Effectiveness of Current Farming Systems in the Control of Dryland Salinity

The way it was...

In most cases Australia’s native vegetation comprises trees or woody shrubs. This perennial vegetation, with its relatively deep roots, has become effective at taking full advantage of any available water. As a result, it can use most of the water entering the soil. The ‘leakage’ of excess water into the deeper soil below the roots is usually quite small, if it happens at all. Various studies have shown that over most of Australia’s current dryland grazing and cropping areas, this leakage was commonly between 1 and 5 mm/year.

Over thousands of years the minimal leakage has allowed the salts introduced through rainfall or rock weathering to build up in the soil below the depth of plant roots.

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Changing the natural conditions

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There is too much leakage

Leakage can be much greater under current agricultural systems than under natural vegetation. It occurs when the plant/soil system cannot cope with the amount of water that has fallen over a period of time. Leakage is caused by a complex combination of climate and rainfall patterns, vegetation cover and soil properties.

Climate

The effect of climate and rainfall on leakage to groundwater can be simplified into two different types, as follows.

1. In wetter areas, normal rainfall can exceed potential evaporation for a period of the year, leading to leakage when the excess water cannot be stored in the soil.
2. In drier areas, leakage is likely to occur mainly as a result of exceptional circumstances, such as intense rainfall and flooding that may only occur once every 3—20 years.

In the winter rainfall dominated southern parts of the Basin, most of the rainfall occurs during the cooler part of the year, when evaporation and hence the amount of water the vegetation is using is likely to be low. Under these circumstances, the rainfall infiltrating the land must be stored in the soil if leakage is to be prevented. If the soil already contains water that was not used by agricultural plants in the growing seasons, leakage will occur more readily.

In the summer rainfall dominated northern parts of the Basin, most of the rain coincides with the period of highest evaporation, thus increasing chances for the vegetation to use the rainfall. However, rainfall can often be distributed in short periods. In this case, water moves through the profile more rapidly than the plants can extract it, causing significant leakage. Thus while leakage is often lower in summer dominated rainfall areas, it is also more episodic, and depends on the rainfall sequence. Summer rainfall can be as effective in causing leakage as winter rainfall if it is concentrated over short periods.

Vegetation

Vegetation affects the amount of leakage to groundwater in two key ways: the depth of plant roots and whether the plants are perennial.

1. Plant root depth. Deeper roots allow plants to extract water from deeper in the soil, reducing the opportunity for water to leak below them. For example, the roots of Eucalypt trees have been found to depths of 30—40 metres. Unfortunately, as the roots of most of our dryland agricultural plants are less than 1—2 metres deep, they have much less opportunity to extract water from the soil before it flows below their reach and becomes leakage.
   
   For example, a water balance study at Katanning in south-west Western Australia showed that shallow rooted clover and deep rooted lucerne both dried out the shallow soil (0—45 cm) to the same extent, while the deeper rooted lucerne was able to dry out the deeper soil (45 cm—1.2 m) to a much greater extent. Leakage from 80 mm under the clover was reduced to 30 mm because the lucerne was helping to dry the deeper soil.
   
   Even with deeper rooted crops, leakage can occur during the early growing stages when they have established only their shallow roots. A study at Wagga Wagga showed that in the first year of lucerne in rotation with wheat, there was a similar amount of leakage past 1 m to that under wheat, but in its second and third years, when the deep root system became established, leakage was dramatically reduced.
2. Perennials. If a plant does not have leaves that are transpiring and using water when the soil is wet, it is difficult for the plant to use the water before it leaks below the root zone.
   
   Annual plants grow, flower, set seed, then die. They use water while they grow. Most annuals are shallow rooted and do not use any of the summer and early autumn rainfall. They do not need roots in the deeper soil because they are not present over the drier period of the year.
   
   In contrast, perennial vegetation grows all year round. It can send down deeper roots to use extra water from deeper in the soil. While perennials may shut down during dry periods, they can respond to occasional rainfall opportunities in the dry season. So overall, they have the chance to use more water than annuals.
   
   Eucalypts in the lower rainfall areas have adapted to this cycle very well, although perennial crops such as lucerne and phalaris have the potential to perform in similar ways in lower rainfall areas. However, it is extremely difficult for even perennial pastures to reduce leakage in winter dominated rainfall areas that have an annual rainfall of more than 600 mm.

Soils and geology

Soils can affect the amount of leakage in three main ways.

1. Water holding capacity. This is the amount of water the soil can hold in between wet and dry conditions. For example, because sandy or rocky soils have a low water holding capacity, we would expect greater amounts of leakage in these types of soil than in heavier clay or loam soils, all other things being equal. While clay soils can store large amounts of water, comparatively little of it may be available to plants because the clay holds the water more tightly than the plants can extract it.
2. Soil permeability. The amount of leakage depends on how easily water can move through the soils and sub-soils. For example, some subsurface clays may be less permeable and reduce the rate that water can leak into the groundwater systems. Under these circumstances more water may move laterally rather than leak vertically into groundwater systems.
3. Impeding plant growth. The physical or chemical characteristics of a particular soil may impede plant growth, thus restricting the depth of plant roots and hence the opportunity for plants to use water before it becomes leakage. Physical factors include soil density, or the presence of hard soil layers. Chemical factors include soil acidity, salinity or nutrient changes.

Clearly, estimating the amount of leakage under different climates, soils and land management systems is important in designing farming practice strategies to control dryland salinity. One difficulty in devising simple ways to determine the precise value of leakage is that it is highly variable from season to season and can vary over small distances because of the factors discussed above. In addition, leakage depends strongly on climate and Australia has large variations in climate, further complicating the measurement and estimation of leakage into our landscape.

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