2 THE IMPORTANCE OF CLIMATE AND WEATHER IN AGRICULTURE

Climate and weather are vitally important in agriculture. Frosts at critical times can wipe out crops. If we do not have rain, then pastures and crops will not grow. Food will need to be transported further to get to where it is needed in a market, and further travel means costs will rise. Consumers pay more. When there is a drought, failed crops will mean farmers will get reduced returns or get nothing for their work. Drought areas means pasture and water are usually in short supply and causes severe hardships for those individuals and communities where it occurs. Ecosystems may be degraded. A better understanding of the variable climate and how the seasonal climate forecasts may be used to make better decisions is essential in the agriculture of today.

2.1 Introduction and definitions
Climate refers to the average weather conditions over a fairly long period of time.

Fig. 2. Location of the Earth and Sun at any particular time will influence the climate we are receiving

Source: QDPI

Climate includes the average rainfall and temperature. These averages are calculated from daily recordings over a lengthy period of time (see Table 1).

The average rainfall for this period of time is 1275/12 =106.2 mm.

If we do this for all months we then get a snapshot of the rainfall for that place for an average year. (see Figure 3).

Fig 3. Average Monthly Rain for Rainsville, 1930-2000

Exercise
Calculate and graph the average monthly rainfall for where you are. Explain what it means about your location eg summer or winter rainfall dominance… Compare your rainfall to rainfall in another place in Australia at least 500km away. Discuss why it varies.

Because the climate is a result of the global circulation, the climate and weather can vary from day to day. (This will not be detailed further as it is assumed to be covered in previous units).

Weather is the daily interaction of the atmosphere with the earths surface.
For instance any day's weather could be shown and described briefly as:

Each days weather has an important impact on agriculture.

For instance a rainy day can help plants to grow. A windy day may mean animals head for shelter. A warm day may mean animals have to drink more water.

Exercise
Measure daily temperatures and rainfall for each day for a week. Make remarks for each day.

Optional Exercise
If near a farm, observe and measure plant and animal behaviour at different times throughout the day. Get the students to answer how plants and animals may respond to the weather at different times of the day, eg in summer, stock head for shelter in middle of the day, plants like sunflowers may face towards the sun etc.

2.2 Solar radiation - the driving force
The sun drives the system for all climate, weather and agriculture to exist and be maintained.(see Fig 2)

2.3 Some important climate and weather features

2.3.1 Rainfall
Rainfall provides the moisture and water for all plants and animals to survive. Moisture is necessary in photosynthesis. Nutrients are only able to be transposed to plant roots via soil moisture.Rainfall variability leads to major management intervention in places like Australia when the rain we think is going to come, that is depended on for crops and livestock, either fails or occurs in smaller amounts than what is required by plants to survive. Fig. 4 shows the rainfall variability of Australia.

Fig 4. Annual Rainfall Variability

Source: BoM

2.3.2 Temperature
All living organisms exist within a preferred range of temperature. Polar bears exist where it is very cold. Tropical rainforests exist where it is warm (and wet). Optimal temperature describes the range of temperatures which living organisms will best function.

Fig 5. Wind chill

Source: Internet

Fig. 5 shows that wind speed and temperature combine to give a wind chill effect. That is a lowered actual temperature, that greatly affects plants and animals. If the wind chill is significant, it is possible vulnerable plants and animals may die. Comments on how going beyond optimal temperature and moisture boundaries affects certain plants and animals is shown in Table 3.

In agriculture, it is important to know the optimal temperature range for different enterprises. This needs to be managed so there is sustainable production.

Exercise Idea.
Investigate temperature requirements for a specific agricultural enterprise. Experiment on a certain crop with optimal, below optimal and above optimal temperature for that crop. Describe and explain what happens. eg. Plant some seeds (eg wheat) and compare growths in locations that are cold, warm and hot. Discuss the findings.

2.3.3 Wind
Because the sun heats the tropics more than the poles (see diagram), it leads to circulation of air and water over all the world (see Figs. 6a, b & 7).

Fig. 6a Differential heating of the Earth's surface leads to global circulation of the atmosphere and Oceans

Source: BoM

Fig. 6b Pacific Ocean

Source: QDPI

As air is warmed it rises, as it rises it cools, as it cools it falls. This happens on a localised scale and a global scale, creating the jet streams, trade winds, sea breezes and local winds.

Fig. 7 The general atmospheric circulation of the globe

Source: BoM

Very strong winds can damage agricultural products by physically pushing crops over ie. lodging. This makes harvesting difficult. Strong winds also makes plants use up more water ( because of increased evapotranspiration) which may lead to wilting and mortality if there is insufficient moisture to replace the moisture taken from the soil which is transpired through the leaves.

Fig. 8 Drought affected sunflowers

Source: QDPI

Wind can also be essential to help many plants to pollinate eg. Corn, jojoba, some pine trees and others.

2.3.4 Humidity
Humidity is a measure of the amount of water vapour in a body of air at a certain temperature. Relative humidity describes the amount of water vapour in the air at a particular temperature in relation to the maximum amount of water vapour the air could hold. At 100% relative humidity, fog will form.

Humidity is an important measure in agriculture because it has an effect on evaporation rates. Where there is high humidity, animals may get stressed, and moulds, mildews and fungus may grow.

Fig. 9 Yellow spot (a fungus) on wheat

Source: QDPI

Exercise Idea.
Experiment. Try growing mould on bread in a continuously humid versus a drier place. What happens? Why does this happen? Ask the students to write up their observations and to consider a project that studies what humidity effects may have on some agricultural enterprises such as vegetables. A guest speaker farmer may also be available…!

Further reading
Ref: DPI Farmnotes on Yellow spot in wheat.