There are a number of benefits that can come from planting trees. Some of these relate specifically to irrigation. Others are more general, but apply to irrigation areas as much as they do to dryland areas.
Reducing and intercepting groundwater recharge
While all irrigated soils are subject to recharge (i.e. water filtering through the soil profile to add to groundwater), specific sites have potentially much greater recharge capabilities. These areas include:
Distinctive landforms
These landforms can be recognised on a regional or farm level and are identified below:
(i) Prior stream formations - soils of these formations are usually of a high permeability with a high recharge potential. They usually have species such as yellow box (
Eucalyptus melliodora) and Murray pine (
Callitris columellaris) growing on them.
(ii) Ancestral rivers - these are localised depressions showing the course of ancient rivers. Because they are lower than the surrounding country and generally consist of coarse sediment they can be significant sites in terms of groundwater accessions.
(iii) Poorly drained areas and depressions - the size of these may vary greatly. They are especially significant to small farms and may be either localised recharge or discharge sites.
(iv) Specific high water table areas - areas where watertables are approaching the soil surface. Any watertable within 2 m of the soil surface is considered serious.
In these areas trees and associated natural vegetation often die when high watertables are present.
Man-made structures
Recharge is even more likely when irrigation structures such as channels or drains are associated with some of the landforms outlined above. This is especially the case when structures have been built on or adjacent to hill slopes, on the lighter textured soils of prior streams or over the courses of ancestral rivers.
It has been estimated that 5% of all water diverted for irrigation purposes in NSW is lost as seepage. It is also estimated that 25% of all groundwater accessions can be attributed to seepage from irrigation structures
Another problem arising from both district and farm irrigation structures can be the obstruction of natural flows of water in an area. This can lead to areas that were previously quite well-drained becoming waterlogged, eventually leading to shallow groundwater levels. The problem also extends to roads and other man-made structures.
All of these areas, both natural and man-made, provide potential sites for the use of trees to intercept underground flows of water.
Targeting specific channel seepage or poorly drained sites for tree planting will have greater benefits in minimising groundwater recharge than planting for general groundwater control.
Lowering of watertables
Tree plantations established in the right place can act as groundwater pumps, lowering the watertable directly below them and in the surrounding area. The size of the area that they draw water from is largely determined by the permeability of the soil .
Re-using drainage water
Instead of letting excess water leave the farm as drainage or by seeping through the soil profile to the watertable, it can be used to irrigate a tree plantation grown for timber or fodder production. Trees can use substantial amounts of water, up to 300 litres/tree/day.
This does not necessarily mean that 300 litres of water should be applied to every tree each day. The actual amount of water that each tree uses depends on climatic conditions, their stage of growth and their species.
If drainage outfalls cannot handle extreme cases of excess drainage (e.g. extended periods of heavy rainfall, or a thunderstorm immediately after watering), it may be possible to divert this water to a tree plantation as a temporary measure.
This at least provides an opportunity for the trees, with deeper roots, to use more of the water and may help to reduce groundwater accessions. If this is to work, on-farm surface drainage has to be well-planned and flexible.
Using drainage water to irrigate tree plantings is becoming common practice in the Sunraysia area of north-western Victoria.
Rehabilitating saline ground
When areas are already badly degraded, trees and other vegetation can be used in conjunction with other techniques to slowly return them to productive use, although probably not to the previous enterprise.
Examples of these techniques are surface and sub-surface drainage (including groundwater pumping) and fencing off affected areas to stop further damage.
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Providing shade and shelter
By using windbreaks designed to reduce stock, pasture and crop stress, productivity can be improved. Protecting animals and plants from the weather lets them devote more of their energy to production rather than to just staying alive. The more intensive the enterprise the more noticeable this is.
To achieve these gains, a minimum of 5% of the farm area should be under strategically designed windbreaks (Kent, 1984).
Producing extra fodder
Tree fodder crops can be grown that will provide nutritional benefit to livestock. Examples of this are saltbush, tree lucerne, allocasuarina, the casuarinas and some species of acacias.
Improving the environment
By planting local species, some of the natural character of the landscape is retained. This helps to maintain the level of diversity of wildlife in an area. One of the benefits that may result from doing this is the preservation of viable populations of insect-eating birds to protect crops and pastures.
Natural vegetation areas can be more effective if they are linked, using corridors of trees of suitable width. These provide pathways for wildlife to follow.It may not always be possible to use native trees. Where watertables are saline and are close to the surface, some species will not survive.
Providing extra income
Planting fast growing species for timber production can be another source of income. Choosing species that are already known to the market and have a wide variety of uses make it easier to market the product when it is mature. Possible uses include firewood, sawn-wood, and fence posts.
Some species can also be grown for their non-wood value. Examples of non-wood products are eucalyptus and melaleuca oil, charcoal, honey and native flowers. At times when the returns from other enterprises are down, a tree crop could provide a much needed cash return. If this is not the case, the trees can be left growing until they are needed or when prices are high. The timing of the harvest can be very flexible.
To make sure that there is a regular source of income from the plantation, the trees should be planted and harvested on a rotational basis, particularly if they are being grown for wood products. This would provide a number of plantations at different stages of growth.
Increasing property and aesthetic values
Trees can improve a property's appearance, making it more attractive to a potential purchaser. Trees improve the working environment, and generally make the property more pleasant to live on.
Summary
Trees can be used in a number of ways. These range from lifting productivity by improving the quality of the environment to providing a direct cash return from the sale of tree products.
Uses of trees in irrigation areas include reducing the amount of water seeping through to the watertable and lowering already high watertables in specific localised areas.
Tree planting is not the only solution for high watertable and salinity management. To be effective, tree planting needs to be used in conjunction with improved soil, water and crop management.
