So if the contribution of water to the regional economy is USD 1 billion, and there is a 10% reduction in its availability, then a first-order guess at the welfare cost would be 0.10*USD lbill. This is just a first-order estimate, since any reduction in available water will affect the marginal value product of water. It is also the kind of estimate which can be generated without resource to an economy-wide model. Of course, such a shortage will also interact with other features of the economy - hence the need for economic modelling in the context of the CIRCLE project.