Right now the Clarence River flow at Newbold Crossing is registering in the >80% stream flow percentile, the Shannon Creek side dam is at 99% capacity and soil moisture is for the most part within acceptable limits across the Clarence Valley which is classified 100% non-drought.
However, the Australian Summer officially begins on 1 December 2024 and air temperatures and water evaporation rates are bound to rise.
So how is the New South Wales Combined Drought Indicator (CDI) Map likely to look come December?
Where we are going?
NSW Dept. of Primary Industries and Regional Development, Climate Branch, Drought Forecasting, 17 November 2022:
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NOTE: The DPIRD drought forecast for NSW presents the ‘Most Likely’ Combined Drought Indicator (CDI) category for the forecast period. The Most Likely CDI category is determined by identifying the 'mode' of the CDI. The mode is the category that appears most frequently across all possible forecast outcomes in the ensemble run. It is the most common prediction for drought conditions in the forecast period based on the model's simulations.
Where have we been along the Clarence Valley drought history continuum, 20 November 2019 to 9 October 2024?
Click on graphs to enlarge
Fine Flower & environs
Heifer Station & environs
Grafton & environs
Maclean & environs
Yamba
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*All maps & graphs were created on 17.11.24 using interactive tools created by NSW Dept. of Primary Industries and Regional Development, Climate Branch |
The NSW Combined Drought Indicator (CDI) includes four indicators for rainfall, soil moisture, plant growth and drought direction which, used together, can indicate the five phases of drought.
Rainfall Index (RI)
The RI is the percentile rank of rainfall aggregated over 12 months. The ranking is made using a 30 year (1980-2010) baseline which captures recent big shifts in climate variability, and factors in climate change. This provides an index between 0 and 100 where values approaching 0 are close to driest, and those approaching 100 close to the wettest, for any given region. Percentile-based indices like the RI have a uniform distribution regardless of their climatic setting, which is an attractive feature in NSW given the presence of rangeland, temperate and sub-tropical climates which have skewed, normal and log-normal rainfall distributions.
Soil Water Index (SWI)
The SI is calculated using the same procedure as the RI, but uses a soil moisture field derived from the DPI AgriModTM soil water balance. 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 calculate 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.
Plant Growth Index (PGI)
The PGI is calculated using the same general procedure as the RI, using the output from DPI’s crop and pasture models. Crop stress and pasture growth data are taken from DPI AgriModTM, and the percentile rank 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 by EDIS.
BACKGROUND
The main water supply (other than the village systems of Wooli and Minnie Water) in the Clarence Valley is sourced from the Nymboida River, flowing through a section of the wider Clarence River catchment area.
At this time of year the Clarence Valley urban water supply is drawing around 14.26ML/per day from the river weir.
The Nymboida River also gravity feeds water to Shannon Creek Dam when required and, if the Nymboida river flow is too low (less than 225 Megalitres a day) or turbid post-flood, the Clarence Valley's principal urban areas receive water sourced from the off-stream storage at Shannon Creek.
Overall, Shannon Creek Dam is used to supply the Clarence Valley’s water about 5% of the time. Right now this dam is at 99% capacity.
This scenario is complicated by the fact that historically the Clarence Valley also supplies water out of the catchment to Coffs Harbour City local government area and this draw on catchment water is constant and always exceeding an optimal sustainability level for average daily drawn down.
