Add PEcAn-to-ILAMB GeoTIFF conversion pipeline (GH-4019)

[Tejas7007]

Adds a Python pipeline in PEcAn.benchmark to convert downscaled SDA reanalysis GeoTIFF ensemble maps into ILAMB-compatible CF-1.8 netCDF for cVeg, cSoil, mrsol, and lai, with unit conversions and 13 unit tests.

Description

This is the first GSoC 2026 contribution toward benchmarking the PEcAn North American carbon reanalysis (Zhang et al. 2026) against TRENDY, CMIP, and observational datasets using ILAMB.

The pipeline (modules/benchmark/inst/ilamb/):

• Reads the 100 GeoTIFF ensemble members per variable per year
• Computes the ensemble mean (memory-safe, one member at a time)
• Coarsens from 1km to ILAMB's default 0.5 degree resolution
• Applies unit conversions to CMOR standards
• Writes CF-1.8 compliant netCDF with time bounds and CF coordinates

Variable mapping and conversions:

PEcAn variable	CMOR name	Source units	Conversion	ILAMB units
AbvGrndWood	cVeg	Mg C ha-1	x 0.1	kg m-2
TotSoilCarb	cSoil	kg C m-2	none	kg m-2
SoilMoistFrac	mrsol	vol. percent	x 9.98	kg m-2
LAI	lai	m2 m-2	none	m2 m-2

Follow-up PRs will add flux variables (GPP, NEE), ILAMB scoring configuration, and ensemble probabilistic diagnostics.

Motivation and Context

Benchmarking the PEcAn carbon reanalysis against other models and observations requires the data in ILAMB-readable CF-convention netCDF. This pipeline produces that from the downscaled GeoTIFF product. Closes #4019.

Review Time Estimate

• Immediately
• Within one week
• When possible

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My change requires a change to the documentation.
• My name is in the list of CITATION.cff
• I agree that PEcAn Project may distribute my contribution under any or all of
  • the same license as the existing code,
  • and/or the BSD 3-clause license.
• I have updated the CHANGELOG.md.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[mdietze]

I'd recommend moving your name down to the bottom of the list. You're definitely not "second author" on PEcAn

[DongchenZ]

Code looks clean, well-structured, and well-documented. Nice work!