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Summary of full report
Water balance and economics of irrigated eucalypts in areas with shallow saline groundwater
by Jim Cox, Jim Morris, Zahra Paydar, Bill Loane, Neil Huth, Val Snow, Kerryn McEwan and Glen Walker.
December 2005
RIRDC Publication No 05/199 RIRDC Project No CSL-9A
However, recent findings that salts accumulate in the rootzone of plants grown over shallow saline groundwater have raised uncertainty as to the sustainability of plantations.
A hydrological modelling exercise using APSIM was undertaken to determine the conditions under which plantations in the Riverina are sustainable and economic. The ability of APSIM to predict tree growth and salt and water balances was validated against data from a number of plantations including Kyabram and Timmering. In total, 2106 scenarios were modelled, including different plantation management, three soil types, three groundwater salinities at two depths, and four irrigation management scenarios. Throughout the report, examples are selected from the scenarios to highlight the most important processes.
An economic model was used to assess the efficacy of plantations and consisted of modules in Microsoft Excel using Visual Basic for Applications. The routines were partly derived from the FARMTREE model but were heavily modified to suit the needs of this project and new data formats available. The economic model takes biophysical data outputs from APSIM such as wood biomass, watertable depth, and irrigation water volume and salinity, converts them to variables of economic relevance, and combines them with monetary value data. It considers the benefits and costs in terms of wood production, agricultural production and river salinity, and outputs the economic results in terms of annual net benefits.
Structure of the report
Summary of findings
Groundwater use by trees
depends on soil hydraulic properties, groundwater salinity and depth, irrigation
and plantation management. In general trees perform best with: lighter
soils, deeper watertables, lower groundwater salinity, higher application
of fresh water and a high-density plantation. Trees preferentially use
fresh water from both surface and groundwater sources. If groundwater is
saline, their use of surface water (rainfall and irrigation) increases.
The sensitivity of trees to groundwater salinity decreases if they use
more surface water. A general exponential form for the groundwater use
by trees is suggested that describes a sharp decrease in the groundwater
use from low (2 dS/m) to medium (5 dS/m) salinity and much less with higher
than 5 dS/m salinities.
Trees can be effective in lowering the watertable and producing forestry products for some years.
Irrigation with fresh water helps with this process as it triggers more growth and water use. However, salt accumulation in the profile over the years, causes a decrease in both water use by the trees and their subsequent capability for lowering the watertable. There seems to be a limit to the effectiveness of trees to perform as a natural sink in the shallow watertable areas, which depends on all the environmental factors (soil, climate, watertable on surrounding land, groundwater salinity) as well as management (species, irrigation, plantation management) factors. As a general guide, we would expect 6-9 effective years of growth and watertable lowering for plantations with no irrigation, increasing to 9-13 years with high rates of irrigation on light soils with good connection between shallow and deeper groundwater systems.
For the majority of the scenarios run, a negative net economic return resulted from the modelling at a 5% real discount rate. This was generally due to the high establishment costs for trees and irrigation, together with salt accumulation and poor growth with more saline water, or high costs for fresh water.
Positive net benefits were only found for a limited range of scenarios. Using irrigation with salinity at 2 or 5 dS/m, on light soils, with deep (4 m) watertables, a near optimal balance was achieved with good wood production (from MAI’s in the range 20 to 35 m3/ha) and high, river salt saving (based on diversion of salt in irrigation water from river to trees). Heavy thinning of trees results in the most economic plantations, with light thinning slightly lower, and even no-thin versions still achieving positive benefits. Trees grown under the above constraints on medium textured soils were a little less economic than trees grown on light soils, but the returns were still positive.
Annuity results ranged from
a maximum of $385/ha/yr (equivalent to an internal rate of return of 8%)
to a loss of $1100/ha/yr. Not too much significance should be placed on
the exact numbers output by the models, but rather on the magnitudes and
relative differences.
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