Underpinning the P2R Projector Grains projects fine sediment savings are summarised constituent generation rates, taken from a large suite of P2R paddock modelling simulations (simulations applied using the HowLeaky? modelling framework). Each district/ catchment has its own unique set of simulations which were developed using input from local experts this is in recognition than farming practice differ between each district.
The responses to the Grains project survey questions have been aligned to practices simulated in HowLeaky. The combination of selected survey questions are used to align the farm’s before and after practices to the most closely matched simulated practice combination available in HowLeaky?. This allows an area-based estimate of constituent generation reduction to be calculated. These constituent generation rates have already considered the proportion of fine sediment in total eroded soil mass, and the ratio of each constituent delivered to stream. Fine sediment is assumed to only be carried to the stream in overland flow (runoff).
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
- Riverine system delivery ratio (RSDR for each constituent, from P2R catchment modelling)
Each P2R Projector Grains project polygon is intersected with the soil, climate and RSDR spatial layers, and constituent generation rates are calculated for each soil intersection (with dominant climate series used to select appropriate simulations). These intersections are proportionately combined to give the project (before and after practice change) constituent generation rates.
Finally, a representative RSDR (dominant RSDR from project polygon intersection) is applied to each pollutant generation rate, allowing calculation of a pollutant load reduction as delivered to the Great Barrier Reef Lagoon.