these in turn provide little useful guidance for management because critical parts of the system have been ignored. For example, the traditional subdivision of water resource issues into those of quality and quantity is now seen as inadequate to structure future research, given that water quality and quantity are intimately, causally, and mechanistically connected. Similarly, theoretical studies of water flows (hydrology) and aquatic ecosystems (limnology) can no longer be viewed as independent subjects, as each materially affects the other in myriad ways. Finally, the physical, chemical, and biological aspects of water cannot adequately be investigated without reference to the human imprint on all facets of the earth’s surface. Thus, the challenge in identifying water resources research needs is to engage researchers in novel collaborations and novel ways of perceiving the research topics that they have traditionally investigated.

Water resources research priorities were recently extensively considered by the Water Science and Technology Board (WSTB) in Envisioning the Agenda for Water Resources Research in the Twenty-first Century (NRC, 2001a). This resulted in a detailed, comprehensive list of research needs, grouped into three categories (Table 3-1); the reader is referred to NRC (2001a) for a detailed description of each research need. The category of water availability emphasizes the interrelated nature of water quantity and water quality problems and it recognizes the increasing pressures on water supply to provide for both human and ecosystem needs. The category of water use includes not only research questions about managing human consumptive and nonconsumptive use of water, but also about the use of water by aquatic ecosystems and endangered or threatened species. The third category, water institutions, emphasizes the need for research into the economic, social, and institutional forces that shape both the availability and use of water.

After review and reconsideration, the committee concluded that the priorities enumerated in the Envisioning report constitute the most comprehensive and current best statement of water resources research needs. Moreover, successful pursuit of that research agenda could provide answers to the central questions posed in Chapter 1. However, the list of research topics is not ranked, either within the three general categories or as a complete set of 43. An absolute ranking would be difficult to achieve, as all are important parts of a national water resources research agenda. Furthermore, the list of research priorities can be expected to change over time, reflecting both changes in the generators of such lists and in the conditions to which they are responding. This chapter, thus, provides a mechanism for reviewing, updating, and prioritizing research areas in this and subsequent lists. It should be noted that the 43 research areas in Table 3-1 are of varying complexity and breadth. In addition, the committee expanded research area #21 (develop more efficient water use) from the version found in the Envisioning report to include all sectors rather than just the agricultural sector.

The increasing urgency of water-related issues has stimulated a number of scientific societies and governmental entities, in addition to the WSTB, to produce



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