11 Snow and Ice
257
simultaneous data for 3 radar frequencies (1.25, 5.3 and 9.6 GHz). The SIR-C also
operated at 4 different transmit/receive polarizations to provide polarimetric data. The
results of snow cover investigations using SIR-C/X-SAR data demonstrated a potential for using multi-frequency polarimetric SAR data to retrieve information such as
the discrimination of wet snow areas (Li and Shi, 1996), snow cover wetness or liquid
water content (Shi and Dozier, 1995; Matzler et aI., 1997), and the water equivalent
of dry snow (Shi and Dozier, 1996).
Although currently the availability of routine SAR data from satellites is limited to
single frequency and single polarization, two polarimetric SAR sensors are planned
for launch in the near future. The Advanced Synthetic Aperture Radar (ASAR),
scheduled to be launched on the European Space Agency's Envisat-l platform in
2000, will provide C-band in a number of alternating polarization modes. Radarsat-2
is the Canadian Space Agency's C-band SAR follow-on to the current Radarsat,
which is planned for launch in late 2001, and will have new advanced capabilities,
including dual or quad polarization options on selected beam modes. The availability
of polarimetric SAR data from these satellite platforms should enhance the use of
active microwave sensors for routine snow cover monitoring in support of hydro log ical applications, especially in small, alpine basins. The appropriate frequency and
polarization combinations for operational monitoring by a single space platform,
however, will be unlikely in the future.
11.4.4 Integration of Various Data Types
Combinations of two different remote sensors can at certain times increase the
information available about the snowpack. As an example, a method has been developed in Finland which combines several different approaches to measure or estimate
snow water equivalent (Kuittinen, 1989). Ground-based point measurements of water
equivalent were made as usual about twice a month. One airborne gamma ray flight
was made at the beginning of the snowmelt season to give line transect values of snow
water equivalent. All available NOAA-A VHRR satellite images in the Spring are used
to provide areal snow water equivalent estimates based on a relationship between the
percentage of bare spots in the snow cover and snow water equivalent (Kuittinen,
1989). The point, line and areal snow water equivalent values were used with a
method of correlation functions and weighting factors, as suggested by Peck et ai.
(1985), to determine an areal value based on all data. Evaluations in the Finland study
have shown that the error of the estimate of areal snow water equivalent is less than
3.5 cm (Kuittinen, 1989). Carroll (1995) also developed a system which combines
ground-based snow observations and airborne gamma ray snow water equivalent
observations to yield gridded snow water equivalent values. Satellite snow cover
extent data are used to constrain the snow water equivalent interpolations to regions
where snow cover is observed to be present. Colour Plate 11.D presents the NOHRSC
snow water equivalent data (1 inch = 2.54 cm) resulting from using a geographic
information system to integrate satellite, airborne, and ground snow data (9-12 April
1997) for Eastern North Dakota during the 1997 Red River of the North snowmelt
flood.
257
simultaneous data for 3 radar frequencies (1.25, 5.3 and 9.6 GHz). The SIR-C also
operated at 4 different transmit/receive polarizations to provide polarimetric data. The
results of snow cover investigations using SIR-C/X-SAR data demonstrated a potential for using multi-frequency polarimetric SAR data to retrieve information such as
the discrimination of wet snow areas (Li and Shi, 1996), snow cover wetness or liquid
water content (Shi and Dozier, 1995; Matzler et aI., 1997), and the water equivalent
of dry snow (Shi and Dozier, 1996).
Although currently the availability of routine SAR data from satellites is limited to
single frequency and single polarization, two polarimetric SAR sensors are planned
for launch in the near future. The Advanced Synthetic Aperture Radar (ASAR),
scheduled to be launched on the European Space Agency's Envisat-l platform in
2000, will provide C-band in a number of alternating polarization modes. Radarsat-2
is the Canadian Space Agency's C-band SAR follow-on to the current Radarsat,
which is planned for launch in late 2001, and will have new advanced capabilities,
including dual or quad polarization options on selected beam modes. The availability
of polarimetric SAR data from these satellite platforms should enhance the use of
active microwave sensors for routine snow cover monitoring in support of hydro log ical applications, especially in small, alpine basins. The appropriate frequency and
polarization combinations for operational monitoring by a single space platform,
however, will be unlikely in the future.
11.4.4 Integration of Various Data Types
Combinations of two different remote sensors can at certain times increase the
information available about the snowpack. As an example, a method has been developed in Finland which combines several different approaches to measure or estimate
snow water equivalent (Kuittinen, 1989). Ground-based point measurements of water
equivalent were made as usual about twice a month. One airborne gamma ray flight
was made at the beginning of the snowmelt season to give line transect values of snow
water equivalent. All available NOAA-A VHRR satellite images in the Spring are used
to provide areal snow water equivalent estimates based on a relationship between the
percentage of bare spots in the snow cover and snow water equivalent (Kuittinen,
1989). The point, line and areal snow water equivalent values were used with a
method of correlation functions and weighting factors, as suggested by Peck et ai.
(1985), to determine an areal value based on all data. Evaluations in the Finland study
have shown that the error of the estimate of areal snow water equivalent is less than
3.5 cm (Kuittinen, 1989). Carroll (1995) also developed a system which combines
ground-based snow observations and airborne gamma ray snow water equivalent
observations to yield gridded snow water equivalent values. Satellite snow cover
extent data are used to constrain the snow water equivalent interpolations to regions
where snow cover is observed to be present. Colour Plate 11.D presents the NOHRSC
snow water equivalent data (1 inch = 2.54 cm) resulting from using a geographic
information system to integrate satellite, airborne, and ground snow data (9-12 April
1997) for Eastern North Dakota during the 1997 Red River of the North snowmelt
flood.