Author: Lee Menhenett, Senior Technical Agronomist, Incitec Pivot  

Budgeting is a fluid process accounting for credits and debits coming into and out of accounts. You can budget using a sophisticated spreadsheet or by writing on the back of an envelope. The main consideration, whichever way you do it is to account for inputs and outputs and to be constantly reviewing/amending numbers as circumstances change. The process is not “set and forget”. 

Nitrogen budgeting requires the same level of review, however the driver for amending these budgets is moisture. Rainfall records should be used to constantly amend water limited yield estimates, and not as bragging rights at the pub. At the start of a new financial year, it is timely to update rainfall data, re-calculated water limited yields and review nitrogen budgets. 

Calculating water limited yield  

French and Schultz (1984) determined a method to predicted yield based on crop available water and further refinement of the calculation has occurred over the years as noted in the GRDC (2009) Water Use Efficiency fact sheet. If you don’t have access to models or find the calculations overwhelming, you can still use rainfall and yield data to provide very useful, localised information and to refine water use efficiencies. 

Table 1 is rainfall data collected from the Bureau of Meteorology website. Shown within the table is 30% of rainfall November 2025 to March 2026, growing season rainfall from April to June and average, 25% below average rainfall and 25% above average rainfall July to October for several locations. Threshold water 100mm (typically lost to evaporation) is subtracted from the total moisture to provide an available water number. 

Water limited yield can then be calculated by multiplying available moisture by an efficiency number. The calculation in Figure 1 uses 20kg grain(wheat)/mm moisture. This number can vary widely based on soil fertility, soil type, soil limitations, rainfall effectiveness (run-off, stored moisture, weed control) and spring temperatures. It shows yield predictions based on three rainfall scenarios from July to October – average rainfall, 25% below average rainfall and 25% above average rainfall. The variations in yield provide a realistic range to assess and review nitrogen budgets. Targeted yield will be determined by individuals based on local knowledge, cash flow, commodity (fertiliser and grain) pricing and seasonal outlook. Risk is involved as nitrogen decisions will be made over the next two months, without knowledge of weather conditions in the spring. 

Table 1: Year to date rainfall accumulation including below average, average and above average rainfall scenarios for the rest of the season.  

 Rainfall mm Average 
 30% GSR 25% below average Average 25% above average. Total Available 
Location Nov – Mar (a) Apr – Jun (b) Jul – Oct Jul – Oct (c) Jul – Oct a+b+c Total – 100mm 
Cummins 30 199 153 204 255 433 333 
Kimba 43 172 111 148 185 363 263 
Kadina AWS 36 160 89 119 148 315 215 
Hallet/Jamestown 54 143 174 231 289 429 329 
Kapunda West (Kaplee) 30 100 166 221 277 351 251 
Keith 41 174 153 205 256 420 320 
Pinnaroo 50 119 106 141 177 309 209 
Sea Lake 74 76 96 128 160 278 178 
Yarrawonga 49 111 125 167 209 327 227 
Horsham 79 133 115 153 191 365 265 
Lake Bolac / Westmere* 28 144 185 247 309 419 319 
Griffith 25 65 104 139 174 229 129 
Wagga Wagga 52 117 130 173 216 341 241 

* includes November rainfall  
Source: Adapted from Bureau of Meteorology 2026. 

Figure 1: 2026 Wheat water limited yield predictions by location using three rainfall outlook scenarios and wheat WUE of 20 kg grain/mm. Source: Incitec Pivot 2026. 

Nitrogen budget  

From water limited yield scenarios nitrogen requirements can then be calculated. In Figure 2, 11% protein and 40% nitrogen use efficiency is used i.e. 48 kgN/t wheat. Using Pinnaroo location as an example the wheat yield variation ranges from 3.5t/ha with below average July to October rainfall, 4.2 t/ha with average rainfall and 4.9 t/ha in an above average rainfall scenario. The total nitrogen requirement for these yields are 168, 202 and 236 kgN/ha respectively. This is the account debit. Account credits are soil mineral nitrogen, mineralisation and nitrogen fertiliser. 

Figure 2: Nitrogen requirements based on three rainfall outlook scenarios and wheat at 11% protein. Source: Incitec Pivot 2026 

Summary 

Moisture banked from summer rain, crops establishing on time and excellent June rainfall, places yield potential in many locations at average to above average even using a below average rainfall July to October outlook. Nitrogen rate is the major influencer for realising yield potential. Using on-farm rainfall records, assessing crop yield potential and reviewing nitrogen budgets is critical. Once a range of yields and nitrogen requirements are established, consideration then turns to risk versus reward.  

Further information 

For more information on the Lab and its services, please contact Incitec Pivot’s Senior Technical Agronomist, Lee Menhenett, at lee.menhenett@incitecpivot.com.au. 

References 

Bureau of Meteorology (2026) Climate Data Online. Commonwealth of Australia 

French RJ, Schultz JE (1984) Water use efficiency of wheat in a Mediterranean-type environment. I. The relation between yield, water use and climate. Australian Journal of Agricultural Research 35, 743-764. 

Grains Research and Development Corporation (2009) Water Use Efficiency. Southern and Western Region Fact Sheet.