THE CLIMATE KELPIE BLOG: How to assess your ‘green date’ probability using the CliMate app

Posted by BCG on 24th August 2020

Matching the nutritional needs of your livestock to the annual pasture cycle is important, particularly in northern Australia where periods of feed deficiency can persist for extended periods.

Many cattle producers use the ‘green date’ to identify the start of the growing season and help time their cattle’s production cycles to match peak feed availability.

But for producers in areas with low rainfall or highly variable rainfall, the chance of achieving their green date can be less than reliable. This is where the CliMate app gives producers a better understanding of these probabilities to help them make well-informed decisions about timing based on the longer-term weather data.

Green date

Definitions of the green date vary depending on local requirements. For example, heavy soils may need a greater amount of breaking rain than light textured soils.

The green date is basically the target date when a minimum amount of rainfall over a set period − such as 50 millimetres over five days, or 30 mm over three days − is ‘likely’ to be achieved, where likely is set by each decision makers’ risk profile.

The green date gives a producer some idea of when they might expect that the soil moisture, temperature and day length has reached the point where pastures can really start to grow.

Test the options

The CliMate app helps users explore rainfall probabilities, providing rich detail of how rain has fallen in the past. By interrogating historic and current seasonal weather data for their own location, users can undertake their own risk assessments.

The How Often? analysis in the CliMate app allows users to test the sensitivity of their chosen green date rules. By adjusting the inputs, the user gets a good feel for the odds (probabilities) of each specified scenario (Figure 1).

The process you need to undertake to interrogate the CliMate App to assess your green date is:

  1. Open the CliMate App and go to ‘How Often’
  2. Select the time of the year that you wish to investigate (e.g. example 1 below shows 1 September to 31 December, while example 2 shows 1 September to 31 January)
  3. Select how much rainfall you think you need to achieve your green date (e.g. 50mm)
  4. Select how many days that rainfall needs to fall over (e.g. five days)
  5. The App will then tell you the historical probability of achieving that since 1980 and the bar charts which years

Once you have done it once, you can vary the month(s), rainfall and timeframe to assess other likelihoods and build up a picture of green date likely outcomes.

Figure 1. The CliMate ‘How Often?’ analysis shows that Gatton Vale in Queensland is likely to achieve a threshold of 50 mm rainfall in five days in about half of the years (55 per cent) between September and December, but if the time frame is extended out to January the odds increase to 77 per cent of years.

Scan the season

A simple alternative is to scan the How’s the past? information for each month. Figure 2 shows an overview of potential growing conditions for each month.

Figure 2. The CliMate ‘How’s the Past?’ analysis provides a quick overview of the average rainfall, evaporation potential and temperatures throughout the season.

The How’s the past? analysis also includes a ‘chequer-board’ chart of monthly rainfall totals for each year.

A quick scan of the monthly rainfall for Gatton Vale in Queensland (Figure 3) shows that totals for September to October have only exceeded 50 mm in six of the past 30 years (20 per cent chance). While November may average 53 mm of rainfall for the month, there are only 10 years out of 30 where it actually exceeds 50 mm.

Figure 3. The CliMate ‘How’s the Past?’ chequer-board shows that September to October rainfall totals for Gatton Vale for have only exceeded 50 mm in six of the 30 years (20 per cent) from 1990 to 2019.

“Many landholders are surprised to find that achieving 50 mm in a week before December is by no means guaranteed,” said Paul Webb, former extension specialist with the University of Southern Queensland. “They have found the ready access to long-term rainfall records provided by CliMate really valuable for decision making.”

The most suitable green date for any given business will depend on more than just location, with soil type, stocking rates and risk profile all playing a role in identifying an appropriate green date. The real strength of CliMate is that it gives the user a simple tool to explore the local historic climate data and provides an objective estimate of the probabilities for a range of scenarios.

This information plays a vital part in planning for farm businesses and their advisors.

Climate Kelpie has more information about using the application or you can access it directly at Australian CliMate app, where you also will find the links to the android and iOS apps.

Contact

David Freebairn, 0408 876 904, david.freebairn@usq.edu.au
Australian CliMate

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