For this reason, the reduction in inflows associated with climate change in the
‘adaptation only’ case is modelled partly as a change in the probability
distribution, which is changed so that only Normal and Drought states occur,
each with probability 0.5. To match the reduction in mean inflows for each
catchment, the change in probability distribution is combined with a
proportional adjustment in flows in each state.
In the ‘mitigation’ case, the probability distribution of states of nature is
assumed unchanged. The reduction in inflows, as shown in Table 1, is modelled
as an equiproportional reduction in each state of nature.
Policy responses
The model is solved to determine the allocation of land and water that yields the
maximum expected return for the Basin as a whole subject to a number of policy
constraints. Some constraints are applied in all runs.
First, the salinity of water supplied to Adelaide is constrained not to exceed 800
EC. This constraint is not feasible in the ‘adaptation only’ projection for 2100 as
there is no flow in drought states.
Second, for each catchment, there is a constraint limiting total use of water for
irrigation. This constraint reflects the existing policy regime, which has included
such restrictions since the imposition, in 1994, of a cap on aggregate water use.
In addition, we consider two water allocation rules that might be adopted in the
‘mitigation’ scenario. Under the first allocation rule, referred to as ‘environment
as residual claimant’, existing constraints on water use are left unchanged. As a
result, changes in land and water use in irrigation are fairly modest, and the
main effect of reduced inflows is to reduce the flow of water through natural
environments in the system, measured here by the outflow at the Coorong.
Under the second allocation rule, referred to as ‘environmental flows take
priority’, constraints are imposed to ensure that environmental flows, as
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