Hence, a differentiation at zone level will not adequately internalise the environmental impact, and therefore charges should be differentiated to reflect this variation. Reflecting such changes in the value of w'ater (environmental or opportunity costs of using w'ater) can drive private investment in water storage. An example from the UK (see Annex 3.A2) provides some guidance on how to incorporate such variations in the value of water. In practice, most OECD countries consider that seasonal variations in abstraction charges add a lot of complexities and seldom use them.