Understanding the CDI phases
The CDI phases
The NSW Combined Drought Indicator (CDI) is used by the Department to classify drought in New South Wales. It integrates weather, climate, soil, and land-use datasets into a single framework to help decision making.
The CDI combines meteorological, hydrological and agronomic definitions of drought using indicators for rainfall (RI), soil water (SWI), and plant growth (PGI). By using multiple indicators, the CDI provides an overall indication of drought conditions in NSW.
The CDI is designed to track long term drought events that form periodically in NSW. These are droughts that are 9-12 months or more in duration affecting large geographic areas. The CDI does not track:
- short term drying or ‘flash drought’ events.
- Large individual storms or flash floods.
- moderate to mild drying events that impact individual farms.
For this reason, the CDI may not match field conditions on the day of observation. This is particularly the case during drought entry and recovery if a short term intense drying or individual rainfall event has been experienced. Use the NSW State Seasonal update to access a wider range of drought and forecast information.
Description of the Combined Drought Indicator Phases
| CDI Phase | Technical definition | Description - typical field conditions |
|---|---|---|
| Non-Drought | All indicators greater than the 50th percentile or all indicators are greater than the 30th percentile AND the previous category is Non-Drought | Production is not limited by climatic conditions. |
| Recovering | All indicators are greater than the 30th percentile AND any indicator is less than the 50th percentile AND the previous category is Drought Affected. | 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. |
| Drought Affected (weakening) | Any indicator is less than the 30th percentile AND Positive 5-month Rainfall Index trend (DDI>0.4). | 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. |
| Drought Affected (intensifying) | Any indicator is less than the 30th percentile AND Negative 5-month Rainfall Index trend (DDI≤0.4). | 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. |
| Drought | Any indicator is less than 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. |
| 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. |
About the underlying indices
The NSW Combined Drought Indicator (CDI) includes four indices of rainfall, soil moisture, plant growth and drought direction that when used together can indicate the different phases of drought.
The rainfall, soil moisture, and plant growth indices are a moving 365-day (12 month) percentile ranking. This is calculated by taking the aggregate of daily values over the last 365 days and ranking it relative to a 40-year baseline period (1981-2020). These calculations are undertaken on every 1km2 grid point across NSW.
An index between 0 and 100 is provided for every grid point, where values approaching 0 are the lowest recorded relative to the baseline, and those approaching 100 are the highest. Percentile-based indices compare current conditions to past conditions at the same location, so they work in different climatic settings which is important in NSW given the presence of rangeland, temperate, coastal (maritime) and sub-tropical climates.
The indices explained
Rainfall Index
The rainfall index (RI; top left) is the ranking against the baseline period of the 365-day sum of daily rainfall as measured by weather stations and interpolated to a 1km2 resolution in the ANUClimate dataset.
Soil Water Index
The soil water index (SWI; top right) is determined using daily plant available soil water from the DPI AgriMod model. In most districts of NSW, an SWI 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 an indicator of conditions for dryland than irrigated agriculture.
In DPI AgriMod plant available soil water is determined for the rooting zone as a function of daily rainfall, evaporation, the level of soil moisture on the day and soil physical properties. The soil physical properties are determined using available observation data on the Australian Soil Landscape Grid, remotely sensed estimates of soil moisture and available soil probe observations.
Plant Growth Index
The Plant Growth Index (PGI; bottom left) is calculated using both relative crop and pasture production from DPI AgriMod. If the predominant land use in a given area is cropping, the PGI uses relative crop production, otherwise it uses relative pasture production. The PGI is an agronomic drought index which provides information about how effective climatic conditions are for production.
Relative crop and pasture production is determined as a function of plant phenotypical responses to available soil moisture, the daily temperature regime and solar radiation, as well as the condition of the crop/pasture canopy on the day. The phenotypical responses are determined for each crop-pasture species using remotely sensed Gross Primary Production data and, where available, field estimates of growth.
It is important to note that the PGI tracks the influence of climate on relative crop/pasture production. This is suitable for deriving an agronomic drought indicator so that drought classification is not determined by rainfall patterns alone. In the paddock, management decisions like fertiliser application and timing, sowing times and stocking rates will affect actual production, the accumulated biomass in the field.
Drought Direction Index
The Drought Direction Index (DDI; bottom right) is used to categorise the direction (strengthening or weakening) of drought. The DDI tracks the trend of the rainfall index over the last 150 days and calculates the linear slope.
If the DDI signals a drying trend (DDI = negative), and the other indicators are below the 50th percentile, the CDI category will indicate Drought Affected (positive, strengthening).
If the DDI signals a wetting trend and other indicators lift above the 5th percentile the region is in a Drought Affected (negative, weakening) phase. The DDI is not a predictive index: DDI values have a low to moderate correlation with what is happening in the immediate future.