Worksheet introduction
The Himalayas are one of the largest mountain chains on Earth. These mountains have the highest peaks and contain one of the largest ice volumes outside the polar regions.
The glaciers of the Himalayas are an important fresh water resource for the many millions of people living downstream. They also include a number of natural threats to these people, for instance glacier lakes (see below) that may burst out.
In this exercise you will work with two satellite images over the main ridge of the Himalayas, between Bhutan and the Tibet plateau (ca. Lon. 90° E, Lat 28.2° N). Familiarise yourself with the location using the GoogleEarth file. The two satellite scene sections contain one glacier and a glacier lake at its front.
The ASTER satellite image below gives you an overview of the area. Some glaciers are very clean, whereas some are heavily covered by debris. This is because the accumulation areas of these glacier types are surrounded by steep, high rock walls. Rock fall and rock avalanches from these walls fall onto the glacier and the debris is incorporated into the ice. The debris then follows the ice flow and emerges again in the ablation area. Glaciers whose accumulation areas are not surrounded by rock walls are without debris and therefore remain clean. Many of the glaciers terminate in moraine lakes. Some of these lakes could burst out, and represent a severe hazard to the population living down in the valleys. The lakes to the middle right have already caused an outburst flood (see GoogleEarth file).
Ice flow is an important element of the glacier system. Knowledge of the flow speed and flow pattern of a glacier helps us understand its health, as well as its potential reaction and sensitivity to climatic changes. Knowing the flow speed of a glacier also helps us estimate future developments of lakes at its front. The growth of such lakes depends on the balance between the glacier’s ice supply (i.e. ice flow) and the ice melt at the ice front. If this ice melt is greater than the ice flow, then the lake can grow. If the ice flow exceeds the ice melt, the lake may shrink.
In order to reach the area shown in the satellite image you would have to walk for about 2 weeks and have a fully-equipped expedition. Many sections of the high-mountain terrain shown in the image are very hard to access. That is why the only way we can compile a map of glaciers in this region is by using satellite images.
Purpose of the exercise
In this exercise you will work with two ASTER images of the Himalayas between Bhutan and Tibet. One image is from 20 January 2001, and the other is from 20 November 2001. The two ASTER images have been processed so that all topographic distortions between the two were removed . This process is called orthorectification, or orthoprojection. Basically, the two images do not have their raw geometry anymore, but a map geometry.
By comparing a section of two repeat satellite images you will be able to assess if it is possible to see and measure the ice flow of a glacier. You will also learn about the limitations of this method.
Data
For this exercise you will be working with:
- ast_20jan2001.tif: Section of channel 3N of an ASTER satellite image of 20 January 2001. Orthorectified, UTM projection. Pixel size: 15 x 15 m.
- ast_20nov2001.tif: Section of channel 3N of an ASTER satellite image of 20 November 2001. Orthorectified, UTM projection. Pixel size: 15 x 15 m.
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