Delivery ratios

Loads are now reported at the end of the catchment

Versions 1 and 2 of P2R Projector reported loads at the point of generation (edge of paddock). These were based directly on the outputs from paddock modelling simulations (APSIM).

Version 3 reports the load at the end of the catchment. This brings projector outputs into alignment with the Reef 2050 Water Quality Improvement Plan (WQIP) targets and reported modelled load estimates.

Version 3 accounts for losses from the paddock and stream network and losses within the river system to estimate an end-of-catchment load as opposed to an end-of-paddock load.

Losses between the end-of-paddock and the stream are estimated by applying a delivery ratio specific to each catchment, each pollutant and each delivery pathway (surface/sub-surface) from GBR Source. These delivery ratios are derived from calibrating the catchment modelled load to instream monitoring data.

Losses between the stream and the end of catchment are estimated by applying a riverine system delivery ratio, derived from GBR Source. These values are unique to each model functional unit (sub-catchment) and each pollutant.

Typically, for coastal streams, around 90% of the dissolved inorganic nitrogen load is delivered to the end of the catchment. For fine sediment, the riverine system delivery ratios are more variable with less sediment delivered from inland areas.

Delivery ratios conceptual model

Example

A site in the Wet Tropics loses 5kg of dissolved inorganic nitrogen (DIN) in surface flows and 20kg of DIN in sub surface flows.

The total DIN loss of 25kg from the site is called the end-of-paddock load.

To estimate how much of this load makes it to the stream network, a paddock to stream delivery ratio is applied to the surface load and a different one to the subsurface load.

In the Wet Tropics, surface delivery ratios are generally 100% (100% of the generated or end-of-paddock load makes it to the stream network) and 30% for sub surface load. The load that reaches the stream network would be 5kg from the surface (100% of 5kg) and 6kg from sub surface load (30% of 20kg).

This means that from the 25kg that were lost from the end-of-paddock, only 11kg makes it to the stream network (load at stream). The rest is lost between the paddock and the stream.

P2R Projector further scales the load at stream by the Riverine System Delivery Ratio (RSDR) to account for losses in the stream network. P2R Projector reports the end-of-catchment load.

2026 Update to Delivery Ratios

What has changed?

P2R Projector has been updated to align with the latest Paddock to Reef model through the incorporation of revised delivery ratios.

This update includes changes to:

Riverine System Delivery Ratios (RSDRs) used in fine sediment calculations.
Surface delivery ratios used in dissolved nutrient calculations.
Subsurface delivery ratios used in dissolved nutrient calculations.

Project activities have not changed. However, estimated load reductions may change because P2R Projector is now using the latest available science to estimate how pollutants move through catchments and ultimately reach the Great Barrier Reef.

Why were the delivery ratios updated?

The updated delivery ratios are based on significant enhancements to the latest Paddock to Reef modelling and the Great Barrier Reef Dynamic SedNet model.

These improvements include:

Updated land use data across the Great Barrier Reef catchment to 2021.
Improved soils and gully datasets.
An extended model run period.
Additional monitoring data, with 39 monitoring sites now used for calibration and validation compared with 31 previously.
Improved representation of river and floodplain processes.

The most significant enhancement was the integration of high-resolution LiDAR data, improving the representation of river channels, floodplains and sediment movement during flood events.

Together, these improvements increase confidence in model outputs and provide a more accurate representation of how pollutants move through catchments.

Why should I trust the updated results?

The updated model has:

Undergone independent peer review.
Been endorsed by the Reef 2050 Independent Science Panel.
Been adopted for use in the current Great Barrier Reef Report Card.

These updates reflect the best available science currently used by the Paddock to Reef program.

What does this mean for my projects?

The updated delivery ratios may result in different estimated pollutant reductions for some projects.

This does not mean project activities have changed. Rather, it reflects improved scientific understanding of how pollutants move through landscapes, waterways and catchments before reaching the Great Barrier Reef.

The effect varies between pollutants, locations and activities. Some projects may show higher estimated reductions, while others may show lower estimated reductions.

Aligning P2R Projector with the latest Paddock to Reef model improves confidence in results and helps delivery partners target on-ground investment where it can achieve the greatest water quality benefits.

Existing projects

Existing projects can either:

Be migrated to the updated delivery ratios and recalculated using the latest science.
Retain previously calculated results.

This option is available to support projects that have already progressed through planning, approval or funding processes, including some Landscape Repair Program projects.

New projects

All new projects created after the update will automatically use the updated delivery ratios.

It is not possible to create new projects using previous delivery ratios.

P2R Projector is a Truii product.

If you are interested in learning more about Truii and viewing their full range of data-driven web applications, follow the link below.

Find out more about Truii >>