Joost van der Sanden, Natural Resources Canada, Canada Centre for Remote Sensing
Figure 1: Mackenzie River at Inuvik, NWT. Picture taken 26 May 2008 (break-up).
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River ice constitutes a major component of the cryosphere that modulates natural processes and can either facilitate or jeopardize human activities. Consequently, the significance of river ice in northern countries such as Canada is substantial and multi-facetted. For instance, river ice:
- controls the winter flow regime of rivers and compromises the operations of hydrometric stations.
- provides seasonal road access to locations that lack a regular land-based road network (e.g. communities, hunting / fishing grounds, and mining operations).
- governs the water intake and discharge activities of municipalities and businesses (e.g. hydropower and oil sands industries).
- is hazardous to shipping and, in particular during spring break-up, can create jams and floods that endanger infrastructure (e.g. locks, bridges, pipelines) and communities but may also nurture aquatic ecosystems.
- affects the habitat of wildlife and represents an erosive force that can reshape fluvial landscapes.
- influences weather patterns and, consequently, weather forecasting and climate modelling.
- may be used as an indicator of climate change
It follows, that information on river ice cover supports various science, engineering and management activities including hydraulic / hydrological modelling, break-up forecasting, and decision making related to, for example, water intake / discharge, ice road routing, wildlife management, and ice jam flood emergency preparedness. Typical river ice variables of interest include the timing of freeze-up / break-up, ice coverage, ice type, ice thickness, and ice condition (e.g. dry and solid versus wet and deteriorating).
Often, the collection of information on river ice cover will be complicated by: unfavourable weather conditions, the large extent and poor accessibility of the river of interest, and the dynamic nature of ice cover during the freeze-up and break-up seasons. The capability of remote sensing satellites to routinely image the Earth’s surface in a systematic, synoptic and repetitive manner make satellites potentially outstanding tools for collecting up to date information on river ice cover. The application of Synthetic Aperture Radar (SAR) satellites rather than optical satellites offers certain advantages that are explained from: the capacity of radar to penetrate dry ice cover and snow (see Figure 2), the sensitivity of radar to the presence of free water, and the capability of radar sensors to image independent of weather and daylight conditions.
Current research and development (R&D) activities regarding the application of SAR satellites to the mapping and monitoring of river ice focus on the Mackenzie River near Inuvik, NWT. The work is carried out in the framework of an International Polar Year (IPY) project in collaboration with partners from, among others, Environment Canada, University of Alberta, and Simon Fraser University. The SAR R&D supports one particular objective of the IPY project, namely the development of a hydraulic model for the Mackenzie River Delta. Examples of information derived from radar images that feeds into the development of the hydraulic model include: channel network layout, ice type distribution, ice jam locations, ice cover break-up sequence, and extent/location/duration of break-up flooding. From the radar remote sensing perspective, the research improves our understanding regarding the river ice / radar wave interaction process which translates in knowledge about the potential of satellite SAR to map / monitor certain river ice cover characteristics. The available radar data set comprises images acquired by RADARSAT-1, RADARSAT-2, Envisat ASAR, ALOS PALSAR and TerraSAR-X.
Results to date have shown that SAR satellites make good tools in support of the mapping and monitoring of river ice cover. Their potential in support of the mapping of ice cover conditions during spring break-up is demonstrated in this RADARSAT-1 derived information product (Figure 3). The product shown, also available in a GIS-ready format, represents one out of 11 maps generated for the 2008 spring break-up season. The approach used to generate these maps is guided by expert knowledge regarding the breakup process and exploits the information contained in both the strength and the spatial variability of the radar return signal. The capacity of second generation SAR satellites (e.g. RADARSAT-2, ALOS PALSAR, TerraSAR-X) to simultaneously acquire images in multiple polarizations considerably improves their potential to map winter ice cover, in particular. The interpretation of polarimetric radar images is complicated by the challenges associated with the collection of ground reference data. R&D in this particular field is ongoing.
Figure 2(a-b). Image subsets showing Mackenzie River near Inuvik, NWT, Canada:
(a) Landsat TM true color image composite acquired 25 March 2009
(b) RADARSAT-2 multi-polarization image composite acquired 8 March 2009. The RADARSAT-2 image can be seen to show many ice cover details that are not visible in the Landsat TM image nor, by extension, to the human eye.
NOTE: RADARSAT-2 Data and Products © MacDONALD, DETTWILER AND ASSOCIATES LTD. 2009 - All Rights Reserved; RADARSAT is an official mark of the Canadian Space Agency