Information source is the basis of ecological environment research, and its quality directly affects the accuracy and scientificity of research results. The selection of ecological remote sensing information sources in Tarim River basin follows the following principles: ① Through the reasonable combination of various information sources, the ecological environment of the basin can be monitored from macro, meso to micro; ② The spatial resolution of the selected information source must meet the accuracy requirements of different monitoring scales, and at the same time, it must have high spectral resolution, which can fully reflect the rich ecological environment information; ③ Try to choose the information source with long-term stable operation to facilitate the long-term dynamic monitoring of the ecological environment; ④ highly selective information source types.
Because this area is located in warm temperate zone and alpine zone, winter is long, spring, summer and autumn are short, and the growing period is short rather than long. In the growing period, vegetation grows luxuriantly, but in the non-growing period, it is all yellow, barren or frozen by heavy snow. Vegetation is an important mirror of ecological environment, which has obvious symbolic characteristics. Therefore, the seasonal stage of remote sensing information source is determined as July-September, when vegetation grows vigorously to meet the consistency of large-scale remote sensing survey results and the feasibility of dynamic analysis. In addition, considering the specific characteristics of soil secondary salinization, the late February to early March of the non-growing season is the salt return period every year, and there is no influence of vegetation cover. Therefore, in order to reflect the rich information of soil secondary salinization to the greatest extent, the images from late February to early March were selected for soil salinization monitoring.
According to the above principles and requirements, the system has built a three-level monitoring system from macro to fine and from whole to local, as shown in Figure 5- 1 and Table 5- 1. Using MODIS satellite data with low spatial resolution and high temporal resolution, macro monitoring was carried out in the whole basin (the overall contour range within the plate 5- 1)1∶ 500,000 scale. TM/ETM(ASTER) data and CBERS data of Landsat-7 are used for monitoring, and the scale is 1∶65438+ million (within the brown boundary of 5- 1). Spot-5 data and QUICKBIRD data with high spatial resolution were used to monitor the key areas in the middle and lower reaches of the main stream (in the blue box in map 5-1:10,000) in detail. From the middle reaches to the lower reaches, there are three key monitoring areas: usman, Calda Yi and Taitema Lake, in which usman and Calda Yi are monitored by Spot-5 data and Taitema Lake is monitored by Quick. In this way, we can not only macro-control the river basin, but also accurately grasp the ecological changes in key areas.
Table 5- 1 Index Table of Remote Sensing Monitoring System for Ecological Environment in Tarim River Basin
(1) Monitoring with high temporal resolution and low spatial resolution in the whole basin
Make full use of the characteristics of MODIS satellite data, such as high time resolution, high spectral resolution and wide landscape, to construct a first-level macro-monitoring system covering the whole basin, and timely grasp the macro-information of ecological environment such as vegetation growth, surface temperature, floods and droughts (X. Zhan et al., 2000).
MODIS (Modem-Resolution Imaging Spectrometer) is the most important sensor installed on TERRA and AQUA, the first satellites of the American Earth Observation System (EOS), which sent data to the ground from199965438+February and 200065438+February respectively. EOS Earth Observation System is a long-term data acquisition system established by the National Aeronautics and Space Administration (NASA) for global change research. It is planned to launch 10 satellites in the next 10 year to form a data acquisition system in 15 year, which is unprecedented in the development history of earth observation satellites (Li Dengke, Zhang Shuyu, 2003). Compared with the previous remote sensing data, MODIS is a new generation of "spectral integration" optical sensor in the world, and its image data has the following outstanding characteristics (,Ge,, 2000):
(1) has high spectral resolution and multi-channel simultaneous observation: MODIS sensor is at 0. 4 ~ 14.4 micron, with wide spectral range, higher than SPOT, TM and AVHRR.
(2) Large-scale multi-frequency observation: the scanning observation width is 2330km, and two satellites can observe the Tarim Basin twice a day and twice a night;
(3) Appropriate spatial resolution observation: MODIS has 2 channels with spatial resolution of 250m, 5 channels with spatial resolution of 500m and 29 channels with spatial resolution of1000 m;
(4) High-precision observation: MODIS sensor is the most accurate radiation observation instrument in the world. The quantization level of each output is 12 bits, and the temperature resolution can reach 0. 03℃. Secondly, MODIS adopts the complex on-board calibration technology of visible light channel for the first time, which ensures the long-term observation stability of the instrument.
(5) NASA's global free receiving policy for MODIS data enables users to obtain cheap and practical data resources.
Therefore, MODIS remote sensing data plays an important role in monitoring large-scale vegetation, drought and macro-dynamic water bodies in Tarim River Basin, and is an ideal remote sensing information source.
(b) Medium spatial resolution monitoring of "four sources and one dry work"
Using TM/ETM data (ASTER data) and CBERS data, four special topics of land use, vegetation types and coverage, desertification and soil salinization in the "four sources and one trunk" area of Tarim River were monitored by remote sensing.
1.TM/ETM and ASTER data
Landsat has accumulated a wealth of image data since 1972 launched its first satellite, and has a long and stable history. The TM of Landsat-5 has seven bands from visible light to thermal infrared, and the e TM of Landsat-7 has eight bands. The band setting is conducive to the acquisition of ecological information. The spatial resolution of the image is high, and the ground resolution of TM and ETM reaches 30m and 15m respectively, which meets the mapping requirements of 1∶65438+ million and below. The sensor has a large scanning width, with a single coverage area of 185km × 185km and a repeated coverage period of 16 days. It is the most widely used remote sensing data in the domestic remote sensing field at present. However, the Landsat-7 satellite broke down in 2005, and the Landsat-5 sensor has been seriously aging, so it can be replaced by ASTER data. ASTER is a sensor on TERRA satellite, which was launched in19991February 8 18. It has three visible and near infrared bands with resolution of 15m, six short-wave infrared bands with resolution of 30m, and five thermal infrared bands with resolution of 90m, with scanning width of 60km and repeated observation period of 16 days. With the operation of EOS series satellites, ASTER data will become a stable remote sensing information source.
2.CBERS data
CBERS- 1 was successfully launched in 1999, ending China's long-term dependence on foreign satellite remote sensing data. The spectral band selection is similar to Landsat-7, and the spatial resolution reaches 19. 5m。 The processed image information can meet the mapping requirements of 1∶65438+ million scale. CBERS data has been sent free of charge at present, so this time we mainly use it for experimental research to explore the feasibility of China's independent intellectual property satellite remote sensing data as a substitute for Landsat data.
According to the actual needs of ecological environment monitoring in Tarim River basin, the remote sensing monitoring scope of each topic is determined.
1. Land use monitoring
Since 1950s, with the increase of population and the development of social economy, under the action of high-intensity human economic and social activities centered on the development and utilization of water resources, the ecological environment of the basin has changed significantly. In Tarim River Basin, new oases replace original oases, artificial vegetation replaces natural vegetation, artificial soil replaces natural soil, artificial ecology replaces natural ecology, artificial rivers replace natural rivers, and artificial reservoirs replace natural lakes. As a result, the land productivity and water resources utilization efficiency are improved, the oasis microclimate is improved, the population capacity of resources and environment is increased, and the ecological environment of the basin is rapidly deteriorating. Unreasonable land development and disorderly water transfer in the source and upstream areas are the main reasons for the cutoff of the lower reaches of Tarim River Basin and the deterioration of ecological environment. Therefore, it is the key to protect the ecological environment to monitor the land use in the "four sources and one dry" area, curb large-scale reclamation and uncontrolled water use, rationally allocate water resources quota according to the land use area and planting structure, and realize scientific water use regulation. The natural conditions of land development and utilization at the source of Tarim River are good, and it is the key area of indiscriminate reclamation. In recent years, driven by the economic benefits of cotton planting, the phenomenon of land reclamation in the main stream area has become increasingly serious, so the coverage of land use monitoring is the "four sources and one dry" area, with an area of about1:65438+100000 standard map.
2. Vegetation coverage and vegetation type monitoring
Since the 1970s, nearly 400 kilometers of the lower reaches of the Tarim River have been cut off, vegetation in the middle and lower reaches has declined, Populus euphratica forest has died in a large area, and desertification has expanded, which is the most serious ecological deterioration area in the Tarim River basin and the key area for comprehensive management and ecological protection. Seven emergency water transfers were carried out from Bosten Lake to the main stream through Daxihaizi Reservoir. In order to scientifically monitor and evaluate the recovery of vegetation coverage and vegetation types in the basin after the implementation of comprehensive control measures, the middle and lower reaches of the main stream were selected as the remote sensing monitoring range of vegetation factors, covering 20 maps of 1∶65438+ 10,000.
3. Monitoring of land desertification
Land desertification refers to the environmental degradation process in which the fragile ecological balance is destroyed by excessive human activities in the arid and windy sandy surface environment, so that desert landscapes similar to sandstorm activities appear in the original non-desert areas, leading to the decline of land productivity.
From the spatial distribution, desertification and vegetation coverage are decreasing each other. The area with the most serious vegetation decline is also the area with the most serious desertification, so the monitoring range of desertification is consistent with that of vegetation monitoring, and it is selected in the middle and lower reaches of the main stream.
4. Soil salinization monitoring
Tarim basin is a closed inland basin with a dry climate and strong evaporation. Due to the unreasonable utilization of water resources and improper irrigation and drainage in Tarim River Basin, soil generally accumulates salt, forming a large area of saline soil. Alar-Shaya area, in particular, is located in the upper reaches of the main stream where the three sources meet, and the groundwater level is high. A large number of plain reservoirs have been built along the Tarim River, and large areas have been reclaimed, and flooding irrigation has been adopted, resulting in low utilization rate of water resources and serious secondary salinization of soil. Therefore, the upper reaches of the main stream of Tarim River are selected as the working area for remote sensing investigation and monitoring of soil salinization. Coverage 1: 13 million sheets.
(3) High spatial resolution monitoring system in key areas of main stream.
The SPOT-5 satellite was successfully launched by the French company SPOT on May 4th, 2002. Remote sensors include: high-resolution geometric imager (HRG), high-resolution stereo imager (HRS) and vegetation detector. The basic resolution of panchromatic image is 5.0m when shooting, and two images with 5m resolution in the same area are obtained by two different sensor channels at the same time. These two images are in the same focal plane and staggered by 1/2 pixels. CNEG superimposes two images and processes them into an image with a resolution of 2.5 meters through its patented Supermode coding technology. The green (B 1), red (B2) and near infrared (B3) of the multispectral image are all 10m, and the short-wave infrared (SWIR) is 20m. The pixel size of stereoscopic image is 5m× 10m. The panchromatic and multispectral image frames are 60km×60km, the stereoscopic image frames are 120km×600km, and the coverage period is 26 days. Because of the high spectral and spatial resolution and strong pertinence of SPOT-5 data information, it can be combined and extracted according to different application purposes, which greatly expands its application scope in environment, geology and other aspects. Considering the economic and technical factors comprehensively, spot-5 panchromatic band (2.5m, 5m) and multispectral data (10m) are selected as remote sensing monitoring information sources in key areas of the main stream.
The middle and lower reaches of the main stream are the focus of ecological environment monitoring and restoration. On the basis of monitoring the vegetation coverage and vegetation types in the middle and lower reaches of the main stream with medium resolution, three key monitoring areas are selected from the middle reaches to the end of the river (Figure 5- 1): the key monitoring area of usman in the middle reaches of the main stream is used to monitor the ecological impact of embankment projects built since the comprehensive management of the Tarim River basin; In the key monitoring area of Calda in the lower reaches of the main stream, the vegetation restoration in the lower reaches of the main stream has been monitored and evaluated since many emergency water transfers. The taitema lake monitoring area at the end of Tarim River monitors and evaluates the changes of taitema lake waters and vegetation under the background of comprehensive water resources management measures.