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C],l.iG Although the South Plalle River and North Plane River drainage baslOS are about Ille same size, the amount of water entering the state vIa the North Plane RIVer or canals at the Nebraska-Wyoming Slate line averages about 1.5 million acre-feet annually while only slightly more than 300,000 acre-feet annually comes into the state through the Soulh Platte River at the Nebraska-Colorado state line. Nebraska benefits in- directly from water Imported IOta the South Platte River basin from the Colorado River and the North Platte River basins. In recent years such Importations have amounted to about 300,000 acre-feet annually. Although this amount approximates the amount enter- ing Nebraska. most of the Imported water is used con- sumptively in Colorado. Natural flows, Irrigation return flow and other sources provIde the water that enters Nebraska. Natural evapotranspiration losses are large in the South Platte River drainage baSin In Colorado where consumptive use ttlrough IrrigaliOn is relatively high. Of the remaining 2 million acre-feet of stream flow entering the state, about tOO,OOO acre-feet enters the state from other streams Originating in Kansas, South Dakota. Colorado, and Wyoming. The flow IOta the state from the Republican River and the Niobrara River and their tributaries constitutes most of the remainder (Ben- tall and Shaffer. 1979). Flows are further discussed in Chapter 3 and AppendIX A of this report. Water in streams can be considered supplemental water especially in the losing reaches of streams. Most streams in Nebraska are groundwater drains, not sources of groundwater recharge, and overland runoff is a major component of flow. Public water supply and Irrigation wells do tap groundwater reservoirs adjacent to the few losing reaches 01 streams. In most of these cases, the streams do not provide Significantly different amounts of annual recharge under conditions of con. tinuous flow or interrupted flow. Some streams of small annual discharge on the semi.arld plains of western Nebraska are probably exceptions to thiS generality. Where subirngated lands are adjacent to streams, the water-level under these lands ordinarily is maintained by groundwater moving toward or parellelto the stream and not be seepage away Irom It. On a statewide baSIS, then, the seepage from streams as a source of ground. water reCharge or supplemental supply is qUite small and prObably cannot be increased by any great signifi. cant amount. The water stored In or the water added to the state's groundwater reservoir IS the largest polentlal source of water available for supplemental use in the state. The supply is estimated to be more than 2 billion acre-feet of good quality water (Engberg and Spalding, 1978) at relatively shallow depths, This is discussed further in Chaplers 2 and 3 of this report and in the Policy Issue Study on Groundwater Reservoir Management (Nebraska Natural Resources Commission, 1982). like preCipitatIon, streamflow and stored surface water, groundwater IS not evenly distributed within the stale or within any region. Geologic conditJons have naturally limited the occurrence and availability of groundwater around the northern and southern margins of the slate and in the glaciated eastern part of the state, The amount of groundwater stored in different areas of the state has been changed because of water use. These changes, which are the results of groundwater Withdrawal by wells and surface-water storage and distribution, have been documented by observation- well moOltoring in a cooperative program of the U.S. Geological Survey and the ConservatIon and Survey DIVision, IANA. UN-l since 1930. Many other federal, state, and local agencies have contnbuted data and resources to the program. Reports containing maps depicllng progressive changes in water levels have been prepared since 1963 and are now published an. nually (Johnson and Pederson, 1982). Although changed in geographical distribution, the total amount 01 groundwater in storage withlO the state has changed little since Ihe first settlers arrived. Awareness 01 Nebraska's large store of the available groundwater developed slowly. Awareness of the op- portunity to use the storage capacity of the underground and 10 integrate that storage with surface- water storage for various uses will await general recognrtion by the pubhc. E. USES OF SUPPLEMENTAL WATER SUPPLIES Irrigation has been and will continue to be the largest user of water in the state. The vagary of the climate was recognized 10 accounts of the early explorers and selllers and has been documented by more than 100 years of weather records. This condition requires sup- plemental water lor sustained crop production in most ollhe state. Groundwater is the largest source of water used for irrigation in Nebraska. Although, as defined / ,-5