Improvements to EDIS and the CDI

Drought events are a stressful time for farmers, communities and those providing services in regional areas. The provision of timely, easily accessible, reliable climatic data and forewarning is an important part of a wider set of actions needed to effectively navigate through drought events.

As part of our ongoing commitment to provide improved climate and weather information, NSW DPI has incorporated Australia’s premium quality and highest resolution climate data, ANUClimate, into the EDIS framework which underpins the delivery of genuine farm scale climate services.

The underlying data informing the Combined Drought Indicator (CDI) is now at a higher resolution of 1km2. This improved resolution underpins the delivery of genuine farm scale climate information service.

The improved resolution and processing of the data means that the accuracy of the CDI maps is improved. There will be minor differences in what you see as the improvements are in the underlying data quality. This upgrade provides you with greater confidence in the maps that are being produced.

The map below is a comparison of the previously operating version of the CDI Version 1 (left) and the new version CDI Version 2 (right; active as of October 2022) with the higher resolution climate data. You will see that the spatial pattern of the drought categories is broadly similar, though you may notice regional differences in some parts of NSW. The differences are a result of the higher quality data, upgrades to the underlying science and changes to the climate baseline period used.

Combined Drought Indicator (CDI)

1st April 2020

CDI comparisons maps

This animation compares the transition between the Combined Drought Indicator categories during the period from January 2015 to December 2020, capturing the major drought event that began in 2017. The first key difference between the CDI in Version 1, the previously operating version (left), and the CDI Version 2, active as of October 2022 (right), is that Version 2 is the produced from the higher 1km resolution climate data. There are some differences in timing of events due to an upgraded evaporation scheme in Version 2, as well as the more robust data.

The NSW DPI Combined Drought Indicator (CDI) includes four indices of rainfall, soil moisture, plant growth and drought direction which, when used together, determine the different drought categories shown in the CDI.

The map below shows a comparison of the rainfall index for the previously operating version (top left; Version 1) and the new version (top right; Version 2 active as of October 2022) for all of NSW. The larger box (bottom) compares a zoomed in regional view of Version 1 and Version 2. The difference in the granularly is easily identified by overlaying Version 2 on top of Version 1.

The indices explained

This animation shows a zoomed-in view of the Soil Water Index in the area near Parkes (yellow star) and Eugowra (orange star) during the period from January 2015 to December 2020, capturing the major drought even that began in 2017. The difference in spatial resolution is highlighted with the light grey lines denoting the grid boxes between Version 1 and Version 2, respectively. For each grid box in Version 1, there are 25 boxes in Version 2. This demonstrates more detailed climate variability in relation to topographic features captured in the climate data.

The NSW DPI 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 calculated by taking the mean of daily values over the last 12 months and ranking them relative to a 40-year baseline period (1981-2020). This captures recent big shifts in climate variability and also factors in climate change.

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 an attractive feature in NSW given the presence of rangeland, temperate and sub-tropical climates.

The indices explained

Soil water index map

The rainfall index (RI; top left) is the ranking against the baseline period of the 12-month sum of daily rainfall as measured by weather stations and interpolated to a 1km resolution in the ANUClimate dataset.

The soil water index (SWI; top right) and plant growth index (PGI; bottom left) are the rankings of daily soil water and daily plant growth as modelled by the DPI AgriMod soil water balance model.

In DPI AgriMod, plant available soil water from the assumed maximum rooting zone (0-45 cm), are aggregated and percentile ranked to calculate the SWI. 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 useful as an indicator of conditions for dryland than irrigated agriculture.

To calculate PGI, daily crop stress and pasture growth data are taken from DPI AgriMod. 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 sensitive to moisture and 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. In-field conditions can be above or below the regional indicator reported by EDIS.

The last index, the Drought Direction Index (DDI; bottom right), is used to categorise the direction (strengthening or weakening) of drought. The DDI tracks the trend in rainfall 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.