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Drought Monitor

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About the Australian Combined Drought Indicator (CDI)

The Australian Combined Drought Indicator (CDI) is based on the U.S. Drought Monitor concept, which was developed at the National Drought Mitigation Center at the University of Nebraska-Lincoln in the late 1990s. It is a scaled down version of the U.S. Drought Monitor using four selected drought indicators.

The Australian CDI uses a combination of rainfall (Standardised Precipitation Index (SPI), soil moisture (SM), evapotranspiration (ET), and Normalized Difference Vegetation Index (NDVI) to produce a Drought Monitor tailored for Australia.

Data to calculate the Australian CDI are sourced from the Australian Bureau of Meterology, the Queensland Department of Agriculture and Fisheries SILO database, and the Copernicus Land Monitoring Service - the European Union's Earth Observation Programme.

Each dataset is percentile ranked over a baseline period and the results combined using a weighted average. Principal Component Analysis (PCA) is used to determine the optimal weighting for the CDI for each grid cell for every month over Australia.

The CDI calculation methodology combines the four data sets as given in the equation: CDI = (a x SPI) + (b x SM) + (c x ET) + (d x NDVI) where a, b, c, d are respective weights assigned to each of the input data.

About this time scale (1-month period)

A short-term 1-month dry period (known as a meteorological drought) can lead to short-term soil moisture and crop stress, especially during the growing period.

Using the Australian Combined Drought Indicator (CDI)

Legend

The CDI is calculated as a percentile value, which is then converted into eleven categories as follows:

Category Percentile Approximate frequency of drought
Exceptional Wet 98% to 100%
Extreme Wet 95% to 98%
Severe Wet 90% to 95%
Moderate Wet 80% to 90%
Slightly Wet 70% to 80%
Near Normal 30% to 70%
Slightly Dry 20% to 30% 1 in 3 years
Moderate Drought 10% to 20% 1 in 5 years
Severe Drought 5% to 10% 1 in 10 years
Extreme Drought 2% to 5% 1 in 20 years
Exceptional Drought 0% to 2% 1 in 50 years

Time scales

The 12-month Drought Monitor is usually a good place to start investigating drought conditions.

To determine if a region is either going into drought, continues to be in drought or is coming out of drought it is recommended to view drought maps using different time scales (1, 3, 6, 9, 12, 24 or 36 months).

The shorter timescales may not be a useful indicator of drought conditions in some climate regions at some times of the year. For example, tropical and sub-tropical Australia during May-October is normally dry so the 1 and 3 month Drought Monitor at this time is calculated against a low rainfall background when rainfall is not usually expected. Therefore, the wet period between November and April is the key period to assess drought conditions in this region.

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