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supplemental water-supply alternatives has been used in evaluating costs and benefits of state and federal grants for water-development purposes. This method also provides a means to evaluate competing water projects: those with the highest net benefits are those best meeting supplemental water supply "needs." This leads to the issue of how supplemental water- supply projects are financed. Generally, direct project beneficiaries (e.g., irrigators purchasing project water) cannot afford or are unwilling to bear total the project costs. Therefore, direct state and federal grants are made for water-development purposes on the theory that the general public (federal, state or regional) in- directly benefits economically and in other ways from these projects. These indirect benefits have been dif- ficult to quantify, however, leading to controversy re- garding whether indirect benefits are as great as project proponents believe they are. This issue will become more prominent when federal financial assistance for water projects is made contingent on substantial state and local financial contributions. Politically, this means that state and local taxpayers will have to be persuaded that the indirect benefits from water projects will justify the significant public expen- ditures (and the associated tax increases) required if additional federal or state water projects are to be con- structed in Nebraska. D. AVAILABLE WATER SUPPLY Just as it is difficult to define water needs, it is equally as difficult to determine if, when, and where an available supply might exist for supplemental use. Available water might be any water in sufficient quan- tity and of satisfactory quality for a specific use that is surplus to existing uses. The most obvious type of surplus water generally considered available for sup- plemental use is flood flow. Flood flows are a primary source of water for storing in most existing surface- 1-4 water reservoirs. A dam on any stream or drainage could store future flood flows. Most surface-water reser- voirs also store base flow of streams, and that is the main source for a few reservoirs. Water is available in streams as both flood and base flow that could be stored and/or transported. Water is available also from the groundwater reservoir for supplemental use. 80th types of available water are identified in this report in Chapter 3. The same test of beneficial and adverse im- pacts as applied in judging uses must be applied in judging the availability. In addition, tests of costs and water rights apply. Obviously there is a limit on the water supply available to provide supplemental water no matter who makes the judgment. Just as precipitation is the ultimate source of all water supplies, the ultimate con- straint or development and management of supplemen- tal water may be the statewide or regional precipita- tion. Amounts of annual precipitation range between 80 and 100 million acre-feet in Nebraska. The distribu- tion of precipitation is uneven across the state and on the long-term average increases uniformly eastward across the state in amounts of about 15 inches in the extreme west to about 35 inches in the extreme southeast. State-wide the longterm average annual precipitation is about 22.8 inches, about two-thirds of which falls during the growing season. Although much of the water from precipitation is evaporated or is transpired by plants (about 90 percent in an average year), temporary storage in the soil reservoir in the state can range between 8 and 25 million acre-feet. Some precipitation moves to streams and is diverted, sfored, evaporated, or leaves the state. The amount of water leaving the state annually ranges widely from below 5 million acre-feet in extremely dry years to more than 15 million acre-feet in extremely wet years. The average is about 7 to 8 million acre-feet. Part of the Nebraska supply originates as precipitation falling in adjacent regions to the northwest, west and southwest. In this study, the availability of water for supplemental uses was considered to start atter precipitation found its way into surface flows or the groundwater reservoir. The flow of the Missouri River and storage and releases from the main-stem reservoirs in Montana, North Dakota, and South Dakota constitute a major potential water supply. The combined storage capaci- ty of the six reservoirs is huge, more than 70 million acre-feet. Water moving into the state as streamflow as a result of precipitation in South Dakota, Wyoming, Colorado, and Kansas also constitutes a potential sup- plemental supply. Nearly 2 million acre-feet of streamflow originates primarily as snow melt in Wyom- ing and Colorado and enters the state through the North and South Platte rivers. However, considered water comes across the state line as storage releases from out-of-state reservoirs which were built for project use by irrigation or irrigation and power districts. Additional water enters the state as return flow from water use in the upstream states.