Short & Long Term Drought Blends

Example map of long-term drought blend for July 14, 2023 from GridMET DROUGHT climate reanalysis dataset.

Description

Blends of multi-temporal drought indices that represent different drought timescales can be useful to assess short- and long-term drought processes and associated impacts across regions. The experimental short- and long-term objective blends produced by the National Weather Service Climate Prediction Center (CPC) are examples of blends that are produced by weighting several key drought indices where the weights are based on expert judgment from drought experts. The metrics that go into the experimental CPC blend are:

Palmer-Z Index (Z)
Palmer Drought Severity Index (PDSI)
Standardized Precipitation Index (SPI)( 30-day, 90-day, 180-day, 1-year, 2-year and 5-year)
Palmer Hydrological Drought Index (PDHI)
Soil Moisture from NOAH (SM-NOAH)

In Climate Engine, we are providing blends that are instead a weighting of the standardized indices coming from drought indices calculated from the gridMET data product (also in Climate Engine and based on 1981-2016). The weightings are the same as the experimental CPC blend with some differences:

the weightings for SM-NOAH (soil moisture from NOAH) and PHDI (Palmer Hydrological Drought Index) are added in with the weights for PDSI. Only PDSI is used in the blend calculation to represent soil moisture.
in the calculation, the Palmer drought indices (i.e Z and PDSI) are divided by 2 to put them on roughly the same scale as the standardized indices
the colors and bins used to visualize the drought blends will be the US Drought monitor colors with non-linear standardized index bins.

The precise details of the construction of the blends is:

Short-term Blend = 0.2 *(PDSI/2) + 0.2 * SPI30d + 0.25 * SPI90d + 0.35 * (Z/2)

where

PDSI = Palmer Drought Severity Index
Z = Palmer's Z-Index
SPI30d = 30-day Standardized Precipitation Index (SPI)
SPI90d = 90-day Standardized Precipitation Index (SPI)

Long-term Blend= 0.35 *(PDSI/2) + 0.15 * SPI180d + 0.2 * SPI1y + 0.2 *SPI2y + 0.1 * SPI5y

where

PDSI = Palmer Drought Severity Index
SPI180d = 180-day Standardized Precipitation Index (SPI)
SPI1y = 1-year Standardized Precipitation Index (SPI)
SPI2y = 2-year Standardized Precipitation Index (SPI)
SPI5y = 5-year Standardized Precipitation Index (SPI)