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Rural Industries Research & Development Corporation
Irrigation management and groundwater uptake in young tree plantations growing over high watertables 
Final report for the RIRDC/Land & Water Australia/FWPRDC/MDBC Joint Venture Agroforestry Program
December 2002
RIRDC Publication No 02/146 RIRDC Project No CSF-54A
A main conclusion from this study is that extensive
plantings of spotted gum (Corymbia maculata) on prior stream beds
in the Deniliquin region would:
Background
In the low rainfall regions of the southern Murray-Darling Basin (<600 mm) tree plantations may need to be irrigated to achieve commercial rates of growth for conventional forest products. Potential irrigation demand is high (> 16 ML ha -1 y -1 for furrow irrigation) and is greater than most landholders or managers would be willing to apply to tree crops. As the cost of fresh water for irrigation rises and supply becomes less certain, the volume of water applied to plantations will be sub-optimal for growth.
In the Deniliquin region a long history of irrigated agriculture has caused watertables to rise, often to within three metres of the soil surface. One mitigation strategy is to plant an appropriate area with trees in the expectation that they may use groundwater, and also need less irrigation than current agricultural crops.
Water applied in irrigation can be lost through direct evaporation, deep drainage (leaching from soil), and run-off. As the availability of water decreases, and its cost increases, developing irrigation practices to minimise loss of applied water is therefore a priority.
The objectives of efficient
irrigation practices are to:
Specific guidance would
include such information as:
This report describes results
from a three-year study of growth, water fluxes and salinity, in young,
flood-irrigated eucalypt plantations near Deniliquin, south-western NSW.
Sites
Five field trials were established on a range of sites that varied in soil type, and depth and quality of the watertable. Studies were initially conducted at‚ Norwood Park™ an irrigation rates trial in Eucalyptus grandis. Four trials at three ‚satellite sites™ were added to the project in the second year (July 1998) to extend the range of site conditions studied. A summary of sites and irrigation treatments is given in the table below. The main feature is the difference in sub-soil properties, Karawatha have deep sandy loam soils, the other sites having very clayey and saline sub-soils. The watertable at all sites was about 3 m below the surface and was relatively fresh at Karawatha, but saline to highly saline at all other sites.
Irrigation to sustain growth and
conserve water
Results showed that the most appropriate irrigation strategy will vary according to soil conditions.
On shallow clayey soils or where sub-soil properties impede root penetration, frequent irrigation in small volumes would maximise growth per megalitre of water applied while minimising losses of the water applied during flood irrigation. About 50-70% was lost to direct evaporation, deep drainage and run-off, with 30-50% remaining in soil. In these soils, only a proportion of this water is available to tree roots due the relatively high tension by which it is held. Thus, irrigation every 10-14 days in relatively small volumes of about 0.5 ML ha -1 is preferred. Over a growing season (November- March) this would add about 4-6 ML ha -1 . Irrigation systems that increase the efficiency of application could apply even less water and more frequently.
In deep, sandy loam soils conducive to root development, less frequent irrigation (e.g. 4 times y -1 ) in larger volumes (e.g. 1.5 ML ha -1 ) would be appropriate. The sandy loam texture of the soil holds water less tightly and a greater volume is stored due to the large rooting depth, making more of the water applied available to trees.
Groundwater uptake
Groundwater uptake was substantial at Karawatha (deep, sandy loams) but minimal or moderate at all other sites, and there was also an indication that C. maculata used more groundwater than E. grandis.
During the 1999-2000 growing season, groundwater use at Karawatha was 793 mm under C. maculata and 450 mm under E. grandis. These volumes represent 82% of total water used by C. maculata and 73% of that used by E. grandis.
Careful selection of sites and species, followed by appropriate irrigation strategies, are required if farm forests in this region are to achieve reasonable growth and help control rising watertables. Factors to consider for site selection are:
In the Deniliquin region, a significant challenge remains to integrate the above information into a management strategy at a regional scale. An effective and inexpensive method is needed to locate and map suitable sites in the region where trees can be grown to lower watertables and produce wood at economically viable rates of growth. Even in the absence of a commercial wood production venture, strategic reforestation of areas of a farm could make a significant contribution to sustainable production of rice.
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