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|Drv pasture residue

1 Cereal |

|Drv cereal residue

1 Grain leaume |

. Drv araln legume residue

Cereal .

FIGURE 10.3. Climatic patterns and sequence of crop rotations in southern parts of the Australian grain belt (Source: Tow, 1991.)

Effective rainfall is related to the moisture available in the plant's root zone, allowing the plant to germinate, emerge, and maintain its growth. Soil moisture levels need to remain above the wilting point of plants, otherwise plants that cannot replace water lost by transpiration through their leaves will collapse or wilt. This results in death if replacement water is not added quickly. As a rule of thumb, the evaporation from an exposed soil surface is about one-third of that from the evaporimeter. By examining local figures for average monthly rainfall and evaporation, the number of months of effective rain can be assessed; this, when combined with temperature figures, shows the main growing season periods (Figure 10.4).

In northern cropping areas of Australia, effective rainfall may occur at different times throughout the year but not necessarily in a "growing season" block of five or more consecutive months. Storing soil moisture through fallows is critical in overcoming these "gaps" in effective rainfall.

Fallowing is the way most farmers make effective use of water. This involves conserving moisture in the soil between crops by killing (either by cultivation or spraying) any plants that would take moisture from the soil.

Roseworthy, South Australia

Roseworthy, South Australia

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

FIGURE 10.4. Rainfall, evaporation, and length of growing season at Rose-worthy, South Australia (Source: Tow, 1991.)

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

FIGURE 10.4. Rainfall, evaporation, and length of growing season at Rose-worthy, South Australia (Source: Tow, 1991.)

The topsoil may dry out in this period, but a "bank" of water will remain in the subsoil for the following crop. The crop is sown when seasonal rain wets the topsoil, or in some cases, sowing machinery may be specially modified to sow into the existing moisture.

Farm Practices Affecting Water Use Efficiency

To make improvements in water use efficiency, the goal is to maximize and make productive use of soil moisture in the root zone while minimizing nonproductive losses from the root zone (Pratley, 1987). Nonproductive losses include evaporation, transpiration from weeds and volunteer plants, runoff, and deep drainage. Soil evaporation is a major component of nonproductive moisture loss (Perry, 1987). The relative amounts of these nonproductive losses will be determined by

• the quantity, variability, intensity, and seasonal distribution of rainfall;

• soil type, soil fertility, aspect, slope, and landform; and

Farm practices that improve water use efficiency include the following:

• Reduced cultivation or no-till practices—any soil disturbance by cultivation leads to increased evaporation.

• Crop residue or stubble retention reduces the impact energy of raindrops and prevents or slows the flow of water across the soil surface, improving infiltration. A mulch of dead material on the soil surface reduces temperature changes, lowering evaporation and preventing the germination of some weeds.

• Use of deep-rooted plant species—The better the plant can explore the soil profile by root growth, the more soil water will be used. Lucerne is the archetypical deep-rooted pasture plant.

• Use of perennial plants species, where possible, that respond to soil moisture year-round—Some native grasses are able to perform this task efficiently, e.g., Danthonia spp. and Microlaena stipoides.

• Maintaining groundcover at 70 percent or greater reduces runoff and erosion.

• An integrated pest management program and good crop nutrition help to maintain crops and pastures at maximum health and growth potential.

• Opportunity cropping rather than fixed rotations will take advantage of stored soil moisture and seasonal rainfall, as well as lowering groundwater levels, minimizing the risk of dryland salinity.

These factors can restrict efficient water use in a cropping system:

• Soil structural degradation or decline reduces water infiltration, soil water storage, and the optimum conditions required for the germination, emergence, and root growth of the cultivated plant.

• Poor crop nutrition slows the establishment and early productivity of a crop or pasture, thereby increasing the potential for evaporation and yield loss from weeds, pests, and diseases.

• Soil erosion removes the layer of the soil that contains the majority of the available essential nutrients—1 mm of topsoil lost through erosion is equivalent to 7.5 to 10 tonnes of soil per hectare.

• Soil acidification causes soil toxicity problems which stunt root growth and reduce the development and yield of the crop.

• The accumulation of free salts in the surface horizons can have a dramatic impact on vegetation, from stunting the growth of a pasture or crop to limiting what plant species can be grown.

• Poor weed control creates ongoing soil moisture and nutrient loss, reducing the productivity and quality of crops and pastures.

• Pests and diseases, depending on seasonal conditions and location, can reduce the vigor and productive performance of crops and pastures, increasing the nonproductive loss of water.

• Seeding significantly after the optimum sowing date can lower water use and the potential yield of a crop.

Assessing Water Use Efficiency

Assessing the water-limited potential yield of a crop is a useful performance benchmark to assess how well stored soil moisture and growing season rainfall are being used. The range of climatic factors in a given location (French, 1987) determines potential yield. Hayman and de Vries (1995) provide water-limited potential yield figures for a range of crop types (Table 10.1). These figures are approximately 75 percent of the "absolute potential" based on the French-Schultz model and are used because they represent a more realistic and obtainable "on-farm" target.

For example, the following calculations can be made for a wheat crop growing in the Narromine district of NSW receiving 266 mm of growing season rainfall (April-October) and assuming that one-third of the fallow period (December-March) rainfall of 176 mm is conserved soil moisture:

[(266 mm + 59 mm) - 110 mm] x 15 kg/ha/mm = (10.1) potential yield of 3.2 tonnes per hectare [(rainfall + fallow) - evaporation]

TABLE 10.1. Water-limited potential yield figures for various crop types

Crop type

Paddock evaporation (mm)

Potential "on-farm" yield (kg/mm/ha)

Wheat

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