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Geography K–10 · Years 9–10

How much wheat can the rain grow: the French and Schultz potential yield model

Biomes and food security (ACARA Year 9 sub-strand); NSW Stage 5 focus area Sustainable Biomes (2015 syllabus) and Biomes and sustainable agriculture (2024 syllabus)

Calculation and dataLow risk

This site has no interactive model of its own. Where a step or a material names a Concept Studio model, simulation or tool, it has not been built; an external simulation a step names (for example PhET) is not part of this site.

The idea

In a dryland wheat biome the growing-season rainfall sets a ceiling on grain yield, because a fixed amount of water is lost from the soil surface before the crop can turn water into grain.

What you need

  • Bureau of Meteorology Climate Data Online: monthly rainfall for the last 20 complete years at one NSW wheatbelt station, copied into a spreadsheet. Before choosing the station, read its mean annual rainfall from the Bureau's climate statistics table for that station and check that it falls inside the 275 to 450 mm average annual rainfall zone in which the model was derived; Hillston Airport (station 075032) and Lake Cargelligo Airport (075039) are candidates to check this way, and a station on the wetter slopes such as Wagga Wagga AMO (072150) or Dubbo Airport AWS (065070) can be checked the same way to see whether it falls outside the zone and what that does to the model
  • Spreadsheet or calculator
  • Whitbread and Hancock (2008), Agronomy Australia conference paper, for the model and one measured comparison

How to do it

  1. For each year add the April to October rainfall (the growing-season rainfall, GSR).
  2. Compute potential yield = 20 × (GSR − 110) kg/ha, and 0 when GSR is 110 mm or less.
  3. Plot potential yield against year and against GSR. Mark the driest and wettest years.
  4. Count the years in which potential yield is below 1000 kg/ha.
  5. Compare with the measured case in Whitbread and Hancock: at 287 mm of April to October rain the measured wheat yield at Minnipa reached about 2.9 t/ha. Compute the model value for 287 mm and the gap.
  6. List reasons a real crop falls short of the model, using the criticisms the paper records: timing of in-season rain, runoff and drainage, rain outside the season, and a soil evaporation that is not constant.

What you should see

A 20-year table and two graphs. Potential yield varies from year to year with GSR, and the relationship is a straight line that starts at 110 mm, so a dry season loses a larger share of its yield than of its rain. The model gives 3540 kg/ha at 287 mm, 0.64 t/ha above the 2.9 t/ha measured at Minnipa; the learner explains the gap from the listed causes. It worked if every year was summed over the same seven months.

What changes

This activity lists no variables to change, measure and keep the same.

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

  • Doubling the rain doubles the harvest.
  • Farm yields are limited only by rainfall.

Safety card

Low riskLearners carry it out

Hazards

No hazard is listed.

Controls

No control is listed.

Note

No chemicals or heat are used. The activity is run under the school risk-assessment procedure; RiskAssess (riskassess.com.au) holds templates for school practicals and fieldwork. The NSW Department of Education Chemical Safety in Schools package is not triggered.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

  • Geography K–10 Syllabus (2015), NESA; current and taught in 2026, replaced from 2027. The outcome codes are not printed on the NESA landing page for this syllabus, which only links the document, so the url given is the syllabus document itself (geography-k-10-syllabus-2015.docx); the code and its outcome text were read in that document on 23 September 2026.GE5-2GE5-3GE5-7
  • Geography 7–10 Syllabus (2024), NESA; implementation from 2027, not yet taught; code read on this page on 22 September 2026GE5-PRI-01GE5-TAP-01
  • Australian Curriculum v9AC9HG9K03AC9HG9K04AC9HG9S03

Sources

The pages the author read to write this activity.

  1. www.agronomyaustraliaproceedings.org/images/sampledata/2008/concurrent/assessing-yield-potential/5803_whitbreadhancock.pdf
  2. www.bom.gov.au/climate/data
  3. education.nsw.gov.au/teaching-and-learning/curriculum/hsie/hsie-curriculum-resources-k-12/hsie-7-10-curriculum-resources/geography-s5-biomes-and-sustainable-agriculture
  4. curriculum.nsw.edu.au/learning-areas/hsie/geography-7-10-2024/outcomes
  5. www.bom.gov.au/climate/averages/tables/cw_075032.shtml
  6. www.bom.gov.au/climate/averages/tables/cw_075039.shtml
  7. www.bom.gov.au/climate/averages/tables/cw_072150.shtml
  8. www.bom.gov.au/climate/averages/tables/cw_065070.shtml

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