From gemini (and it said it might be wrong, check ...)
The model used in the MyPaddock tool from the Department of Primary Industries and Regional Development (DPIRD) is documented in a publication by Darshan Sharma (2017) titled "Guidelines for wheat yield loss" (Sharma, 2017).
While MyPaddock is a modern web-based tool, it is part of a lineage of Western Australian decision-support tools that incorporate earlier modeling concepts.
The core modeling approach in MyPaddock is not a single complex simulation like APSIM, but rather a "rules of thumb" and constraint-based model (Sharma, 2017). It functions as follows:
Potential Yield Foundation: It often utilizes the French and Schultz model (or variations of it) as a baseline to determine the maximum water-limited potential yield (DPIRD, n.d.).
Constraint Multipliers: The specific innovation in MyPaddock is the quantification of yield loss from roughly 40 different production constraints (e.g., soil acidity, compaction, diseases, and nutrient deficiencies) (Sharma, 2017). It uses a "traffic light" system to visualize these impacts.
The "SYN" Connection: STIN (STress INdex) or SYN (Synthetic) models were developed by researchers like David Stephens at the Department of Agriculture (now DPIRD) in the 1980s and 90s. While MyPaddock is a diagnostic tool for specific paddocks, the historical yield forecasting systems in WA were indeed built on the STIN model, which uses a stress index based on rainfall and soil evaporation (Stephens & Lyons, 1998; Van Burgel et al., n.d.).
The tool has strong links to the MyCrop suite of mobile applications. Specifically, the "MyCrop Wheat" app (and previously the iPaddock or similar iPad-based diagnostic tools) shares the same diagnostic logic and "rules of thumb" for calculating potential yield loss based on field observations (Sharma, 2017).
The primary reference for the specific logic and intervals used in the MyPaddock wheat module is:
Sharma, D. (2017). Guidelines for wheat yield loss. Agricultural Science, 29(1), 28–39. https://library.dpird.wa.gov.au/j_article/54
References
Department of Primary Industries and Regional Development (DPIRD). (n.d.). Potential yield tool. https://www.dpird.wa.gov.au/online-tools/rain-and-irrigation-calculators/potential-yield-tool/
Sharma, D. (2017). Guidelines for wheat yield loss. Agricultural Science, 29(1), 28–39.
Stephens, D. J., & Lyons, T. J. (1998). Variability and forecasting of Australian wheat yields. Agricultural and Forest Meteorology, 90(1-2), 43–50.
Van Burgel, A., et al. (n.d.). Towards improved wheat yield forecasting for Western Australia. Department of Agriculture and Food Western Australia.