sidered to provide only a crude approximation and is not generally recommended (Barry, 2006), it was used for many years for the basin of the Grosse Aletsch in Switzerland (PSFG, 1967, 1973). The hydrological approach is difficult to use in the HKH area because of limited precipitation and discharge measurements. However, the approach does hold promise as more basins become instrumented with automatic weather stations and automated measurements of discharge height, as demonstrated for the Langtang Basin in Nepal by the recent International Centre for Integrated Mountain Development (ICIMOD) field class on glacial mass balance measurements.
REMOTE SENSING MEASUREMENTS
Remote sensing measurements are based on information gained by aerial sensing technologies installed on aircraft and satellites. Scientists are increasingly able to measure glacier properties over larger areas and longer time spans because of the increased availability of imagery from remote sensing platforms. Remote sensing yields information about glacier properties including glacier area, length, surface elevation, surface flow fields, accumulation/ablation rates, albedo, equilibrium line altitude (ELA), accumulation area ratio, and the mass balance gradient. The ELA, accumulation area ratio, and mass balance gradient respond to annual changes in temperature, precipitation, and humidity and are therefore important for mass balance monitoring. Changes in glacier area and terminus positions over timescales of several decades, which are relatively easy to determine from remote sensing imagery, have been used as indicators of a glacier’s response to climate forcing (Barry, 2006).
Remote sensing uses sensors that detect solar energy reflected by Earth’s features as well as the emission of the Earth’s own thermal energy (Figure C.1). Wavelength ranges (or bands) of interest to remote sensing of glaciers generally include visible light (VIS ranging from about 0.45 to 0.75 µm), near-infrared (NIR ranging from 0.75 to 1.4 µm), short-wavelength infrared (SWIR ranging from 1.4 to 3 µm), and thermal infrared (TIR ranging from about 8 to 15 µm). Sensors record the brightness temperature within a defined band that depends on the characteristics of the specific sensor, as illustrated for two different sensors in Figure C.1. For example, Band 2 of Landsat1 7 records the brightness temperature from 0.53 to 0.61 μm, often mapped as the blue band.
Snow and ice are characterized by (1) high reflectivity in the visible wavelengths, (2) medium reflectivity in the near-infrared, (3) low reflectivity and high emissivity in the thermal infrared, and (4) low absorption and high scattering in the microwave (Racoviteanu et al., 2008). Increasing amounts of black carbon quickly reduce the albedo of snow and ice.
The spatial resolution of a remote sensing image is critical to obtaining glacial properties in mountainous terrain, such as the ELA. A pixel size of 500 m on a side (e.g., MODIS)2 means that, in general, the remote sensing information can only detect changes that occur in lengths greater than 500 m.
Newer sensors can acquire data at medium spatial resolutions (5 to 90 m in multispectral mode), with larger swath widths (185 km for Landsat and 60 km for ASTER)3 and short revisit times (16 days for ASTER). These capabilities allow regular glacier mapping over extensive areas. The thermal band of Landsat Enhanced Thermal Mapper Plus (EMT+)4 (10.4 to 12.5 μm, at 60-m pixel size) and the multispectral thermal bands of ASTER (8.125 to 11.65 μm, at 90 m pixel size) could potentially distinguish debris cover on glaciers. The ASTER, SPOT5,5 IRS-1C,6 and CORONA KH-4, KH-4A and KH-4B7 can acquire stereoscopic images, which in turn provide elevation data that can be used for geodetic mass balance estimates (Racoviteanu et al.,
1 Landsat is a series of Earth-observing satellite missions jointly managed by the National Aeronautics and Space Administration and the U.S. Geological Survey. More information about the pro gram is available at http://landsat.gsfc.nasa.gov/.
2 The Moderate Resolution Imaging Spectroradiometer is an instrument aboard the Terra (EOS AM) and Aqua (EOS PM) satellites. More information about the instrument is available at http://modis.gsfc.nasa.gov/.
3 The Advanced Spaceborne Thermal Emission and Reflection Radiometer is an instrument aboard the Terra (EOS AM) satellite. More information about the instrument is available at http://asterweb.jpl.nasa.gov/.
5 A fifth-generation Earth observation satellite launched in 2002, the series of SPOT satellites were initiated by the French space agency and are run by Spot Image base in Toulouse, France.
6 India’s second generation remote sensing satellite.
7 A series of U.S. reconnaissance satellites launched between 1959 and 1972.