Procedure 1

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In the arid zone, rainfall and soil moisture are the major factors limiting both plant growth and pastoral production.

Rainfall in the arid zone is highly variable and for the most part lacks distinct seasonality, although there is a tendency towards winter dominance in the south and summer dominance in the north.

Another characteristic is the marked variation in both monthly and yearly rainfall. This variation reduces the usefulness of averages in describing the rainfall at any particular location. Typically, for annual or monthly rainfall, the average is considerably higher than the median value, as it can be inflated by a few very wet years.

This is a difficult environment for decision making as there is no clear signal marking the beginning or the end of the feed production period. Seasonal risk assessments based on the Sothern Oscillation Index (SOI) system provide useful information over much of the region in the late winter-spring period but at other times there is only historical climate data to use to determine ‘trigger points’ for decision making on pasture growth (Hacker et al., 2005).

Use of the SOI and historical data is systematically being replaced by the Predictive Ocean Atmosphere Model for Australia (POAMA) which is a seasonal forecast system based on a combined ocean/atmosphere model and ocean/atmosphere/land observation system. Unlike existing statistical forecasting, this system is not limited by historical relationships and so it can forecast a new set of climatic conditions. For example, because they simulate the real world they have the potential to predict how the impacts of one El Nino might be different to those of another.

Infiltration and run-off

In a well-functioning landscape, rain either infiltrates the soil surface or runs off to lower-lying parts of the landscape. In larger rainfall events some of this run-off may leave the local landscape through major drainage systems. However, much of it is absorbed in other parts of the landscape, creating a mosaic of ‘run-off–run-on’ areas that are important in determining the amount of soil moisture available for plants and the overall productive capacity of the land.

Concentrating water (and nutrients) in restricted parts of the landscape can result in higher plant production in localised areas rather than an even spread. The scale of this mosaic in different land types varies widely. In some areas it is very evident in the formation of groves of dense vegetation separated by relatively bare areas. The redistribution of rainfall through run-off–run-on patchworks is an important part of the functioning of rangeland systems and underpins their production (Hacker et al., 2005).

While run-off is also important for enabling dams to fill, it is important that this does not result in water erosion and that as much water as possible infiltrates the soil, in turn contributing to plant growth.

Temperature

Temperature is an important feature, as it determines both the effectiveness of rainfall and the growth response of plants. In dry environments evaporation is one of the main means of water loss.

Temperature influences evaporation and so is an important factor in determining how much rainfall is available for plant growth (effective rainfall). It also determines the amount of water required by plants to maintain their growth and how long growth can be sustained by a given amount of water contained in the soil profile. The growth rate of plants is influenced by temperature (relates also to soil temperature), so that slower growth may be observed in winter than in spring or summer regardless of the amount of soil moisture available.

The immediate effect of rain on grazing animals

Livestock need particular attention following drought-breaking rain, as the sudden change in diet arising from the new pasture growth may cause nutritional imbalances. Stress due to wet conditions and a period of low nutrient intake needs to be managed carefully if the transition is to be successful.

Prolonged wet conditions will often cause stock to limit their intake or ‘go off their feed’. If rain is coupled with windy conditions, stock will also need sheltered areas, particularly if stock are in poor condition. Low temperatures compound the problem; the nutritional stress experienced by stock in cold, wet, windy conditions is likely to increase substantially. An increased intake of high quality nutritious feed is likely to be required in order to prevent stock losses but also keep in mind that sudden dietary changes can severely disrupt the functioning of the rumen.

Particular attention should also be given to the rapid germination of potentially toxic plants following rainfall events, such as spurge and pimelea. Often these are some of the first plants to emerge following rain and may be consumed by cattle if there is little else on offer.

Soil surface management

Soil erosion by wind and water is one form of land degradation affecting pastoral productivity. Erosion of topsoil reduces nutrient availability to the plants growing in that soil and often exposes subsoils that are impervious to water and inhospitable to plant growth. Sealed surfaces on sloping areas can result in excessive run off and reduce the capacity of the landscape to produce pasture. Restoration of eroded areas is often difficult and may require mechanical intervention and changes to grazing management to ensure retention of ground cover.

Animal production is closely linked to the availability of feed and continuity of the feed supply can only be maximised by the presence and relative abundance of perennial forage species. These include grasses, forbs and shrubs, particularly chenopod shrubs (saltbush, bluebush and others).

Facilitating soil surface restoration using palatable perennial species requires the adoption of grazing management that adjusts the level and timing of grazing in relation to the needs of the vegetation and the opportunities or threats imposed by climatic conditions.

Total grazing pressure

Non-domestic herbivores, including rabbits, feral goats, camels, donkeys, horses and kangaroos, can account for a substantial portion of the total forage demand. When combined, the pasture consumed by non-domestic herbivores can represent a significant level of competition to livestock. Under good seasonal conditions the demand by all species can be satisfied but when conditions deteriorate, the level of competition increases and producers may realise a negative economic impact. Also, the increased competition means greater potential for land degradation. The control of total grazing pressure therefore is a fundamental management requirement aimed at addressing arid zone land degradation issues, as well as declining livestock productivity.

What to measure and when

Use historical rainfall records to assess the pattern and variability of rainfall:

  • collect and analyse historical records (Tool 2.01 provides a range of sources and methods to complete this analysis)
  • measure the long term trend in condition for native perennial pastures, including all palatable species (see Module 3)
  • look for any evidence of soil erosion or sediment deposition; this should be observed routinely after rain