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, , Ii '" " recreation. Some also supply water for irrigation and most are used for fishing and provide a habitat for wildlife. Many of these reservoirs were constructed by the U.S_ Army Corps of Engmeers in the Salt and Papillion Creek drainage casins. Other medium.size reservoirs have been bUilt by the U.S. Soil Conserva- tion Service and the natural resources districts Thousands of small.size reservoirs of less Ihan 1,000 acre-feet of storage have been constructed as farm ponds. grade-stabilization structures and Irrigation reuse pits. A few smail-size reservOirs have been con. structed lor mUnicipal use. Although all of Ihese small. size and medium-size reservOirs provide benefIts In. eluding supplemental water supply. they also deplete annual streamflow because they lose water by seepage and evaporation, However, the losses by seepage recharge the groundwater reservoir. Many other potential surface sites for storage exist in the state. Those which have been studied to date are identified In Chapter 3 of this report. Although sur. face sites are numerous, perhaps the greatest poten. tial for water storage is underground. Storage in the groundwater reservoir to provide supplemental water probably is of greater Importance In Nebraska than in most other states. Because of unique geologic condi. tions that led to an accumulation of thick deposits of sill. sand. and gravel. much of Nebraska is underlain with a vast underground reserVOir. Water stored in groundwater reservOirs or space above such reservoirs can provide storage for later use. Water thus stored is less vulnerable to loss from evaporation than in water stored in surface.water reservoirs. The concept of reservoir operation. both surface and underground. is that of storing during periods of wet years and large overland runoff and of use through drawdown during peak demands andlor dry years. The opportunity and challenge are to integrate and manage water in bolh surface and groundwater reservoirs. That water can be stored in groundwater reservoirs has been established in Nebraska and elsewhere, Ex. perimental recharge through wells has been attemp.- ted by the City of Uncoln and in Hamilton County. One reservoir has been constructed in the Little Blue Natural Resources District with groundwater reCharge as a main benefit. Others have been proposed. The most dramatic example of potential lor underground storage is the groundwater mound Ihat has developed south of the Platte River from near Sutherland to Minden. In. advertent seepage losses from the Nebraska Public Power District's Sutherland Reservoir and canal system through Lake Maloney. coupled with that from canals. reservoirs and Irrigated lands of the Central Nebraska Public Power and Irrigation Districts proJect. account lor an estimated increase in groundwater slorage of 10 million acre.feel. Most other surface.water projects also have con. ttlbuted to groundwater Storage. One area in particular provides an example of the prinCiple of conjunctive use, i.e., integrated use of surface water and groundwater. Irrigation in Dawson County had an early (1890s) reliance on water diverted Irom a limited Plalte River supply. Irrigators there progressed to using water from private wells constructed since Ihe 1930s which are supplemented with water stored in Lake McConaughy through rights that were acquired. The net effect has been to balance water supplied from streamttows and storage releases WIth intensive groundwater withdrawal by wells and by crops through subirrigation. Water levels have been maintained by the distribution of the surlace water. Drainage problems have been minimiz- ed by groundwater use. C. "OPPORTUNITY" AND "NEED" FOR SUPPLEMENTAL USE Supplemental water cannot be viewed without con- sideration of the available water supply and oppor- tunIties for use. For purposes of this report "opportuni- ty" is defined as a situation where a party would be likely 10 utilize water not currently available at a par- ticular location. Whether it is wise to take advanlage 01 that opportunity. in terms of cost or other factors is a marter 10 be evaluated in connection with the Chapter 4 discussion of constraints and impacts. Need refers to the degree of good that water can do for a party or group if supplied. Water that is judged available and needed by one group may not be judged in a like fashion by another. Competition for an available water supply may result. Water needs come about because someone or some group perceives a water Shortage for one or more particular uses in a specific area. Thus, need may be identified. The need may range by degree from that which is wanted or desired 10 that which is a requisite. Some needs may continue to exist indefinitely necessitating adjustment to an existing supply. Another need might await its turn. Implementation 01 an action 10 provide a supplemen- tal supply might be delayed until one or more condi- tions change or are changed and favorable cir- cumstances permit the action to lake place. In other cases a need, identified as a water quantity or quality problem, might not be sufficiently serious to warrant action until some tIme in the future as conditions worsen. Another factor in deciding whether to fulfill a need is evaluation of the constraints and impacts identified in Chapter 4. Economic (or financial) evaluation com. pares the economic cost of fulfilling a need to the economic benefil It provides. Cost of developing the supplemental supply plus the costs reflecting the adverse Impacts can be compared to the willingness of project beneficiaries to pay for the supplemental water. ThiS willingness to pay presumably reflects the economic benefits conferred by the project. (Payment In this context includes direct water-use charges and Indltect charges through increased property taxes in the project area.) This financial approach to evaluating '-3