Lake Ice Phenology: A New Analysis Approach Yields New Insights

Large lakes freeze less reliably in the winter. Modeling weather conditions can help predict the likelihood of freezing on different sized lakes.

The seasonal freezing of lakes is important for the ecosystems supporting fish, wildlife, and human activities, but a warming climate is likely to impact the timing and reliability of lake ice cover. To understand how lake ice might change, scientists can use sophisticated mathematical representations that require detailed measurements of the physical characteristics of the lakes being studied, but detailed measurements of physical characteristics are rarely available for Alaskan lakes. To overcome this challenge, National Park Service scientists from the Southwest Alaska Network used satellite imagery and a statistical technique called “survival analysis” to predict lake ice formation and breakup. Using only a few meteorological variables and geographic information, they were then able to project ice cover patterns back in time as far as the early 1980s using historic weather data. Their findings reveal interesting patterns in the seasonality of ice cover in Southwest Alaska.

The scientists found volume of a lake is an important predictor of its freezing behavior, with low-volume lakes freezing very regularly and for long periods, and high-volume lakes having shorter, more variable ice seasons and some winters without complete freeze over. Ice-cover season has also shortened in most, but not all, of the lakes studied over the past 40 years. While the largest lakes in this study experience years with incomplete freezing throughout the entirety of the record, the smallest lakes continue to reliably freeze.

The success of this analytical approach opens the door to future applications. With climate model projections, it is possible to project the future of lake-ice seasons for the lakes in this study. Sophisticated machine learning algorithms may be able to generate lake-ice seasonality data for more lakes in Alaska, which, in turn, could be analyzed in a similar manner. Other spatial analyses may allow even more reliable predictions of the ice season at each lake. All of these can help us better prepare for a changing winter season in the future. It’s important to note that the findings of this work only pertain to the surficial cover of lake ice and do not provide any information about ice thickness; thus, they don’t provide guidance for safe travel over ice.

A graphic summary of the abstract that shows the main components of the study.

Volume-Mediated Lake-Ice Phenology in Southwest Alaska Revealed through Remote Sensing and Survival Analysis

Abstract

Lakes in Southwest Alaska are a critical habitat to many species and provide livelihoods to many communities through subsistence fishing, transportation, and recreation. Consistent and reliable data are rarely available for even the largest lakes in this sparsely populated region, so data-intensive methods utilizing long-term observations and physical data are not possible. To address this, we used optical remote sensing (MODIS 2002–2016) to establish a phenology record for key lakes in the region, and we modeled lake-ice formation and breakup for the years 1982–2022 using readily available temperature and solar radiation-based predictors in a survival modeling framework that accounted for years when lakes did not freeze. Results were validated with observations recorded at two lakes, and stratification measured by temperature arrays in three others. Our model provided good predictions (mean absolute error, freeze-over = 11 days, breakup = 16 days). Cumulative freeze-degree days and cumulative thaw-degree days were the strongest predictors of freeze-over and breakup, respectively. Lake volume appeared to mediate lake-ice phenology, as ice-cover duration tended to be longer and less variable in lower-volume lakes. Furthermore, most lakes < 10 km³ showed a trend toward shorter ice seasons of −1 to −6 days/decade, while most higher-volume lakes showed undiscernible or positive trends of up to 2 days/decade. Lakes > 20 km³ also showed a greater number of years when freeze-over was neither predicted by our model (37 times, n = 200) nor observed in the MODIS record (19 times, n = 60). While three lakes in our study did not commonly freeze throughout our study period, four additional high-volume lakes began experiencing years in which they did not freeze, starting in the late 1990s. Our study provides a novel approach to lake-ice prediction and an insight into the future of lake ice in the Boreal region.

Kirchner, P. B. and M. P. Hannam. 2024. Volume-mediated lake-ice phenology in Southwest Alaska revealed through remote sensing and survival analysis. Water 16(16): 2309.