Reductions in the duration of annual lake ice cover in Alaska are expected to produce profound environmental changes. These include biological productivity changes in aquatic and terrestrial ecosystems, reduced albedo, and shifts in the timing of ice-jam flooding – all of which impact human activities including transportation and subsistence. The duration and seasonality of lake ice is sensitive to wind, air temperature, and snow accumulation and is correlated with climate variability and change. To help understand these dynamics, the Southwest Alaska Network of the NPS Inventory and Monitoring Program is using Moderate Resolution Imaging Spectroradiometer (MODIS) satellite data to analyze lake ice phenology-- the freeze and break-up timing of lake ice--for 17 southwestern Alaska lakes and lake clusters from 2001 to 2016.
Near-daily 250-m resolution visible, near-infrared (NIR), and shortwave infrared (SWIR) reflectance data from the MODIS Terra and Aqua sensors are used in this analysis. The frequent imagery collection allows the detection of ice cover on lakes as it forms, collects snowfall, intermittently thaws, refreezes, and clears when lake extents are free of cloud cover. A grid-based mapping approach to estimate percentages of lake area covered by ice is accomplished through supervised classification using a 1-km2 grid in which each grid square is given a value of ice-covered (50–100% ice cover) or ice-free (0–49% ice cover) status for each day of the year that the areal lake extent is visible. Calibration and validation of results is accomplished using ground-based observations from lake temperature arrays and remote automated weather stations. Correlation is apparent between increased air temperature and reduced lake ice cover and duration.
The analysis produced a dataset of daily percent lake ice cover for each of 17 lakes or lake clusters with dates for initial freeze-up (days with greater than 10% but less than 90% ice cover), complete ice cover (days with greater than 90% ice cover), break-up (days after complete cover with less than 90% but more than 10% ice cover), and complete exposure (days with less than 10% ice cover). Data for 2001–2016 will be published and available from the Southwest Alaska Network. The use of these data in collaborative research to investigate the interaction between lake ice phenology on aquatic and terrestrial ecosystems is ongoing and encouraged.
http://science.nature.nps.gov/im/units/swan/monitor/lake_ice_about.cfm
MODIS images of Lake Iliamna February 25, 2015, with 48% ice cover. True color (bands 1,4,3) (top), NIR and SWIR (bands 7,2,1) (middle), and estimation of total ice cover using 1-km2 grid masked to the lake’s margin (bottom).