Rainfall, evaporation and effective rainfall
Rainfall
Tocal receives more summer rainfall than winter. In January to March there is more than twice the rain of July to September. There are only slightly more rain days in the summer/autumn period, however there are more thunderstorms in summer, bringing heavier falls. Most summer rainfall comes from storms caused by moist air extending southwards from northern Australia and east from the Tasman Sea. Winter rainfall is mainly caused by cold fronts from the mid latitudes. The development of low pressure systems in the Tasman Sea in winter can also bring heavy rain and strong winds (see wind).
The lower monthly rainfall figures from July to September are made much worse by the incidence of strong westerly winds causing high evaporation rates. The district rarely receives really good rainfall in spring compared to southern New South Wales. Reliance on the ryegrasses and clovers as the main feed source is therefore difficult and explains the need for summer growing species such as kikuyu.
Graph of average and median monthly rainfall at Tocal (1965-2006)
Click here to view base rainfall data 1965 - 2006
This graph shows both average monthly rainfall and the median at Tocal. The average is calculated by adding rainfall totals for each month over the period from 1965 to 2006 and dividing by the number of years. This figure can be misleading, because a couple of very low or very high rainfall years can alter the average significantly. A more useful figure to use for farm planning is the median. Median rainfall is calculated by ranking totals from highest to lowest, the middle figure being the median. The median will often be lower than the average, and is a more realistic figure to use, although it is not often quoted in rainfall statistics.
When looking at long-term trends, it is more accurate to use figures from as far back as possible. The graph below is a combination of the figures available from Tocal (beginning 1965) and Paterson Post Office (beginning 1902) which are approximately 5km apart. While the data is not all strictly Tocal, it is a more useful record than the 1965-2006 Tocal records alone. In comparing the graphs, you can see that while the average varies slightly from the Tocal figures for each month, the strong seasonal trend is constant.
Graph of average and median monthly rainfall taken from records at Tocal and Paterson (1902-2006)
Click here to view base rainfall data 1902 - 2006
Rolling five-year rainfall averages
The rolling five-year average is calculated by averaging the rainfall figures for the previous five years to get the figure for that year. For example, the 1970 figure is the average of 1966, 1967, 1968, 1969 and 1970. The rolling five-year average is useful because the effects of rainfall (either high or low) do not stop at the end of the calendar year, so a graph of annual rainfall for each year can be misleading. Importantly, the rolling five-year average is a better indicator of trends in rainfall.
Graph showing rolling five year average rainfall for Paterson and Tocal (1906-2006)
The graph clearly shows above and below average periods throughout the 20th century, with the 1940s and early ’80s being poor rainfall years, while the early ’50s, early ’70s and late ’80s experienced high rainfall.
Evaporation
Evaporation is the reverse of rainfall. It is a measurement that can be related to the loss of moisture from the soil and from plants. Evaporation rises as temperatures increase, and as monthly temperatures fall, so do evaporation rates. The graph below clearly shows this relationship. The incidence of westerly winds increases evaporation rates.
Measurements of evaporation have only been recorded at Tocal from 1974, and never at Paterson. It must be remembered that the figures for evaporation are based on evaporation from an open pan (evaporimeter) that is continually replenished. This is why most months of the year get more evaporation than rain. In reality you can never get more evaporation than rain.
Monthly pan evaporation 1974-2003 - Tocal
Click here to view base pan evaporation data
Effective Rainfall
Effective rainfall is the rainfall that is available in the plant root zone, allowing the plant to germinate or maintain its growth. This can be related to rainfall versus evaporation. When combined with temperature figures, effective rainfall will indicate the main growing seasons for plants. As a rule of thumb, the evaporation from an exposed soil surface is about one-third that from the evaporimeter.
In the following two graphs, evaporation is compared with rainfall at Tocal. The first graph shows that evaporation from the pan evaporimeter is always higher than rainfall.
Monthly rainfall pan evaporation comparison 1974-2006 Tocal
Click here to view base rainfall data and evaporation data
When the 'rule of thumb' described above is applied, as shown in the graph below, evaporation from the soil surface indicates that effective rainfall occurs between February and June. This is the main growing season at Tocal. It would be hoped that spring would also be a major growing season, but the success of spring growth is limited by low rainfall, even though temperatures are rising.
Average monthly moisture availability based on soil evepaoration 1974-2006 Tocal
Using Climate Data for Irrigation Scheduling
Irrigation schedules are used to ensure efficient water use. Using moisture budgets a farmer can ensure that water is applied to a crop or pasture when it will be of most benefit. Moisture budgets are calculated using rainfall and evapotranspiration .
Evapotranspiration is used because while effective rainfall is a good indication of seasonal conditions, plant water use needs also to be considered. Evapotranspiration combines water used by plants as well as water lost from the soil through evaporation.
Plant water use is influenced by environmental conditions, including wind, temperature, humidity and solar radiation. If conditions are hot,dry and windy, crop water use and evaporation will be high. However, if the day is cool and overcast, water use will decrease. The chart below shows the evapotranspiration measurements and trends at Tocal in 2003.
Chart of the evapotranspiration at Tocal
A standard or reference crop, (usually a well-watered, actively-growing grass pasture), is used to calulate pasture evapotranspiration (ETo) (the maximum amount of water that such a pasture would use on a particular day). This reference evapotranspiration is calculated and published in tables by the Bureau of Meteorology (www.bom.gov.au). When pasture evapotranspiration is related to pan evaporation at Tocal (using figures from 2003), pasture evapotranspiration was 62% of pan evaporation which is close to the rule of thumb we gave in the graph 'Average monthly moisture availability based on soil evaporation'.
Daily water use is also influenced by a crop factor (Kc) (also a published figure) and the stage of growth that the pasture is in.
Soil acts as a reservoir for moisture for growing plants. You can calculate how much water you need in the soil if you have the following information:
- Effective root zone of the crop in the soil
- Effective rainfall for the crop. This depends on the crop and soil texture
- Readily available water (RAW). This depends on the soil texture and the effective root zone of the crop. No matter how much water you add, the RAW value is the amount that the soil can hold for the crop can readily use without suffering yield loss.
Here is an example of how to calculate RAW from 'The Soils of Tocal (www.tocal.com)' in the All About Tocal series, Book 5.
Lucerne grown on River flat alluvial soil
- Effective root zone for lucerne=0.80m
- RAW factor for loam = 85mm/m
- RAW for lucerne in this loam = 68mm.
Kikuyu grown on hill soil
- Effective root zone for kikuyu =0.42m
- RAW factor for sandy clay loam =70mm/m
- RAW for kikuyu in this sandy clay loam = 29.4mm.
When the soil moisture declines to these levels (the RAW), irrigation is needed. As shown in the graphs below, during 2003 irrigation was needed 10 times to replenish soil moisture on the lucerne, in the same period, the kikuyu needed irrigation 23 times but less water was applied. This is a reflection of the difference in soil type and the effective root zones of the pastures.
chart of the ETo for lucerne on alluvial loam
