Metadata | NDMC Composite Drought Indicators

The new Composite Drought Indicators were developed by using machine learning (ML) to compare the U.S. Drought Monitor data over 12 years to various drought indicators/indices.

The input “mixes” were developed by focusing the ML on the relative accuracies of the different input aggregate timeframes (e.g., SPI-1 vs. SPI-2 vs. SPI-3).
The transition between “standard” and “winter” scenarios was determined by comparing the accuracies of the vegetation, potential evapotranspiration, and surface water equivalent inputs against the mean 1-month temperature values.
The inputs to test were chosen to cover the drought dynamics of water supply and storage, evaporative demand, and vegetation health.

Ranking of the inputs and indicators was determined using the Gaussian percentiles associated with a standardized distribution, as opposed to a manual calculation based on the minimum/maximum values in the history.

Flash Drought Conditions Indicator

The Flash Drought Conditions Indicator (FDCI) is an automated assessment of the rate at which conditions are changing toward drought or abnormal wetness. The FDCI is calculated using a weighted average of four drought indicators, along with a weighted average of their strongest category changes. The “delta” values are determined using the greatest change between the valid date and the previous four weeks. The following indices and indicators are used along with their respective deltas:

Standardized Precipitation Index (SPI) using 1-month aggregates
Standardized Evapotranspiration (SPEI) using 1-month aggregates
Standardized 10cm Soil Moisture anomalies using 1-week aggregates
Standardized 40cm Soil Moisture anomalies using 1-week aggregates

Composite Drought Indicators (CDIs)

The three CDIs (short-term, mid-term, and long-term) are automated assessments of drought and abnormal wetness conditions. The CDIs are calculated using weighted averages of indices and indicators for both “standard” and “winter” scenarios. The scenario is determined per cell using the 1-month mean temperature.

NDMC Short-Term Composite Drought Index

Standardized Precipitation Index (SPI) short-term mix comprised of 1-3 months.
Standardized Precipitation-Evapotranspiration Index (SPEI) short-term mix comprised of 1-3 months.
Standardized Potential Evapotranspiration (PET) short-term mix comprised of 1-3 months.
NOAH Soil Moisture short-term mix is comprised of 40 cm values out to 1 month and 100 cm values out to 2 weeks.
Vegetation short-term mix is comprised of Normalized Difference Vegetation Index (NDVI) and Enhanced Vegetation Index (EVI) values out to 2 weeks.
Standardized Snow-Water-Equivalent (SWE) short-term mix comprised of 1-3 months.

NDMC Mid-Term Composite Drought Index

Standardized Precipitation Index (SPI) short-term mix comprised of 4-6 months.
Standardized Precipitation-Evapotranspiration Index (SPEI) short-term mix comprised of 4-6 months.
NOAH Soil Moisture short-term mix is comprised of 100 cm and 200 cm values out to 2 months.
Vegetation short-term mix is comprised of Normalized Difference Vegetation Index (NDVI) and Enhanced Vegetation Index (EVI) values out to 2 weeks
Standardized Snow-Water-Equivalent (SWE) short-term mix comprised of 4-6 months.

NDMC Long-Term Composite Drought Index

Standardized Precipitation Index (SPI) short-term mix comprised of 7-24 months.
Standardized Precipitation-Evapotranspiration Index (SPEI) short-term mix comprised of 7-24 months.
NOAH Soil Moisture short-term mix is comprised of 200 cm values out to 12 months.
Vegetation short-term mix is comprised of Normalized Difference Vegetation Index (NDVI) and Enhanced Vegetation Index (EVI) values out to 3 months.
Standardized Snow-Water-Equivalent (SWE) short-term mix comprised of 7-24 months.

Notes

“Winter” CDI scenarios occur when the 1-month mean temperature is less than 2.25C.
Standardized SWE is included to account for above-ground water storage that is not always captured by traditional precipitation or soil moisture indices.
Potential Evapotranspiration (PET) used in the new composite drought indicators is based on the FAO-56 Penman-Monteith equation (alfalfa reference) provided by the University of California, Merced.
To account for climate non-stationarity, the NDMC developed the short-term indices based on a 40-year period of record (POR) with a 30-year reference mean.
To account for climate non-stationarity, the NDMC developed the mid-term indices based on a 50-year period of record (POR) with a 30-year reference mean.
To account for climate non-stationarity, the NDMC developed the long-term indices based on a 60-yr period of record POR and 30-year mean reference.
The grid size for the CDI products is 1/25° x 1/25° (4km x 4km).

Sources

Precipitation and temperature data are from the Applied Climate Information System (ACIS) PRISM model (current) and the Northeast Regional Climate Center (NRCC) model (archive).
PET/SWE values are from the University of California, Merced GridMET model (current), and the LIVNEH model CDR (archive).
Soil moisture is from the North American Land Data Assimilation System (NLDAS) NOAH model (current), and the Global Land Data Assimilation System (GLDAS) NOAH model (archive).
NDVI/EVI are from EROS Visible Infrared Imaging Radiometer Suite (EVIIRS) (current), and the Advanced Very High Resolution Radiometer (AVHRR) Climate Data Record (CDR) (archive).