Modeling the Distribution of Mountain Permafrost in Chile

Abstract. Mountain permafrost is an important feature affecting slope stability and hydrological processes, yet its distribution remains poorly understood in many parts of the world, including Chile. This study develops the first countrywide high-resolution (30 m x 30 m) model of mountain permafrost distribution in mainland Chile, using geomorphological evidence from intact (active and inactive) and relict rock glaciers, along with empirical indicators of permafrost presence/absence primarily derived from borehole temperature records, test pits, and surface temperature measurements. We employ a generalized additive model representing local and regional trends by incorporating mean annual air temperature, potential incoming solar radiation, and latitude as predictors. This model achieved an area under the receiver operating characteristic curve (AUROC) of 0.70 (0.74) in spatial (non-spatial) cross-validation. The model's predictions generate a Permafrost Favorability Index (PFI), which expresses the potential of permafrost occurrence conditional on the predictor variables. Excluding glaciers, rock glaciers and vegetated surfaces, areas with PFI values ≥0.75 were classified as having favorable conditions for permafrost development. Under this criterion, approximately 1.06 % (8,042 km²) of mainland Chile exhibits conditions suitable for mountain permafrost, concentrated in the Atacama, Antofagasta, Coquimbo, and Santiago Metropolitan regions (21–32° S and 33–34° S). In contrast, permafrost is scarce or absent from the Maule to the Magallanes regions (south of ~36° S). The interpretation of PFI values should consider local environmental factors not included in the model, such as snow cover duration, clast size, soil properties, and surface albedo. These variables may influence the presence or absence of permafrost locally and should be accounted for using an interpretative guide. This first version of the permafrost distribution model provides a baseline for understanding its general distribution in Chile, which should be refined as new empirical evidence and improved subsurface temperature records become available in the future.