Sugarcane project explanatory notes on this page:
Sugarcane project computation
November 2023 release
Underpinning the P2R Projector Sugarcane project fine sediment, dissolved inorganic nitrogen (DIN) and pesticide savings, are summarised constituent generation rates, taken from a large suite of P2R paddock modelling simulations (simulations applied using the ApSIM modelling framework).
The responses to the Sugarcane project survey questions provide numeric values, and based on expert weightings these are combined to provide survey scores for soil, nutrient, irrigation, and pesticide management. Using defined ranges, the weighted scores are used to align the farm’s before and after practices to the most closely matched simulation in the available ApSIM simulation suite. This allows an area-based estimate of constituent generation reduction to be calculated. These constituent generation rates have already considered proportion of fine sediment in total eroded soil mass, and the ratio of each constituent delivered to stream. Fine sediment and pesticides are assumed to only be carried to the stream in overland flow (runoff). DIN is conceptually provided via two pathways – runoff and through deep drainage seepage.
Recognising the spatial complexity built into the P2R paddock modelling simulation design, there are spatial layers used in P2R Projector to provide locational context for:
- Soil type (distribution of P2R paddock modelling soil attribute collections), and
- Climate series (Identifier of P2R paddock modelling collection), and
- District (Identifier of P2R paddock modelling collection), and
- River System Delivery Ratio (RSDR for each constituent, from P2R catchment modelling)
Each P2R Projector Sugarcane project polygon is intersected with the soil, climate, district and RSDR spatial layers, and constituent generation rates are calculated for each soil intersection (with dominant climate series and district based NUE used to select appropriate simulations). These intersections are proportionately combined to give the project (before and after practice change) constituent generation rates. Any calculations that require a specific fertiliser or yield value use weighted averages from the survey responses, with plant and ratoon components of the crop cycle considered.
Finally, a representative RSDR (dominant RSDR from project polygon intersection) is applied to each constituent generation rate, allowing calculation of a constituent load reduction as delivered to the Great Barrier Reef.
New band calculation method based on Nitrogen Loss Risk Index (NiLRI)
November 2022 release
The November 2022 release applies the Nitrogen Loss Risk Index (NiLRI) across all bands. NiLRI is the ratio of Nitrogen (N) fertiliser applied to crops and the cane yield expected (i.e. kg N / t cane). The pre-November 2022 version (version 3) employed this approach to assign sites into bands one to six. This release also includes sub-steps [link] to provide greater sensitivity of load reduction to changes in N application rates.
This approach simply allows representation of any farm management improvements that increase expected yield, and/or reduce N fertiliser rate for a given yield. The tool can now represent relatively small, estimated dissolved inorganic nitrogen (DIN) load reductions.
| Band | NiLRI/NUE |
|---|---|
| 1 | Less than 0.900 |
| 1.1 | 0.940 |
| 1.2 | 0.980 |
| 1.3 | 1.020 |
| 1.4 | 1.060 |
| 2 | 1.100 |
| 2.1 | 1.144 |
| 2.2 | 1.188 |
| 2.3 | 1.232 |
| 2.4 | 1.276 |
| 3 | 1.320 |
| 3.1 | 1.380 |
| 3.2 | 1.440 |
| 3.3 | 1.500 |
| 3.4 | 1.560 |
| 4 | 1.620 |
| 4.1 | 1.684 |
| 4.2 | 1.748 |
| 4.3 | 1.812 |
| 4.4 | 1.876 |
| 5 | 1.940 |
| 5.1 | 1.952 |
| 5.2 | 1.964 |
| 5.3 | 1.976 |
| 5.4 | 1.988 |
| 6 | Greater than 2.000 |
References
Peter J. Thorburn, Jody S. Biggs, Kevin McCosker, Adam Northey. 2022. Assessing water quality for cropping management practices: A new approach for dissolved inorganic nitrogen discharged to the Great Barrier Reef. Journal of Environmental Management. Volume 321. https://doi.org/10.1016/j.jenvman.2022.115932
Sugarcane industry mill mud management will not report nutrient saving for individual projects
November 2022 release
The dissolved inorganic nitrogen (DIN)savings from mill mud management are represented in catchment models rather than the P2R projector tool. Mill mud is a by-product of the sugar milling process and the majority of it is spread across cane paddocks. There are significant amounts on nitrogen (N) in mill mud and by reducing the application on one farm, it may reduce DIN generation and loss from that farm.
However, the remaining mill mud can be applied elsewhere in the catchment, meaning landholders cannot guarantee that there is a net reduction in DIN to the Reef by farm scale mill mud management programs.
Previous versions of P2R Projector (up to Nov 2022) accounted for the use of mill mud as part of the farm scale N balance for determining potential N surplus. P2R Projector would account for reported changes in ‘before’ and ‘after’ mill mud application rates to determine the overall N budget and potential DIN savings. Changing the mill mud application rate between the ‘before’ and ‘after’ surveys results in no estimated DIN load reductions in the P2R projector tool. The ‘before’ mill mud application rate is the default value used for both the before and after case (as of Nov 2022 release).
The mill mud use questions have been retained in the sugarcane practice survey because they are used in the collective Paddock to Reef Integrated Monitoring, Modelling and Reporting program (Paddock to Reef program). The Paddock to Reef program level modelling considers the collective impact of all previously reported practice change, and as such the modelling approach has the capacity to include the impact of spatial patterns of changes in in mill mud use.
Sugarcane industry finer resolution of results
November 2022 release
Additional steps in DIN reductions are calculated using a linear interpolation equation. For example, if a site moves halfway to the next existing band, the estimated DIN load reduction will be half of the impact of that change.
Previous versions of P2R Projector (up to Nov 2022) estimated DIN savings by calculating the difference in DIN loss between the before band and the after band. As an example, the before band of five (original bands) might have a DIN loss of 10kg N/ha and the practice change moves the project to band four which might have a DIN loss of 6kg N/ha. Hence, a DIN saving of 4kg N/ha (10kg/ha – 6kg/ha).
