Introduction
Sowing a suitable wheat variety in April can help to optimise farm yields, increase farm area sown and reduce the risks of later season heat shocks across much of WA without significantly affecting frost risk. Research at DPIRD is underway to identify varieties with suitable maturity and good yield potential to match sowing time. The purpose of this paper was to identify if historically the frequency of opportunities to sow in April differs between locations in Western Australia’s wheat belt. Early sowing opportunities can result from two things – good stored soil moisture at seeding in the form of good summer rainfall and/or a certain amount of rainfall in April.
Historical data can generally be assumed to provide a good representation of likely future events unless major climate changes occur. However historical data will not tell you what is going to happen but only what the chances (or probabilities) are of an event happening based on how many times it happened in the past. Rainfall records from 1975 onwards were used in the analysis because this was when there was a major shift in rainfall, particularly winter rainfall in the south west WA (IOCI, 2012) which includes parts of the grainbelt.
Climate forecasts of March and April rainfall have limited skill given that the climate drivers such as ENSO and Indian Ocean Dipole are inactive at this time of the year. ENSO phase transition often occurs during April and May and most ENSO forecasting models have their lowest skill at this time (Webster and Yang, 1992) thus this period has been described as the ‘predictability barrier’. Therefore looking at past years data, is an alternative way to assess the chances of rainfall events occurring at this time of the year.
Methods
Climate data from the years 1975-2017 were obtained from Patched Point Data for the locations Mullewa, Merredin, Katanning and Esperance Downs. These locations were chosen to link with the research trial series investigating wheat variety’s responses to sowing time at Mullewa, Merredin, Katanning and Esperance Downs (Shackley etal 2018). 'Summer' rainfall was classed as November of the previous year to March of the current year. Total rainfall amount of 80mm or greater was considered to give good soil water reserves at 1 April for all four locations and all soil types.
If there had been summer rainfall in a given year, daily April rainfall events 10mm or greater were considered as a sowing opportunity. If no summer rainfall (less than 80mm) then daily April events greater than 20mm were considered a sowing opportunity.
Each of the four weeks in April (week 4 includes 29 and 30 April) were assessed separately to determine the sowing opportunities in April.
Results
In Mullewa, 12 of the 43 years had greater than 80mm of rain in November to March (Table 1). This indicates that usually there is limited amount of stored soil water at the start of seeding. In 12 years with stored moisture, seeding opportunities (10mm or more daily rain in April) occurred in five of these years (Table 2). Of the 31 years which didn’t have more than 80mm of rain before April, only eight had greater than 20mm in April (Table 2). The total number of years with early sowing opportunities at Mullewa is 13 out of 43, or 30% of years.
| November to March rainfall | Mullewa | Merredin | Katanning | Esperance Downs |
|---|---|---|---|---|
| >80mm | 12 (28%) | 24 (56%) | 26 (60%) | 35 (81%) |
| <80mm | 31 (72%) | 19 (44%) | 17 (40%) | 8 (19%) |
In Merredin more than half of the 43 years had greater than 80mm of rain fall between November and March (Table 1). Less than half of these had greater than 10mm in a day in any of the weeks in April (Table 2). Of the 19 years which didn’t have greater than 80mm prior to April, only three had greater than 20mm daily rainfall in any week in April (Table 2). Total number of years with early sowing opportunities is 14 out of 43, or 33%, of years.
| Location | Summer rainfall (November-March) | Rainfall event in April | Number of years with seeding rainfall in April | Total number of years |
|---|---|---|---|---|
| Mullewa | >80mm | >10mm | 5 | 12 |
| Mullewa | <80mm | >20mm | 8 | 31 |
| Merredin | >80mm | >10mm | 11 | 24 |
| Merredin | <80mm | >20mm | 3 | 19 |
| Katanning | >80mm | >10mm | 7 | 26 |
| Katanning | <80mm | >20mm | 14 | 17 |
| Esperance | >80mm | >10mm | 18 | 35 |
| Esperance | <80mm | >20mm | 2 | 8 |
In Katanning more than half of the 43 years had greater than 80mm of rain fall between November and March (Table 1). About a quarter of these had more than 10mm of daily rain in any week in April (Table 2). Of the 17 years with less than 80mm of rain prior to April, the majority had greater than 20mm of daily rain in any week in April (Table 2). The total number of years with early sowing opportunities is 21 out of 43, or 49%, of years.
In Esperance Downs the majority of past years (35 out of 43 years) had received more than 80mm of rain before April and therefore stored soil water at 1 April (Table1). About a half had greater than 10mm April rain (Table 2). Only two of the eight years with less than 80mm before April had greater than 20mm of daily rain over the four weeks (Table 2). Total number of years with early sowing opportunity was 20 out of 43, or 47%, of years.
Discussion
Sowing opportunities differed between locations. There were more years with early sowing opportunities at Katanning and Esperance (21 and 20 years respectively) than at Mullewa and Merredin (13-14 years respectively) between 1975 and 2017. Thus historically, 30% of years had seeding opportunities in April at Mullewa and Merredin. At Katanning and Esperance there were 50% of years with seeding opportunities in April.
Growers seeking to take advantage of stored moisture and seeding opportunities should match the maturity of a variety with sowing date to maximise the yield potential: – 'If sowing early, sow long' (Shackley 2018). Frosts during flowering may affect wheat yields if the sowing date of a variety doesn’t match its maturity. If sowing into soil moisture in early April, sow long maturing wheat varieties.
Conclusion
Looking at past years of rainfall does not guarantee that the rainfall trend will continue into the future but it can give us some idea of what might happen.
Historically, 30% of years had seeding opportunities in April at Mullewa and Merredin. At Katanning and Esperance there were 50% of years with seeding opportunities in April.
Past agronomy research has indicated that to maximise the yield potential at seeding growers should match the maturity of a variety with sowing date. If sowing into soil moisture in early April, sow long maturing wheat varieties.
References
IOCI (2012), milestone report 4 Project 1.2 south-west Western Australia’s regional surface climate and weather systems
Shackley, B, Zaicou-Kunesch C, Curry, J and Nicol, D. (2018) Capturing the best sowing opportunities for wheat in WA. GRDC Research Updates, 26-27 Feb 2018. Perth, Western Australia
Webster, P. J., and S. Yang, 1992: Monsoon and ENSO: Selectively interactive system, Quart. J. Roy. Meteor. Soc., 118, 877−926.
Acknowledgments
Appreciation to Andrew Blake at Department of Primary Industry and Regional Development and Glenn Cooke at Bureau of Meterology for review of the manuscript.