Indices

With a database of weather stations with long periods of record and mostly complete data, drought indices can be calculated from the data along with precipitation statistics.  The various climate statistics and drought indicators are all found in the Data section of the Drought Risk Atlas.

Datasets and Data Values

Data Numbers

In order to compile the indices for the atlas, millions of records were calculated for each index:

Index Total Records
SPI 600,040,360
SPEI 335,343,360
PDSI Monthly 2,796,276
Self-calibrated PDSI Monthly 2,796,276
Deciles 584,262,788
SSI 120,514,480

Temperature and Precipitation Data

Millions of daily temperature and precipitation values were collected and aggregated to in order to be able to calculate each of the indices at the various time steps:

Data Types Total Records Start Date
 Raw Data (daily data)  85,581,506 10/2/1849
 Serially Complete (daily data)  113,413,059 1/1/1908
 Aggregated data (weekly, monthly, etc.)  37,221,373 n/a

Streamflow Data

Millions of daily temperature and precipitation values were collected and aggregated to in order to be able to calculate each of the indices at the various time steps:

Data Types Total Records Start Date
 Raw Data (daily data)  17,315,052 8/1/1889
 Aggregated data (weekly, monthly, etc.)  3,081,646 n/a

Timesteps

Most of the indices were calculated for the following timesteps:

  • 1 Month
  • 2 Month
  • 3 Month
  • 4 Month
  • 5 Month
  • 6 Month
  • 7 Month
  • 8 Month
  • 9 Month
  • 10 Month
  • 11 Month
  • 12 Month
  • 18 Month
  • 24 Month
  • 36 Month
  • 48 Month
  • 60 Month
  • 72 Month
  • 84 Month
  • 96 Month

Standardized Precipitation Index

The SPI is available for all 3059 stations in the Drought Risk Atlas and was calculated using both weekly and monthly aggregates of data. The drought levels are as follows:

  • -1.00 to -1.49 = Moderately Dry
  • -1.50 to -1.99 = Very Dry
  • -2.00 and below = Extremely Dry

See more information about the Standardized Precipitation Index

Standardized Precipitation-Evapotranspiration Index

The SPEI is available for all 3059 stations in the Drought Risk Atlas and was calculated using both weekly and monthly aggregates of data. The drought levels are as follows:

  • -1.00 to -1.49 = Moderately Dry
  • -1.50 to -1.99 = Very Dry
  • -2.00 and below = Extremely Dry

Deciles

The Deciles rankings were done for all 3059 stations in the Drought Risk Atlas.  These rankings were completed using both weekly and monthly aggregates of data and the drought deciles of the following:

  • 1st Decile
  • 2nd Decile
  • 5th Decile
  • 10th Decile

See more information about Deciles

Palmer Drought Severity Index (standard and self-calibrated)

With a need for a database with little or no missing data to calculate the scPDSI, even the stringent criteria we used in separating out the stations for the Drought Risk Atlas led to issues in calculating the scPDSI.  Of the 3059 stations in the Drought Risk Atlas, not all of them had the necessary data to calculate the scPDSI.  Temperature and precipitation data are needed, and many stations used in the SPI and Deciles were stations that historically had recorded just precipitation.  Stations recording both precipitation and temperature were identified and the gaps in the data were noted.  It was noted that even small gaps would cause problems in the calculations, and a serially complete dataset using data estimations would be ideal.

The High Plains Regional Climate Center (HPRCC) has developed several methodologies to provide data estimates as part of their data quality control and assurance programs.  Using these techniques, the HPRCC provided estimates for the missing Drought Risk Atlas data, making the data serially complete.  For the scPDSI, the following stations were obtained:

  • 2173 stations where 90% or more of the temperature data were available, leaving 10% or less estimated.
  • 2296 stations where 80% or more of the temperature data were available, leaving 123 stations where up to 20% of the data were estimated.

See more information about the Palmer Drought Severity Index

Standardized Streamflow Index

The SSI is a variant of the Standardized Precipitation Index. Instead of precipitation data, streamflow data are used. The drought levels are as follows:

  • -1.00 to -1.49 = Moderately Dry
  • -1.50 to -1.99 = Very Dry
  • -2.00 and below = Extremely Dry