Climate change and water resources in the Murray Darling Basin ...

ABARE CONFERENCE PAPER 02.11
the analysis presented here, 78 individual land management units are defined according
to the characteristics of the ground water system — that is, they are classified according to
whether they are local, intermediate or regional flow systems.
Within each land management unit, economic models that optimise land use are integrated
with a representation of hydrological processes in each catchment. The hydrological component
incorporates the relationships between irrigation, rainfall, evapotranspiration and
surface water runoff, the effect of land use change on ground water recharge and discharge
rates, and the processes governing salt accumulation in streams and soil. The interactions
between precipitation, vegetation cover, surface water flows, ground water processes and
agricultural production are modeled at a river reach scale. In turn, these reaches are linked
through a network of surface and ground water flows.
In the agroeconomic component of the model, land is allocated to maximise economic
return from the combined use of agricultural land and irrigation water. Each land management
unit is managed independently to maximise returns given the level of salinity of available
land and surface and ground water resources, subject to any land use constraints.
Incorporated in this component is the relationship between salinity and yield loss for each
agricultural activity. Thus, land use can shift with changes in the availability and quality of
both land and water resources. The cost of salinity is measured as the reduction in economic
returns from agricultural activities from those that are currently earned. Some key features
of the model are described briefly below. A full description is given in Bell and Heaney
(2001).
Hydrological component
The hydrological component of the model consists of three parts. The first determines the
distribution of precipitation and irrigation water between evaporation and transpiration,
surface water runoff and ground water recharge. Within this component of the model there
are two climatic drivers that are specified uniquely for each hydrologically defined land
management unit — average annual rainfall and evapotranspiration — specified as a function
of rainfall and land cover.
In the initial specification of the SALSA model, the Holmes–Sinclair relationship was
used to specify the link between ground cover and evapotranspiration. For a given ground
cover, the Holmes–Sinclair relationship (estimated by Zhang et al. 1999) relates evapotranspiration
to precipitation. The relationship does not include variation in potential evaporation,
as it was not found to be a significant discriminator. However, in the climate change
scenarios that were evaluated, projected changes in potential evaporation were large in
comparison with projected changes in precipitation using the Holmes–Sinclair relationship.
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