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SUPPLEMENTAL WATER SUPPLY In Nebraska both surface water and groundwater are potential sources of supply for supplemental water. Total outflow from the state's streams averages from about 5 million acre-feet in dry years to more than 15 million acre-feet in wet years. The fact that close to half of this flow originates in the generally lower elevation eastern third of the state and that areas with higher economic potential generally occur in upstream areas to the west presents definite economic, technologic, and environmental constraints to its use. However, some of this supply could be stored and/or diverted for use at other times and in other places. Groundwater reservoirs which are recharged by losing reaches of streams represent a unique source of surface water that has become groundwater. Even with a projection of 200 years of maximum development of groundwater for irrigation on overlying land, there would be groundwater in portions of cen- tral and northwest Nebraska available for supplemen- tal supplies. Additionally, the mounding of groundwater south of the Platte River in central Nebraska is another source for such supply. In addition to a supply, most supplemental water development requires that the water be transferred over or under the ground. In Nebraska there still is limited potential for gravity transfer systems. Because much of the remaining source of supply is at the lower eleva- tions in the eastern portion of the state, many sup- plemental water developments in the future will require pumping and lifts. These methods of transfer may in- troduce many technological, economic, and en- vironmental constraints. There are potential sites across the state for the storage of water on the surface. Over 200 sites which would have a normal capacity of 1,000 acre-feet or more have been studied previously by others. The facilities which might be built on these sites could not only directly supply water, but in many cases have the potential to recharge the groundwater reservoir. In some cases they could also serve such uses as flood control, power generation, or fish, wildlife and recreation. Underground storage of water in many areas of the state represents an important potential for the future development of supplemental water systems. These areas include locations where there have been water- level declines in the groundwater reservoir and where there are layers of unsaturated materials which could contain and release \AJater. IMPACTS AND CONSTRAINTS When an impact from a supplemental water develop- ment is proven, or is perceived to be a hindrance to development, then the impact is considered a con- straint. Impacts and constraints can result from a varie- XII ty of environmental, technological, social, legal, institu- tional, or economic factors. The results of this study seem to indicate that often there are net environmental impacts which would be considered negative in the area where groundwater is withdrawn for use as a supplemental water supply. By contrast, the net environmental impacts would usually be positive in areas where the groundwater reservoir might be used for storage purposes. Both surface storage and transfer of water could face environ- mental constraints or have positive environmental im- pacts, depending on the site and management of the development. The debate over which effect might oc- cur is most often a result of the evaluator's perspec- tive and of a lack of environmental data for a specific instance or site. Most technological factors which are obstacles to the development of supplemental water supply systems can be overcome. However, overcoming the technological obstacles may be very costly - beyond the benefits of the development. Social constraints to supplemental water develop- ment are many, varied and closely dependent on the type of development and its specific location(s). The attitudes of people become the major social constraint. Their attitude may be formed in reaction to social im- pacts on land use, population or the local economy. These attitudes also may be in relation to how tax dollars are spent and who receives benefits. Or, their attitudes may be related to traditions and mores. In any event, the expression of peoples' attitudes in a democratic society through social pressures, the political process and the legal system can present a constraint that cannot be ignored in developing supplemental water supplies. The two aspects of Nebraska surface and ground- water law most constraining supplemental water-supply development are: (1) the preference for junior natural- flow appropriators over senior storage appropriators during the irrigation season, and (2) the absence of clear agricultural groundwater-transfer authorities. The major federal institutional constraints to supplemental water project development are en- vironmental and financial. One major federal environ- ment constraint is that many proposed projects could cause environmental problems and thus may be politically challenged because of material contained in the environmental impact statement or legally challeng- ed because some parties may not believe the statement is adequate. Another instance where institutional pro- tection of the environment may constrain project development in Nebraska is the Platte Whooping Crane Critical Habitat designation by the Federal Government. A major state environmental constraint is the require- ment that state actions not jeopardize the continued existence of threatened or endangered species or their critical habitat. The major financial constraints seem to be (1) the availability of federal water resources