The NSW State Government, The Department Primary Industries defines Drought in NSW using the Combined Drought Indicator or CDI Tool. This definition does not account for or encompass the impacts of drought on business, environment, health and other areas but merely the physical symptoms. This method of assessment is one of the first steps in determining the health of the land and deciding when to trigger certain actions from government.

DPI CDI tool method

This methods of assessing Drought combines meteorological, hydrological and agronomic definitions of drought using four scores,

1. Rainfall (RI)
   RI is determined by comparing the current rainfall information aggregated over 12 months, with the historical rainfall over the previous 30-year (1980-2010) to determines the current rainfalls score between 1 and 100 compared to previous records. this is also called an index. 0 is closet to the driest, and those scores approaching 100 closets to the wettest, for any given region.

2. Soil Water (SWI)

   The SWI is determined using the same procedure as the RI. Plant available soil water from layer one (0-10cm) and layer two (11-45 cm), the assumed maximum rooting zone, are aggregated and used to determine the SWI. Similar to the RI, the SWI is an index between 0 and 100. In most districts of NSW, a value of 0 means there is no plant available water held in the profile. The SWI is a hydrological index, but its configuration means that it is more useful as an indicator of conditions for dryland than irrigated agriculture.

3. Plant Growth (PGI)
   The PGI is determined using the same general procedure as the RI. Crop stress and pasture growth data are taken from satellite imagery, and the scale rank is calculated for each day. If the predominant land use in a given area is cropping, the PGI uses the crop-derived data, otherwise it uses the pasture growth indicator. The PGI is an agronomic drought index which is not only sensitive to moisture but also temperature variation and seasonal events such as frost. It is important to note that the PGI tracks the influence of climate on production potential across broad areas only. This provides a regional indicator of conditions. In the paddock, management decisions like fertiliser application and timing, sowing times and stocking rates drive outcomes on the ground, and in-field conditions can be above or below the regional indicator reported.

   These three scales are then used to develop a fourth and final index, the:

4. Drought Direction index (DDI)
   The DDI is a categorical index that tracks the trend in meteorological conditions over the last 200 days. If the DDI signals a drying trend, and the other indicators are below the 50th percentile, a drought warning is issued. If the DDI signals a wetting trend and other indicators lift above the 10th percentile the region is in a drought recovery phase.

After these scores are determined each Parish is categories into one of the following 5 Drought levels similarly to the RFS’s fire scale.

1. Intense Drought-

   All three indicators (rainfall, soil water, plant growth) are below the 5th percentile. Ground cover is very low, soil moisture stores are exhausted, and rainfall has been minimal over the past 6-12 months.

2. Drought-

   At least one indicator is below the 5th percentile. Conditions may be very dry, or agronomic production is tight (low soil moisture or plant growth). It is possible to be in Drought when there has been some modest growth, or a few falls of rain.

3. Drought Affected (intensifying)

   At least one indicator is below the 30th percentile and the rainfall trend is negative over the past 90 days. Conditions are deteriorating; production is beginning to get tighter. Ground cover may be modest, but growth is moderate to low for the time of year. When indicators are close to the Drought threshold drought conditions are severe.

4. Drought Affected (weakening)

   At least one indicator is below the 30th percentile and the rainfall trend is positive over the past 90 days. Production conditions are getting tighter, but there have been some falls of rain over the past month. It is rare to enter the Recovering phase from the Non-Drought category; Usually there is a quick (1-2 week) transition into Drought Affected or Drought. When indicators are close to the Drought threshold drought conditions are severe.

5. Recovering

   All indicators are below the 50th percentile but above the 30th percentile. Production is occurring but would be considered ‘below average’. Full production recovery may not have occurred if this area has experienced drought conditions over the past six months.

6. Non-drought

   At least one indicator is above the 50th percentile. Production is not limited by climatic conditions.

For further and more detailed information please visit the NSW DPI Drought Hub https://www.dpi.nsw.gov.au/climate-and-emergencies/droughthub.