In short, it appears to be quite important that water supplies be modelled at the river basin level, as opposed to the national scale, as might normally be done in a global CGE model. Since ‘unassisted’ water naturally flows downhill, a ‘flow routing model’ is required to understand how different users are spatially connected with supplies. However, it is common in economic models to use the ‘tank modelling’ approach in which routing is ignored (Bell et al.