NLDAS GES DISC Point Download

Overview

Use this skill to retrieve NLDAS_FORA0125_H version 2.0 primary hourly forcing ("File A") from NASA GES DISC for a single point. The user-provided location is always longitude first, latitude second. Output must contain only the requested location or the nearest native NLDAS grid point, never a spatial grid.

Prefer the bundled script for repeatable downloads and point extraction. By default, it uses GES DISC OPeNDAP constraint expressions to request only scalar values for the nearest NLDAS grid cell; it does not download full gridded NetCDF granules unless --access-method https-granule is explicitly selected. In this repository, use .venv-cmip6/bin/python when extracting values because it includes the NetCDF stack needed by the fallback granule path:

.venv-cmip6/bin/python .agents/skills/nldas-gesdisc-point-download/scripts/download_nldas_fora_point.py \
  --lon -72.1715 \
  --lat 42.5378 \
  --auto-span complete-calendar-years \
  --allow-large-download \
  --output result/nldas/US-Ha1_nldas_fora_primary.csv

If credentials are not provided as command-line arguments, the script checks USR_NLDAS and PASSWD_NLDAS. A .netrc file is used only when explicitly passed with --netrc-file. Never write credentials to output files, manifests, logs, or shell snippets shown back to the user.

When USR_NLDAS and PASSWD_NLDAS are defined in ~/.bashrc but are not visible in the current shell process, the script sources ~/.bashrc and reads only those two variables into memory.

Inputs

• --lon: longitude in decimal degrees, west negative. Must be in the NLDAS domain [-125, -67].
• --lat: latitude in decimal degrees. Must be in the NLDAS domain [25, 53].
• --start, --end: UTC timestamps. Accept YYYY-MM-DD, YYYY-MM-DDTHH, or YYYY-MM-DDTHH:MM. The script downloads all whole UTC hours from start through end, inclusive.
• --auto-span complete-calendar-years: infer the first complete UTC calendar year and last complete UTC calendar year from the GES DISC archive. Use this for EcoSIM annual forcing.
• --auto-span available-years: start at the first actual product hour and end at the last complete UTC calendar year. For NLDAS_FORA0125_H, this includes partial year 1979 starting at 1979-01-01T13:00Z.
• --variables: optional native NLDAS variable list. Defaults to the EcoSIM climate-native set: Tair, Wind_E, Wind_N, Rainf, Qair, SWdown, PSurf.
• --variable-set: ecosim-climate by default; use all-primary-file-a only when the user explicitly asks for non-EcoSIM File A fields.
• --username, --password: optional Earthdata credentials that override USR_NLDAS and PASSWD_NLDAS.
• --credential-profile: bash profile to source for credentials, default ~/.bashrc.
• --access-method: opendap-point by default for scalar point API access; use https-granule only when debugging or when the OPeNDAP service is unavailable.
• --keep-raw: keep full hourly NetCDF granules after point extraction. By default, newly downloaded raw granules are deleted after selected-location CSV extraction.
• --allow-large-download: required when a request exceeds the large-download threshold. Without it, the script writes a manifest with the inferred span and aborts before issuing many hourly API requests.
• download_nldas_fora_point.py --allow-invalid and download_nldas_giovanni_ecosim.py --allow-invalid: allow outputs even when validity checks flag out-of-range values. By default, validity failures make the command exit nonzero after writing the manifest or quality report.

Data Source Facts

Load references/nldas_gesdisc_api.md when you need endpoint details, official links, file naming, authentication notes, or variable definitions.

Core facts:

• Collection: NLDAS_FORA0125_H_2.0.
• OPeNDAP point API base: https://hydro1.gesdisc.eosdis.nasa.gov/opendap/NLDAS/NLDAS_FORA0125_H.2.0/.
• HTTPS archive base: https://hydro1.gesdisc.eosdis.nasa.gov/data/NLDAS/NLDAS_FORA0125_H.2.0/.
• File pattern: YYYY/DDD/NLDAS_FORA0125_H.AYYYYMMDD.HH00.020.nc, where DDD is day of year.
• Spatial domain: west -125, east -67, south 25, north 53.
• Native temporal resolution: hourly UTC.
• Dataset summary: https://disc.gsfc.nasa.gov/datasets/NLDAS_FORA0125_H_2.0/summary?keywords=NLDAS.
• CMR reports the first product time as 1979-01-01T13:00:00Z; the GES DISC archive currently has year folders through 2026, so on 2026-06-26 the complete calendar-year span is 1980-01-01T00:00Z through 2025-12-31T23:00Z.

EcoSIM climate-native variables downloaded/extracted by default:

• Tair: maps to TMPH after K to degC conversion.
• Wind_E, Wind_N: map to WINDH as scalar wind speed.
• Rainf: maps to RAINH; native kg m-2 hourly accumulation is numerically equivalent to mm water.
• Qair, PSurf: used to derive atmospheric vapor pressure for DWPTH.
• SWdown: maps to SRADH.
• PSurf: maps to PATM after Pa to kPa conversion.

EcoSIM annual auxiliary variables written by the Giovanni EcoSIM writer:

• Z0G, IFLGW, and ZNOONG are derived directly from site/grid assumptions and longitude.
• For US/NLDAS-domain grids, precipitation chemistry variables are added from the same NADP source used by US AmeriFlux workflows: PHRG, CN4RIG, CNORIG, CSORG, CCARG, and other supported template variables when source rasters exist.
• The Giovanni EcoSIM writer defaults to --add-us-chemistry auto, which adds NADP chemistry for coordinates in [-125, -67] longitude and [25, 53] latitude. Use --add-us-chemistry never to skip or --add-us-chemistry always to force.
• Use --chemistry-input to point to an alternate NADP raster directory and --chemistry-output to choose the intermediate extracted-chemistry JSON path.

Other primary File A variables, only when explicitly requested:

• LWdown: downward longwave radiation, W m-2.
• PotEvap: potential evaporation, kg m-2.
• CAPE: convective available potential energy, J kg-1.
• CRainf_frac: fraction of total precipitation that is convective.

Workflow

1. Confirm the user supplied longitude then latitude. If a value looks swapped, stop and ask for clarification because the grid-cell choice affects water and energy forcing.
2. Confirm the requested period and variables. Interpret all times as UTC unless the user explicitly says otherwise.
3. When the user asks for "first to last available year" for EcoSIM, use --auto-span complete-calendar-years; this drops the partial first year (1979) and incomplete trailing year (2026 as of June 26, 2026).
4. Ensure Earthdata credentials are available when GES DISC requires authentication. The standard repo workflow is to set USR_NLDAS and PASSWD_NLDAS in ~/.bashrc; the script sources that file automatically when needed. The user may still pass --username and --password to override.
5. Run the script from the repository root.
6. Inspect the generated CSV and manifest:
   • Check requested lon/lat and selected grid lon/lat.
   • Check that every requested UTC hour is represented.
   • Check that the variable set is ecosim-climate unless the user explicitly requested another set.
   • Check units before converting to EcoSIM forcing names.
   • Check the quality report's validity_checks.all_passed field before using the NetCDF in an EcoSIM run.
   • For EcoSIM NetCDF outputs, check annual_forcing.auxiliary_climate_variables to confirm whether NADP chemistry was added, skipped, or default-filled.

Output Rules

• Put outputs under result/ unless the user provides another path.
• The EcoSIM climate NetCDF is named by the forcing years it contains, <SITE_ID>_nldas_ecosim_climate_<first>-<last>.nc.
• The default OPeNDAP access path writes only scalar point values for the selected nearest native NLDAS grid cell. The HTTPS archive fallback serves full hourly NetCDF granules; when that fallback is used, the script extracts the point and removes newly downloaded raw granules unless --keep-raw is supplied.
• Long automatic spans are request-heavy. For 1980-2025, expect 403,248 hourly point API requests. The script refuses such runs unless --allow-large-download is explicitly supplied.
• Write a JSON manifest next to the CSV. Include source URLs, selected grid cell, variables, units, and download timestamps, but never include username or password.
• Preserve native units. Do not convert precipitation, pressure, or temperature for EcoSIM until a downstream conversion step is explicitly requested.

EcoSIM Unit Cautions

• Tair is Kelvin; EcoSIM climate tools often expect degC for TMPH.
• PSurf is Pa; EcoSIM climate tools often use kPa for PATM.
• Rainf is hourly accumulated water mass per area (kg m-2), numerically equivalent to mm water depth for liquid water density. Do not treat it as a kg m-2 s-1 rate unless the file metadata for a future product says so.
• Wind_E and Wind_N are vector components. Compute wind speed as sqrt(Wind_E**2 + Wind_N**2) only when the downstream workflow needs scalar speed.

Validity Checks

The bundled point downloader and Giovanni EcoSIM downloader write explicit validity_checks to the manifest or quality report and fail by default when a variable is outside its accepted range.

Native NLDAS checks:

• Tair: 183.15 to 333.15 K (-90 to 60 degC).
• Wind_E, Wind_N: -75 to 75 m s^-1.
• Rainf: 0 to 500 kg m^-2 h^-1.
• Qair: 0 to 0.05 kg kg^-1.
• SWdown: 0 to 1400 W m^-2.
• PSurf: 50000 to 110000 Pa.
• LWdown: 0 to 800 W m^-2.
• PotEvap: 0 to 500 kg m^-2 h^-1.
• CAPE: 0 to 10000 J kg^-1.
• CRainf_frac: 0 to 1.

EcoSIM checks:

• TMPH: -90 to 60 degC.
• WINDH: 0 to 75 m s^-1.
• RAINH: 0 to 500 mm h^-1.
• DWPTH: 0 to 15 kPa.
• SRADH: 0 to 1400 W m^-2.
• PATM: 50 to 110 kPa.

Annual auxiliary checks:

• Z0G: positive measurement height, 0.01 to 100 m.
• IFLGW: allowed flags 0 or 1.
• ZNOONG: 0 to 24 hour.
• PHRG: 0 to 14 pH.
• Precipitation chemistry concentrations: non-negative.

NADP chemistry values must be finite and non-negative, with PHRG constrained to pH 0-14. Annual gaps are filled by linear interpolation with nearest-edge filling. If no valid NADP pH is available, use neutral PHRG=7 and record the fallback in the quality report. Do not silently apply NADP chemistry outside the US/NLDAS auxiliary domain.

Validation

For any delivered extraction, report:

• Product short name and version.
• Requested longitude/latitude and selected grid-cell longitude/latitude.
• UTC start/end and number of hourly files.
• Variables extracted and their native units.
• Whether validity_checks.all_passed is true and which variables failed if false.
• Whether authentication was supplied via CLI arguments, USR_NLDAS/PASSWD_NLDAS, or not needed. Do not reveal credential values.