through these long-term withdrawals. In particular, the greatest available storage for development of managed underground storage (MUS) systems may be in unconsolidated and semiconsolidated sand and gravel aquifers. Thus, in the appropriate circumstances, managed underground storage will offer communities and regions throughout the world an opportunity to address problems of overdraft as well as another tool that can be integrated into a balanced system for managing water scarcity.

The growing scarcity of water will require that water be managed more carefully and that it be used more intensively. More intensive use implies that the productivity of water in existing uses will have to be increased and also that waters that currently are not used, or are underutilized, must be the object of more intensive exploitation. Examples include flood flows (Boxes 5-4 and 5-5), urban stormwater (the stormwater-to-drinking water project in Salisbury, Australia described in Chapter 6) and reclaimed wastewater (numerous examples throughout the report).. Similarly, water will have to be managed in an integrated unified way that acknowledges explicitly the interrelatedness of the hydrologic cycle and the interrelatedness of water and other natural resources. This represents a departure from the way water resources have been developed and managed in the United States.

Until the late to mid–twentieth century the primary means of responding to water scarcity was to build surface water storage and conveyance projects. Surface water storage ultimately fell from favor because: (1) the low-cost sites were soon all developed, leaving only opportunities that were considerably more expensive; (2) the costs of constructing civil works projects grew faster than other costs in the economy; (3) the competition for public funds became keener, making it more difficult to secure the financing necessary to construct large surface water facilities; and (4) the environmental damages and social impacts associated with the construction and operation of surface water storage facilities became fully manifest at a time when public environmental awareness was growing.

There followed a period in which the emphasis shifted away from surface storage toward programs of conservation and more intensive management of water supplies. Improved techniques for managing water on-farm were devised and disseminated. Improved technology, including closed conduit irrigation systems and water saving appliances, were developed and adopted relatively widely. The public became more aware of water and more aware of behaviors that economize on water use. Water transfers began to be accepted. Transfers included the trading of water rights and entitlements, the purchase of water in spot markets, and the development of contingent markets for water. Such transfers have the capacity to reallocate water away from relatively low-valued uses to relatively higher-valued use, thereby ensuring that the productivity of water is optimized. They have the added advantage of being voluntary so that no one is coerced into participating in a water transfer.

There has also been a returned appreciation for the importance of some form of storage as a means of capturing and holding water that is available only

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