Risk Management and Rainfall

Introduction

Rainfall is clearly a key driver of agricultural productivity. Depending upon the timescales that are important to operations, there are a variety of tools that can be of use. In short timescales radar rainfall images provide current intensity and coverage in your local area. Climate maps give an indication of recent and long term rainfall, and hence the likelihood that there is sufficient soil moisture or dam water available for your needs. Weather forecasts provide the short term outlook needed to plan the week ahead, and seasonal information on rainfall enables a longer term plan to be laid out. Of course climate change may well determine the future direction of farming operations in Australia over the coming century. Managing risk, at all timescales, is one of the keys to a successful enterprise.

Rainfall variability and local climate

Rainfall is often described by meteorologists as a very noisy phenomenon. In other words, it can vary a great deal not only on very short timescales, but also over very small distances. For instance, a thunderstorm may deluge one location for a few minutes then cease abruptly, while just next door they receive no rainfall whatsoever. Over large parts of Australia, we also know that seasonal rainfall can vary greatly from one year to the next due to climate drivers such as El Niño and La Niña.

With such great variability in rainfall, it is worth becoming familiar with the local characteristics of rainfall at your own location.  Average annual rainfall can vary over quite short distances due to a variety of local factors, for instance the nearby topography and distance from the sea. Decisions such as the range of possible crop choice may be influenced by studying such detailed information.

Average annual rainfall (millimetres) on the Fleurieu Peninsula south of Adelaide, showing local variation due to presence of hills and proximity to the sea. The shaded square is 10 km by 10 km in size for purposes of illustration. Source: Bureau of Meteorology and 'Farming a Sunburnt Country'.

Such variability in rainfall means there are many and varied associated risks (as well as opportunities). These risks can range from those associated with local flooding to long term drought, and in turn, each has its own coping strategies.

Some typical rainfall events and episodes

Some typical rainfall events and episodes and associated rainfall type, risks and opportunities are described below. Also shown are sources of real-time information for these events.

Cold Fronts

Rainfall type and typical season of occurrence	Often long bands of steady rainfall lasting less than a few hours; sometimes followed by cold showery rain. Northern end of frontal band usually gives less rain than the thicker middle region of the front.
Risks	Short-lived localised flooding
Opportunities	Maximise planting opportunity, Timing of haymaking. Increased effectiveness of superphosphate spreading.
Possible climate change risks and opportunities	Possibly fewer cold fronts penetrating well inland
Information	Cold Fronts Weather forecasts Rainfall outlooks Satellite loops Radar

Mid-latitude low pressure systems

Rainfall type and typical season of occurrence	Mostly occur only on southern coastal regions.  Can give consistent long-lived light to moderate falls or heavier flooding rains, depending on intensity of low pressure and speed of travel (slower travel usually equates to more rainfall)
Risks	Heavy flooding
Opportunities	Rains to fill dams and soil profile Autumn break
Possible climate change risks and opportunities	Fewer systems but occasionally more intense
Information	Cut-off Lows East Coast Lows Weather forecasts Synoptic charts Rainfall outlooks

The Australian Monsoon

Rainfall type and typical season of occurrence	Broad areas of cloud and rain, including thunderstorms and patchy heavy showers. Occur across northern Australia in summer, and occasionally extend further south. Can provide the impetus for significant rainfall in southern parts
Risks	Thunderstorms, lightning.  Flooding, wind damage and storm surge associated with tropical cyclones, weed growth in winter cropping regions In some years, e.g., El Niño years, trough may not be well developed or stay further north, producing less rainfall than in other years
Opportunities	Pastoral and summer cropping opportunities
Possible climate change risks and opportunities	Not well understood but possibly more variability
Information	The Australian Monsoon Weekly tropical statement Rainfall pages Satellite images Maps of cloudiness Experimental RMM index

Madden-Julian Oscillation (MJO)

Rainfall type and typical season of occurrence	Broad areas of active and inactive tropical rainfall, particularly during summer, tracking from the Indian Ocean eastward across Australia to the western Pacific Ocean. Not a reliable indicator of rainfall over the continent, but can sometimes lead to good falls particularly in northern and parts of eastern Australia.
Risks	MJO, while occasionally producing very good rains, is somewhat erratic in the rain that occurs over the continent.
Opportunities	One of the main triggers of rainfall during the wet season (may give several rain episodes during a single wet season.) Research on the MJO provides potential for better tropical seasonal and intra-seasonal forecasting.
Possible climate change risks and opportunities	Impact of climate change on MJO not well understood
Information	Madden-Julian Oscillation (MJO) Weekly tropical statement Rainfall pages Satellite imagery Maps of cloudiness Experimental RMM index

North-west cloud band

Rainfall type and typical season of occurrence	More often in the cooler seasons; long band of cloud often tracking from NW Australia towards the southeast. Some of these cloud bands give good soaking rains
Risks	Possible short-term flooding
Opportunities	Good opportunities for rainfall through the centre of the continent, as well as through parts of SE Australia
Possible climate change risks and opportunities	Climate change impact on frequency not well understood
Information	North-west cloud band Satellite images Weekly rainfall statement

Monthly, seasonal and annual total rainfall

Rainfall type and typical season of occurrence	The sum of rainfall from the various events occurring at a location. Rainfall may be highly seasonal or more uniform through the year depending on geographic location
Risks	Long term dry conditions during unfavourable years; possibly flooding in La Niña events for some locations
Opportunities	Chance to make significant profits in more favourable years
Possible climate change risks and opportunities	Enhanced likelihood for drier conditions particularly south of the tropics
Information	Seasonal climate outlook ENSO Wrap Up Past climate maps Annual summaries Climate change maps and graphs

Examples of risk analysis and decision points

Rainfall exhibits variability on all timescales, from minutes through to years and beyond. This variability produces patterns which can affect a broard range of farm operations, such as:

• dry spells lasting days enabling in-field operations;
• daily rainfall sufficient to suspend irrigation;
• flash flooding lasting a few hours;
• widespread flooding taking days or weeks to dissipate;
• showery type weather interrupting some operations and minor irritation for tactical planning purposes;
• regular rains of a good season providing good soil moisture;
• deficient rains of a poor season or year leading to soil moisture and general water stress.

There are a number of techniques available to assist decision-making in uncertain environments and where information about the future is not given in an exact format but rather as a likelihood or probability. One simple and easy to use technique is the decision tree. The decision tree starts with the decision to be made, branches or boxes are added containing various options and further branching explores costs, benefits or consequences of these options in order to assist the choice of a reasonable and preferable option.

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