Landsat-based mapping of thermokarst lake dynamics on the Tuktoyaktuk Coastal Plain, Northwest Territories, Canada since 1985

• A new Landsat water extraction technique outperforms traditional methods.
• Trends in Landsat water are computed from 1985 to 2011 over the Tuktoyaktuk Plain.
• Results show expansion for most lakes, while few lakes exhibited rapid drainage.
• The net result of expansion and drainage is an overall increase in lake area.
• Results are consistent with previous studies, but not with a study of the same region.

Several remote sensing studies have documented widespread thermokarst lake expansion in continuous permafrost regions of North America over the past few decades. Other studies have found no long-term trends in water body extents, but large intra- and inter-annual changes driven by precipitation. These differences could be due to geographic variability in physical conditions (geology, climate, permafrost and hydrology) or in the data and methods used to extract water bodies. This study tested water extraction methods over the Tuktoyaktuk Coastal Plain, Northwest Territories (NWT), Canada, based on the Landsat 5 shortwave infrared (SWIR) channel and validated them using water extents obtained from 0.5 m resolution orthophoto imagery. Methods included applying thresholds to generate binary land/water classifications, as well as deriving 30 m water fractions from both linear unmixing and a new histogram breakpoint method. Results indicated that the histogram breakpoint method outperformed other methods, underestimating overall water fraction by 0.26% and overestimating the number of pure 30 m water pixels by 0.66% compared to water fractions calculated from orthophotos. The breakpoint method was then applied to a stack of 17 near peak-of-season Landsat 5 and Landsat 7 images acquired between July 4 and August 18 from 1985 to 2011 to create a water fraction time-series for examining both trends and inter-annual variation in water extent. Results showed an overall expansion of lake area along margins by 55 km2 with isolated lakes experiencing rapid drainage totalling 15 km2, leading to a net water area increase of 40 km2 over the 26-year period.