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;~ '1 .~ ; recreation. Some also supply water lor Irrrgatlon and most are used for fish 109 and provide a habitat for wildlife. Many of these reservoirs were constructed by the U.S. Army Corps of Engmeers 10 the Salt and Papillion CreeK dramage basms. Other medium.size reservoirs have been built by the U S Soil Conserva- tion Service and the natural resources distncts. Thousands of smail-size reserVOirS of less than 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 for municipal use. Although all of lhese small- size and medium-size reservOirs prOVide benefits in- cluding Supplemental waler 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 sur1ace sites for storage exist in the state. Those which have been studied to date are identified 10 Chapter 3 of thiS repon. Although sur- face sites are numerous, perhaps Ihe greatest poten- llal for water storage is underground. Storage 10 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 silt. sand. and gravel, much of Nebraska IS underlam with a vast underground reservOir. Water srored 10 groundwater reservOirs or space above such reservoirs can provide storage for later use. Water rhus stored IS less vulnerable to loss from evaporation than in water stored in surface-water reservoirs. The concept 01 reservoir operatIon, both surface and underground, is that of stormg durmg periods of wet years and large overland runoff and of use through drawdown durmg peak demands and/or dry years. The opportunity and Challenge are to integrate and manage water in both surface and groundwater reservOirS. That water can be stored in groundwater reservOIrs has been established in Nebraska and elsewhere. Ex. perlmental reCharge through wells has been attemp- ted by the Cily of Lmcoln and in Hamilton County. One reservoir has been constructed In the Little Blue Natural Resources Distnct WIth groundwater recharge as a main benefit. Others have been proposed. The most dramatIc example of potential for underground storage is the groundwater mound that 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 PubJic Power and Irrigation Districts project, account for an estimated mcrease in groundwater storage of 10 million acre-feel. Most other surface-water prOjects also have con- tnbuted 10 groundwater storage. One area in partIcular provides an example of the pnnclple of conjunctive use, i.e.. integrated use of surface water and groundwater. Ir"gatlon in Dawson County had an early (1890s) reliance on water diverted from a limited Plalte River supply. Irrigators there progressed to using water from prtvate wells constructed slOce the 1930s which are supplemented With water stored in Lake McConaughy through rights that were acqUired. The net effect has been 10 balance water supplied from stream/lows and storage releases With intensive groundwater withdrawal by wells and by crops through subirrigation. Water levels have been maintalOed by the distribution of the surface water. Drainage problems have been mmimiz- ed by groundwater use. C. "OPPORTUNITY" AND "NEED" FOR SUPPLEMENTAL USE Supplemental waler cannot be viewed without con. slderalton of the available water supply and oppor- tunities for use. For purposes of this report "opportuni- ty" is defmed as a situation where a party would be likely to utilize water not currently available al a par- ticular location. Whether It is wise to take advantage of that OPportuOlly, 10 terms of cost or other factors is a matter to 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 waler supply may result. Water needs come about because someone or some group perceives a water shortage lor 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 to that which is a requisite. Some needs may contmue to exist indefinitely necessitating adjustment to an existmg supply. Another need might await its tum. ImpJementation 01 an action to 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 faclor in decidmg whether to fulfill a need is evaluation of the constramts and impacts identified m Chapter 4. Economic (or financial) evaluation com- pares Ihe economic cost 01 fulfilling a need to the economiC benefit it provides Cost of developing the supplemental supply plus the costs rellecting the adverse impacts can be compared to the willingness of prOject beneficiaries 10 pay lor the supplemental water. This willingness to pay presumably reflects the economic beneftts conferred by the project. (Payment in this context inCludes direct water-use charges and mdlrect charges through increased property taxes in the project area.) This financial approach to evaluating ,.3