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MODISSNOW.tif - README
Long-term mean MODIS fractional snow cover, Copiapo watershed
(Atacama Region, northern Chile)
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1. PURPOSE OF THIS FILE
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This README describes the raster file MODISSNOW.tif and its auxiliary files. The
raster is the long-term (2000-2022) mean fractional snow cover of the Copiapo
watershed, derived from MODIS daily snow-cover imagery. It is the snow indicator
used to build the snow term of the Normalized Cryospheric Index (NCI) in the
associated article:

  Ulloa, C., Garcia, A. & Beniest, A.
  "Introducing a new normalized cryospheric index (NCI) to categorize
   sub-watersheds on arid environments."
  Manuscript egusphere-2025-1198 (EGUsphere / Copernicus).

For the full methodology, refer to the article (Section 3.2, "Survey of snow
cover: MODIS-derived snow permanence").

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2. WHAT THE RASTER REPRESENTS
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Each pixel stores the per-pixel time-mean fractional snow cover (FSC) computed by
averaging the complete daily MODIS/Terra MOD10A1 Fractional Snow Cover time series
over the period 2000-2022 (23 years).

  - Values are a dimensionless fraction between 0 and 1:
      0   = pixel never/effectively never snow-covered over the record,
      1   = pixel permanently snow-covered over the record.
  - The value can be read as the long-term average proportion of snow within the
    500 m pixel, i.e. a measure of snow permanence / recurrence.
  - Multiplying this fraction by the pixel area and summing over a sub-watershed
    yields the mean snow-covered area (km2) used as the short-term cryospheric
    water-resource indicator in the NCI.

Note: In the article this long-term snow signal is described both as a time-mean
FSC and as "snow permanence" (the two are derived from the same daily FSC series).
Please confirm which of the two definitions this specific raster stores before
publication if the distinction matters for reuse.

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3. FILE AND GRID SPECIFICATION
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  Format .................. GeoTIFF (GTiff), single band
  Data type ............... Float64 (64-bit floating point)
  Bands ................... 1
  Value range ............. 0 to 1 (dimensionless fraction)
  NoData value ............ -1.7976931e+308 (Float64 sentinel);
                            in this file 100% of pixels are valid (no gaps)
  Columns x rows .......... 415 x 441 pixels
  Pixel size .............. 0.0044915764 degrees (~500 m; MODIS native resolution)
  Coordinate ref. system .. Geographic, WGS84 (EPSG:4326)
  Extent (WGS84, degrees):
     West  (min lon) ...... -70.944450
     East  (max lon) ...... -69.080445
     South (min lat) ...... -28.642783
     North (max lat) ...... -26.661998

  Band statistics (over all valid pixels):
     Minimum .............. 0.0000
     Maximum .............. 0.9960
     Mean ................. 0.0227
     Std. deviation ....... 0.0633
  (The low mean reflects the arid, high-Andean setting, where persistent snow is
   restricted to the highest elevations.)

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4. ACCOMPANYING FILES
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The following sidecar files are distributed with the raster and should be kept in
the same folder:

  MODISSNOW.tfw ........... ESRI world file (affine georeferencing; redundant with
                            the GeoTIFF internal georeferencing).
  MODISSNOW.tif.aux.xml ... ESRI/GDAL auxiliary metadata (band statistics and
                            histogram).
  MODISSNOW.tif.ovr ....... Reduced-resolution overviews (pyramids) for faster
                            display; not required to read the data.

The GeoTIFF is self-contained (it carries its own CRS and geotransform); the
sidecar files are optional conveniences.

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5. SOURCE AND PROCESSING
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  Source product .......... MODIS/Terra Snow Cover Daily L3 Global 500 m
                            (MOD10A1), Fractional Snow Cover (FSC), NASA NSIDC.
  FSC derivation .......... Sub-pixel fractional snow cover from a spectral
                            unmixing algorithm (Rittger et al., 2013).
  Temporal coverage ....... 2000-2022 (full daily series).
  Processing .............. Per-pixel temporal average of daily FSC over the
                            record to produce the long-term mean FSC (this raster).
  Study area .............. Copiapo watershed, Atacama Region, northern Chile.

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6. CITATION
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Dataset:
  Ulloa, C., Garcia, A. & Beniest, A. (2026). Long-term mean MODIS fractional
  snow cover of the Copiapo watershed (Atacama, Chile) [Data set]. Zenodo.

Article:
  Ulloa, C., Garcia, A. & Beniest, A. Introducing a new normalized cryospheric
  index (NCI) to categorize sub-watersheds on arid environments.

MODIS source (please also cite the MOD10A1 product and the exact collection
version you used, e.g. Collection 6.1):
  Hall, D. K. & Riggs, G. A. MODIS/Terra Snow Cover Daily L3 Global 500 m SIN
  Grid (MOD10A1). NASA NSIDC DAAC. https://doi.org/10.5067/MODIS/MOD10A1.061

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7. LICENSE
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Creative Commons Attribution 4.0 International (CC BY 4.0).
https://creativecommons.org/licenses/by/4.0/

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8. CONTACT
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Corresponding author (article): Christopher Ulloa - christopher.ulloa@uda.cl
University of Atacama, Geology Department, Avenida Copayapu #485, Copiapo, Chile.

Dataset file: MODISSNOW.tif
README last updated: 2026-07-27
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