The problem the index addresses is both scientific and political. In arid zones, water policy often proceeds without any regulatory framework for identifying which frozen landscapes matter most, and existing watershed classification schemes rely on morphometric analysis, general hydrology, or ecological indicators that rarely account for the cryosphere. Yet in the Copiapó basin, the cryosphere is everything. Long-term records from Chile’s water authority show that the river’s discharge, typically between roughly 1.49 and 1.82 cubic meters per second at the La Puerta gauging station, is strongly modulated by meltwater from its high-altitude tributaries. Centuries of tree-ring reconstructions reveal declining rainfall in northern Chile, and megadroughts have intensified since 2010, making the structural dependence of the river on snow and ice melt more consequential than ever.

Building the index required an enormous mapping effort. The researchers compiled a GIS-ready inventory of 1,862 cryospheric landforms across the watershed: 62 debris-free glaciers, 16 debris-covered glaciers, 173 rock glaciers, 848 gelifluction taluses, and 763 protalus lobes. The numerical dominance of periglacial features is itself a revelation, since gelifluction taluses and protalus lobes together account for 87 percent of inventoried landforms while glaciers make up less than 4 percent. Applying objective criteria, at least one active cryospheric landform plus measurable seasonal snow permanence, the team classified 12 of the basin’s 27 sub-watersheds as cryospheric. These 12 units, all situated at high elevation with a mean of 3,617 meters above sea level, contain 97 percent of the mapped landforms and overlap almost perfectly with high-probability permafrost zones predicted by an independent global permafrost model.

Quantifying the ice hidden in these landforms demanded a unified physics-based framework rather than simple area-volume scaling. For debris-free glaciers, the team integrated the global consensus ice-thickness product of Farinotti and colleagues over Randolph Glacier Inventory outlines. For rock glaciers, protalus lobes, and gelifluction taluses, they applied a viscoplastic creep inversion that links surface velocity, basal shear stress, and ice rheology to estimate thickness, constraining the model with surface velocities derived from Sentinel-1 radar interferometry. The DInSAR processing, with interferometric pairs selected for short temporal and perpendicular baselines, achieved velocity uncertainties of roughly 3 to 4 millimeters per year, sufficient to detect the slow creep of frozen debris slopes. Ice-content fractions were treated as bounded priors, 0.5 plus or minus 0.10 for rock glaciers and 0.37 plus or minus 0.12 for the other periglacial landforms, and all errors were propagated quadratically through to ice mass.

Fieldwork in the Montosa and Del Potro sub-watersheds provided crucial ground truth. Electrical resistivity tomography surveys, using 600-meter profiles with electrodes spaced 5 meters apart, revealed high-resistivity zones exceeding 4 to 10 kilo-ohm meters, the signature of massive or ice-saturated permafrost. The geophysics indicated an ice-rich core roughly 15 meters thick in a gelifluction talus and 20 to 30 meters thick, with local maxima above 35 meters, in an active rock glacier. These thicknesses are an order of magnitude greater than values assumed for analogous landforms in parts of the existing literature, and they suggest that periglacial ice reserves in the arid Andes are being systematically undercounted. Notably, Chile’s national periglacial inventory currently systematizes only rock glaciers, omitting the gelifluction taluses and protalus lobes that dominate the Atacama cryosphere by extent.

The snow component of the index drew on 23 years of daily MODIS fractional snow cover imagery from 2000 to 2022. Rather than relying on isolated satellite scenes, the researchers computed snow permanence, the fraction of days each 500-meter pixel remained snow-covered, and integrated it into mean snow-covered area per sub-watershed. Critically, they validated this variable hydrologically: September snow extent correlated robustly with October-to-March discharge at the Pastillo gauging station, with a coefficient of determination of 0.58 across 22 years. The regression intercept, 0.55 cubic meters per second, represents melt-season flow sustained independently of the current year’s snowpack, presumably by longer-residence ice storage. The ratio of this intercept to each year’s observed discharge yielded the empirical weighting factor that balances ice versus snow in the index, averaging 0.43 but shifting to 0.27 in wet years and 0.62 in dry years.

The resulting rankings reveal a watershed split into two hydrological personalities. The Del Potro River sub-watershed holds the largest ice mass at 0.901 gigatons, followed closely by Montosa with 0.873 gigatons and Manflas with 0.218 gigatons. Under average and dry conditions, Montosa tops the NCI at 0.72 plus or minus 0.12, followed by Estero Come Caballos at 0.62 plus or minus 0.23, Del Potro at 0.61 plus or minus 0.18, Manflas at 0.59 plus or minus 0.17, and Figueroa at 0.50 plus or minus 0.21. In dry-year simulations, Montosa and Del Potro occupy the top two positions in practically every Monte Carlo run, and together the top-ranked basins concentrate roughly 78 percent of the inventoried ice mass. In wet years, however, the picture inverts: northern, snow-rich basins such as Estero Come Caballos surge up the ranking on the strength of their persistent snow cover, despite hosting relatively few cryospheric landforms.

This dual behavior is the central management insight of the index. Southern, ice-rich sub-watersheds function as drought insurance, sustaining baseflow when snowfall fails, while northern, snow-driven basins generate the bulk of runoff in high-precipitation years and may require infrastructure to capture the increased meltwater that warming will deliver. The authors also traced the geography behind the pattern to the South American Arid Diagonal, the belt of extreme aridity that fragments the cryosphere in the north into small, isolated bodies. In the central sector between Cadillal and Caserones hills, orographic blocking by the higher Argentine Andes starves the Chilean slopes of snowfall, producing a conspicuous gap with no cryospheric landforms at all. The very slow DInSAR velocities of Copiapó rock glaciers, millimeters per year, one to two orders of magnitude slower than counterparts near 30 degrees south, are physically consistent with colder, drier permafrost and the cryogenic, rather than glacigenic, origin of these landforms.

The implications extend well beyond one Chilean basin. Projections for the Southern Andes indicate that glacier meltwater contributions may decline markedly during future megadroughts, which raises the urgency of protecting all cryospheric water reserves, including the periglacial stores that global inventories routinely ignore. The NCI framework, with its explicit Monte Carlo propagation of uncertainty and hydrologically grounded weighting, offers a reproducible template for any arid mountain region where frozen landscapes sustain downstream populations. The authors caution that the index is calibrated for the Copiapó watershed and requires complete resource data to transfer elsewhere, and that under the ongoing megadrought the ice term should gain weight in future applications. Still, the message is unambiguous: five sub-watersheds, Montosa, Del Potro, Manflas, Estero Come Caballos, and Figueroa, host 83.8 percent of the basin’s cryospheric ice volume and constitute the foundation of continuous water production for the Copiapó River. Identifying them, the researchers argue, provides the technical basis for integrating glacial and periglacial reserves into Chile’s water management strategy, and potentially into a legal instrument to protect the country’s cryosphere before the frozen reserves that keep a desert river flowing quietly disappear.

Subject of Research: A normalized cryospheric index for ranking sub-watersheds by ice storage and snow permanence in arid Andean environments

Article Title: Introducing a new normalized cryospheric index (NCI) to categorize sub-watersheds on arid environments

Article References: Ulloa, C., García, A., & Beniest, A. (2026). Introducing a new normalized cryospheric index (NCI) to categorize sub-watersheds on arid environments. The Cryosphere, 20(10), 5829-5866. https://doi.org/10.5194/tc-20-5829-2026

Image Credits: AI Generated

DOI: 10.5194/tc-20-5829-2026

Keywords: cryosphere, Normalized Cryospheric Index, Copiapó watershed, Atacama Desert, rock glaciers, permafrost, snow permanence, water resources, DInSAR, Andes, drought, watershed management