at least more than 88.46%,the kappa coefficient are at least more than 0.82,the results of the classification are better in general.At the same time classification area of HJ1 A and Landsat in 2011 and 2016 years,the data extraction in the middle reaches of Heihe oasis crop and 23 field observation data and 64 eddy covariance system,automatic weather stations and large aperture scintillometer point surface was verified,analysis results show that the classification of remote sensing image area and field observation data have good consistency,the accuracy of not less than 85.06%.By using HJ1 A and Landsat satellite data and statistical data,this paper comparison of crop classification result,we found that although the cities and counties accounted for a slight difference,but in the overall distribution proportion is relatively good.Generally speaking,this method has high classification accuracy in the middle reaches of Heihe.Meanwhile,it shows that it is feasible to classify the main crops in the middle reaches of Heihe by transformation the NDVI of Landsat data.(2)From the change of the oasis scale,when the value of NDVI is 0.23,the oasis boundary is more reasonable and the range of precision is higher.From 1982 to 2015,the size of the oasis in the oasis area of the middle reaches of Heihe was increasing,and the total area increased by about 718km2.the change rule was large in the early stage,and the increase and decrease were all very large.After 2000,it entered a steady rise period and decreased slightly after 2013.(3)From the point of view of the land classification,the classification of farmland,water,urban land and bare land is basically consistent with the actual situation,and small pieces of land have also been well differentiated.The farmland is mainly concentrated in the oasis area along the middle reaches of Heihe and the southwest of Zhangye;The proportion of grassland and woodland is not large,so synthesized themto a kind of ground objects,which are mostly concentrated in the mountain area and the alluvial fan in the front of the mountain and nearthe field;Water is mainly concentrated in the middle reaches of Heihe River,reservoir and pond and wetland in the study area.During 2001 and 2016 years,the area of cultivated land increased by 236.7km2;The forestland and grassland were increased for about 53.6 km2;The water body shows the characteristics of fluctuation because of the change of precipitation over the past years and the specific time of the selection of remote sensing data.But on the whole,the average water area of four years is 71.75 km2,and the fluctuation is not obvious;Because of the increasing population,the area of urban construction land has increased by 67.4 km2;The area of unused ground is reduced by 236.7km2.In general,the oasis in the middle reaches of Heihe is more stable.(4)2001-2016 years,the sown area of wheat and maize were decreased,the wheat planting area decreased for 27.1 km2,corn acreage dropped for 30.4 km2;The planting area of vegetables and fruit is increased greatly,among which the planting area of vegetables increased for 103.6 km2,and the area of fruit and fruit increased for 65.1km2.According to the overall temporal and spatial distribution of major crops,corn is the most important food crop in the region.It has a large scale distribution in three cities and counties in Ganzhou,Linze and Gaotai,accounting for 52~60% of the total cultivated land area.In proportion,Ganzhou > Linze > Gaotai.The distribution of wheat was more average,and it accounted for about 10% of the total arable land.The ratio of the planting area of vegetable and forest fruit was higher than 15%.(5)Based on the Landsat-TM satellite data and the HJ1 A satellite data,this paper fit and analyze the NDVI of the two kinds of satellites.The results show that there is a linear relationship between the Landsat-TM data and the NDVI of the HJ1 A satellite data,and the linear formula is y=1.0028x+0.0454.With this method,the NDVI of the converted Landsat-TM satellite data can be used in the NDVI time sequence curve of the HJ1 A satellite data.This method provides a theoretical and practical basis for prolonging the time of study. |
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