The use of remote sensing data in arctic and alpine areas is
especially useful because of the inaccessibility of these areas. Snow
cover mapping is particularly important in relation to hydropower
applications in areas where the catchment is covered in snow during part
of the year. If the Snow Water Equivalent (SWE) of the snow pack can be
estimated, the amount of runoff can be assessed. Local adaptations of
algorithms are usually necessary in order to obtain the best results.
GRAS is currently involved in several snow cover monitoring projects.
Remote sensing data from satellites can
be used to assess snow water equivalents (SWE) using two different
approaches. Either directly by using passive microwave data or by
mapping changes in snow cover from optical satellite data.
SWE can be estimated directly
from passive microwave remote sensing data, since the density and
thickness of the snowpack is proportionally related to the temperature
of the snow. Two brightness temperatures (apparent temperatures in
different wavebands) are used and these are assessed from 37 and 18 GHz
passive microwave signals. Data are available from the American
satellites Scanning Multispectral Microwave Radiometer (SMMR) (from 1978
to 87) and Special Sensor Microwave Imager (SSM/I) (since 1987). The
spatial resolution is approximately 25 x 25 km for these types of data
and 10 day composites or daily data can be obtained. All optical remote
sensing data can be used to assess snow cover; however, data with a high
temporal coverage are preferred.
Data from NOAA AVHRR and Modis are
being used at the present. NOAA AHVRR has a spatial resolution of 1 x 1
km and Modis has 250-1000 m resolution. Both systems can provide daily
and 10 day composite data. Time series of snow cover can be used to
compute the number of days each grid cell is covered with snow. Applying
a simple degree-day model along with a stratification of snow days in
different altitude intervals, allows to assess SWE (must be calibrated
on run-off data). NOAA AVHRR data are available since 1981 in 10 day
composites aggregated to 8 x 8 km grid cells. Studies where snow cover
mapping from NOAA AVHRR 8 km data have been compared to mapping using
high resolution Landsat data (30 m grid cells) shows a good agreement.
Modis data are only available since 2000.
SMM/I, NOAA AVHRR and Modis
data can be obtained free of charge; however, processing must be done.
Both of the suggested methods, the direct assessment of SWE using
passive microwave data and the assessment of SWE through the mapping of
changes in snow cover from optical data, can be applied in near real
time (NRT) mode and are therefore suitable for forecasting purposes. NRT
processing will be based on an application where remote sensing data
are uploaded from a data server within a period of 3 to 12 hours upon
reception. The data are then processed and made available for the
hydrological modelling. The process can be fully automated once the SWE
estimation method has been calibrated and the remote sensing data source
has been identified. Usually it is necessary to analyse 25 to 30 years
of inflow data to the upstream reservoir (and if available the inflow
from sub-basins) and satellite images to calibrate the model.