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OU0381 from irrigation canals and excess application of water to irrigated crops. A rise in the water table of more than 90 feet has been measured in the Tri-County service area in the Middle Platte River Basin. This area of ground water rise is shown in Figure 7. Other Problems. Flooding, degradation of drainage channels, bed- load sediments, and di lap i dated or obsolete canal structures are other problems which reduce the efficiency of irrigation systems. Floods interrupt water deliveries by destroying diversion works, and by over- topping canals causing breaks or fil I ing them with sediment. This problem occurs in all parts of the State and these flood damages by watershed areas are included in Chapter 7. Degradation of drainage channels, a serious problem in the North Platte Valley, causes under- mining and destruction of crossing structures. These damages are 1ncluded in the gully and streambank erosion sections of Chapter 7. Most irrigation projects constructed 50 or more years ago require a heavy schedule of maintenance to repair di lapldated and obsolete struc- tures. Many of the smaller systems are faced with continuous emergency maintenance problems because funds are not available at anyone time to do a complete renovation. This problem is expected to increase on the sma I I systems operating without a permanent maintenance force. Ground Water Systems The most serious problem affecting pump irrigators is decl ining water tables. This condition is significant in four main areas contain- ing over a mi Ilion acres of land irrigated from ground water. These areas are shown in Figure 7. In each area the lowering of the water table is ascribed to pump irrigators withdrawing ground water faster than it is being recharged (Appendix B, Inventory of Water Resources). The largest area affecting well over one-half mi I lion acres of irrigated land is located in portions of the Big and Little Blue River Basins. It covers parts of Polk, Butler, Hal I, Hamilton, York, Seward, Adams, Clay, Fillmore, and Saline Counties. The declines from assumed normals are generally less than ten feet, but in over one-fourth of the area it is greater. The largest decline is 20 feet. Presently less than half of the land suitable for irrigation in this area is being irrigated. Additional irrigation pump installation wi I I increase the rate of water table decline. About 150,000 acres along the north side of the Platte Val ley in Dawson, Buffalo and Hall Counties In the Middle Platte River Basin hav. experienced a declining water table. The amount of decline from assumed normals is generally ten feet or less. There are two areas of declining water tables In the Niobrara River Basin. The Alliance area in the upper portion of the Basin has the most severe problem with several locations showing declines of 30 feet or more. The area contains over 40,000 acres of Irrigated land. A small area In the vicinity of O'Neil I has declines of considerable variation, but generally less than 10 feet. However, the installation of pumps in this area has been relatively recent. r 39