{
  "cdh_schema_version": "v0.5.0",
  "extensions": [
    "https://cgiar-climate-data-hub.github.io/cdh-metadata-standard/v0.5.0/extensions/cdh/schema.json",
    "https://cgiar-climate-data-hub.github.io/cdh-metadata-standard/v0.5.0/extensions/climate/schema.json"
  ],
  "id": "africa-spei-drought",
  "title": "3- and 12-month SPEI from CHIRPS v3 and CHIRTS-ERA5 (Africa)",
  "version": "1",
  "series": {
    "name": "Africa Agriculture Adaptation Atlas",
    "url": "https://adaptationatlas.cgiar.org/"
  },
  "description": "Standardized Precipitation-Evapotranspiration Index (SPEI) at 3-month and 12-month accumulation, as per-pixel monthly Cloud Optimized GeoTIFFs over Africa, 1981 to present. The index is computed from the climatic water balance (CHIRPS v3 precipitation minus Hargreaves potential evapotranspiration from CHIRTS-ERA5 temperature) on the 0.05° CHIRPS grid, fitted to a log-logistic distribution over the 1991-2020 reference period. SPEI-03 tracks seasonal and agricultural drought; SPEI-12 tracks hydrological and multi-year drought.\n",
  "resource_type": "dataset",
  "license": "CC-BY-4.0",
  "attribution": "Derived from CHIRPS v3 and CHIRTS-ERA5, Climate Hazards Center, UC Santa Barbara.",
  "keywords": [
    "drought",
    "SPEI",
    "Standardized Precipitation-Evapotranspiration Index",
    "climatic water balance",
    "potential evapotranspiration",
    "Hargreaves",
    "log-logistic",
    "ENSO"
  ],
  "note": "Files for the first months of each accumulation window exist but hold no data (1981-01 to 1981-02 for SPEI-03, 1981-01 to 1981-11 for SPEI-12). In arid cells the log-logistic fit is poorly constrained: non-finite results are stored as NoData, and magnitudes well beyond ±3 can occur where the fit is degenerate rather than indicating record drought or wetness.\n",
  "contact": [
    {
      "name": "Peter Steward",
      "organization": "Alliance of Bioversity International and CIAT",
      "email": "p.steward@cgiar.org",
      "roles": [
        "producer",
        "processor",
        "point-of-contact",
        "maintainer"
      ]
    },
    {
      "organization": "Alliance of Bioversity International and CIAT",
      "url": "https://alliancebioversityciat.org/",
      "roles": [
        "licensor"
      ]
    }
  ],
  "citation": {
    "authors": [
      {
        "family": "Steward",
        "given": "P."
      }
    ],
    "date": "2026",
    "publisher": "Alliance of Bioversity International and CIAT"
  },
  "related_publications": [
    {
      "doi": "10.1175/2009JCLI2909.1",
      "citation": {
        "title": "A Multiscalar Drought Index Sensitive to Global Warming: The Standardized Precipitation Evapotranspiration Index",
        "authors": [
          {
            "family": "Vicente-Serrano",
            "given": "S. M."
          },
          {
            "family": "Beguería",
            "given": "S."
          },
          {
            "family": "López-Moreno",
            "given": "J. I."
          }
        ],
        "date": "2010",
        "publisher": "Journal of Climate"
      }
    },
    {
      "doi": "10.13031/2013.26773",
      "citation": {
        "title": "Reference Crop Evapotranspiration from Temperature",
        "authors": [
          {
            "family": "Hargreaves",
            "given": "G. H."
          },
          {
            "family": "Samani",
            "given": "Z. A."
          }
        ],
        "date": "1985",
        "publisher": "Applied Engineering in Agriculture"
      }
    }
  ],
  "created": "2026-08-28",
  "updated": "2026-10-14",
  "spatial": {
    "bbox": [
      -20,
      -40,
      55,
      40
    ],
    "crs": "EPSG:4326",
    "geography": [
      "africa"
    ]
  },
  "temporal": {
    "start_date": "1981-01",
    "end_date": null
  },
  "climate": {
    "baseline": {
      "start_date": "1991-01-01",
      "end_date": "2020-12-31"
    }
  },
  "structures": [
    {
      "name": "monthly",
      "dimensions": [
        {
          "name": "cell",
          "type": "xy",
          "description": "0.05 deg (~5.5 km, CHIRPS native).",
          "step": 0.05,
          "unit": "degree"
        },
        {
          "name": "time",
          "type": "temporal",
          "description": "Last month of the accumulation window (YYYY-MM); the {time} token of the monthly asset.",
          "extent": [
            "1981-01",
            "2026-04"
          ],
          "step": "P1M"
        }
      ],
      "variables": [
        {
          "name": "SPEI-03",
          "description": "SPEI over the 3 months ending in the labelled month: the standardized climatic water balance (precipitation minus potential evapotranspiration) relative to the 1991-2020 distribution for that calendar month. Zero is the reference median; negative is drier than the reference climatology, positive wetter. Values below -1 indicate moderate and below -2 extreme drought.\n",
          "data_type": "float32",
          "unit": "1",
          "nodata": "NaN"
        },
        {
          "name": "SPEI-12",
          "description": "SPEI over the 12 months ending in the labelled month, standardized as for SPEI-03. The longer window responds to sustained deficits in soil moisture, streamflow and storage.\n",
          "data_type": "float32",
          "unit": "1",
          "nodata": "NaN"
        }
      ]
    }
  ],
  "processing": [
    {
      "id": "source",
      "description": "Monthly CHIRPS v3 precipitation and CHIRTS-ERA5 monthly maximum and minimum temperature, cut to the Africa window.\n",
      "derived_from": [
        {
          "id": "africa-precipitation-monthly-seasonal",
          "version": "1"
        },
        {
          "title": "CHIRTS-ERA5 monthly Tmax GeoTIFFs (Climate Hazards Center)",
          "url": "https://data.chc.ucsb.edu/experimental/CHIRTS-ERA5/tmax/tifs/monthly/"
        },
        {
          "title": "CHIRTS-ERA5 monthly Tmin GeoTIFFs (Climate Hazards Center)",
          "url": "https://data.chc.ucsb.edu/experimental/CHIRTS-ERA5/tmin/tifs/monthly/"
        }
      ]
    },
    {
      "id": "spei",
      "description": "Monthly potential evapotranspiration by Hargreaves-Samani from Tmax, Tmin, their mean and extraterrestrial radiation at mid-month; climatic water balance = precipitation minus PET. Per-pixel SPEI at 3- and 12-month scales with the SPEI R package, log-logistic fit on 1991-2020. Pixels with fewer than scale + 24 valid months and non-finite fits set to NoData. Written as COGs with internal overviews.\n",
      "code": {
        "url": "https://github.com/AdaptationAtlas/hazards_prototype/blob/develop/R/observational/2_calculate_obs_spei.R",
        "version": "4d89638"
      }
    }
  ],
  "data": [
    {
      "name": "monthly",
      "description": "Per-pixel monthly SPEI, one COG per variable per month.",
      "locations": [
        {
          "url": "https://digital-atlas.s3.amazonaws.com/domain=climate/type=observational/source=chirps-chirts-era5/region=africa/processing=monthly/",
          "title": "HTTPS"
        },
        {
          "url": "s3://digital-atlas/domain=climate/type=observational/source=chirps-chirts-era5/region=africa/processing=monthly/",
          "title": "S3"
        }
      ],
      "href_template": "variable={variable}/{variable}-{time}.tif",
      "media_type": "image/tiff; application=geotiff; profile=cloud-optimized",
      "file_size": "6 MB",
      "processing_steps": [
        "source",
        "spei"
      ]
    }
  ],
  "cdh": {
    "domain": [
      "climate"
    ],
    "usage": {
      "intended_uses": [
        "seasonal (SPEI-03) and multi-year (SPEI-12) drought monitoring over Africa",
        "comparing drought conditions across ENSO phases or analogue years",
        "drought context for vegetation, crop water-stress and food security indicators"
      ],
      "not_recommended_for": [
        {
          "use": "Reading SPEI as an absolute rainfall or soil-moisture deficit.",
          "reason": "SPEI is a standardized anomaly against the 1991-2020 fitted distribution, not a quantity of water.",
          "use_instead": "Monthly and seasonal precipitation totals (catalog record africa-precipitation-monthly-seasonal)."
        },
        {
          "use": "Combining with SPEI from other producers as one series.",
          "reason": "Values depend on the input data, the PET method and the reference period, which differ between products."
        }
      ]
    }
  }
}