Key messages

DPIRD produces many climate and weather summaries for the South West Land Division. In 2020 three new products were produced:

• Thermal time graphs, which allow comparison of crop growth for the current growing season with historical averages.
• Break of season maps, which inform industry about the timing of sowing opportunities.
• Extreme wind event maps, which show duration and severity of erosion strength winds.

Aims

The Climate and Weather team at the Department of Primary Industries and Regional Development provide summaries of weather relevant to the agricultural industry as graphs and maps.

This paper provides details of three new products in the context of the 2020 growing season. These products gave growers, agribusiness and the government better understanding of challenging climate conditions.

Introduction

The drying and warming climate of the South West Land Division (SWLD), together with extreme weather events, can make grain and pasture production across the region challenging. Knowledge of how the current season is unfolding and the impacts of extreme weather events allows suitable response from growers and agribusiness.

2020 was a challenging year right from the start, with large regional variation in early season soil water due to variable summer rainfall, and a late break for the majority of the SWLD. A severe wind erosion event in late May caused widespread soil movement with some areas requiring reseeding. Growing season temperatures were about 1°C warmer than normal (Bureau of Meteorology), resulting in accelerated crop growth. November rainfall was very much above average (Bureau of Meteorology), delaying harvest and causing stained grain in some locations. Despite this, the 2020 harvest figures were estimated to be 16.5 million tonnes of grain, five million tonnes higher than 2019 (GIWA 2020).

The Climate and Weather team at DPIRD create in-season and multi-season summaries of weather that give useful information about the current season, and allow for informed decision making. Existing products include soil water, rainfall to date, rainfall decile and potential yield maps, and several climate and weather-related decision support tools.

Method

Weather data from 332 Bureau of Meteorology (BOM) and 175 DPIRD weather stations across the SWLD is sourced from DPIRD and SILO (Scientific Information for Land Owners) databases using APIs (application programming interface). Station data is interpolated over the SWLD using kriging.

Cumulative thermal time was calculated from 1 April to 31 October, for each year since 1975, as the sum of the daily average temperatures for the season to date. The current season was then plotted against the historical distribution (1975-2019) to give an indication of the expected crop growth rate. This was prototyped using a few locations, including pasture and oat field trials.

Traditionally the break of season is the date of first sowing opportunity (although many growers dry sow before this date) and is identified as being the first date when the total rainfall over a three-day period reaches a required threshold. DPIRD's standard threshold is 15mm between 25 April and 4 June and 5mm from 5 June. The break of season was calculated using BOM daily rainfall data for all years 1975-2020. This was then used to calculate the median season break across the SWLD.

Wind speeds of greater than 28.8km/hr are considered a soil erosion risk (Bagnold, 1973), as this speed is needed to move soil particles and lift dust for significant distances. Data is available for DPIRD stations only. Duration of erosion strength wind (weekly summary) was sourced from the DPIRD extreme weather API. Severity data uses 15 minute summary data for a 24 hour period.

Results

Thermal time

These graphs provide a snapshot of the current season allowing growers to assess water requirements of their crops and to gauge potential water stress. Including historical data puts the season in context in terms of the likely growth rate of crops or pasture. In 2020, some locations, (e.g. Pingelly West) had average temperatures while others, (e.g. Ravensthorpe) were much warmer (Figure 1). As of 31 August 2020, Ravensthorpe’s thermal time was greater than any in the reference period; this extra heat caused crop growth to be two to three weeks ahead of expected.

Figure 1. Example thermal time graphs for Pingelly West and Ravensthorpe showing the progress of the season as at 31 August 2020 compared to historical data (1975-2019).  The red line is the current year (2020 data), the dashed line is the historical median, and the grey shading is the historical range.

Break of season

Generally, in the SWLD, the season break occurs in mid to late May (Figure 2a). Knowing how early or late the break is gives information about crop establishment and potential yield. In 2020, the break was later than usual for the majority of the grainbelt (Figure 2b). While parts of the south-west corner had received enough rain in early May, others (e.g. Ravensthorpe) were still too dry as of 14 June.

Figure 2. Season break maps showing a) the median break of season date for years 1975 to 2020 and b) for the 2020 growing season as of 14 June. The grey dots in the second map show locations that had yet to receive enough rain for a break.

Wind erosion

Wind erosion is common where cover is minimal, especially in years with late breaks or dry growing seasons (DPIRD). Maps of extreme wind events can alert industry and growers to areas of soil erosion risk and the scale of damage can be estimated. Maximum wind strength (Figure 3a) and duration of erosive winds (Figure 3b) are significant factors in erosion. In late May 2020 a severe weather event occurred across much of the SWLD. This event caused significant top soil movement, most notably in the northern grainbelt where soil amelioration is practiced.

Figure 3 Summary maps for the wind erosion event in late May 2020 a) maximum wind gust (km/hr) on 24 and 25 May 2020 and b) number of hours with erosion strength (> 28.8 km/hr) 20-26 May 2020.

Conclusion

By increasing our range of maps and graphs of automatic weather station data, DPIRD is helping growers understand how the current season is unfolding. Knowing when the break of season occurred and how thermal time is tracking in relation to historical values allows growers to better gauge the potential yield of their crops. Mapping extreme weather events, such as the storm in late May 2020, allows industry and government to stay informed of the challengers facing growers as the season unfolds. This potentially allows timely responses to weather events and growers’ concerns.

Planned developments of these products include within season interpolated break of season maps, and maps of current season thermal time against historical deciles.

References

Bagnold R.A. 1973 The effect of Sand Movement on the Surface Wind. In: The Physics of Blown Sand and Desert Dunes. Springer, Dordrecht

Bureau of Meteorology temperature and rainfall maps http://www.bom.gov.au/jsp/awap/temp/index.jsp (accessed 19 January 2021)

DPIRD website Wind Erosion https://www.agric.wa.gov.au/wind-erosion/diagnosing-wind-erosion-risk (accessed 19 January 2021)

GIWA Crop Report December 2020 http://www.giwa.org.au/2020 (accessed 19 January 2021)

Ritchie, J.T. 1972 Model for predicting evaporation from a row crop with incomplete cover.

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