WSP06696 - Laserfiche WebLink

Home Browse Search

the depletion of streams caused by holding water on the land by land treatment including small farm and watershed ponds. Some studies suggest that the deple- tion of stream flow caused by land treatment may be significant (Edgar and Sunada, 1977, and Leonard and Huntoon, 1974). Thus, it is desirable and probably necessary to quantify that depletion in order to forecast future water supply and need. Although the methods of supplementing water storage in the soil zone are recognized as an im- portant element of balancing or integrating water from all sources, the subject is beyond the scope of this study. Rather, this study, in considering agricultural use, will be concerned with supplementing the natural or conservation-augmented supply as outlined above with water from either surface or underground sources. Three methods of reducing evapotranspiration losses and thereby indirectly increasing the water supply are (1) substitution of crops requiring less water for crops with higher water demand, (2) scheduling of irrigation to meet crop needs, and (3) lowering the water table by pumping from wells in shallow water-table areas, thereby salvaging water which would have been lost to the atmosphere through the processes of evapotranspiration. All three of these methods are to be considered in the water-use efficiency policy issue study (Nebraska Natural Resources Commission, 1984). Evapotranspiration (ET) salvage in shallow water-table areas may be an important element of water management in Nebraska. Evidence suggests that ET salvage may already be an effective though unplann- ed management practice in the state. The lowering of water levels in shallow water-table areas by deep- rooted plants or by wells reduces evaporation losses from soils wetted by capillary water and by plant transpiration. Space is also provided for recharge from some precipitation events. Both conditions of ET salvage are believed to prevail in large portions of the Platte River basin. Too little research has been done and too little data collected to understand the ET component of the hydrologic cycle as it relates to water management in shallow water-table areas of river valleys and portions of the Nebraska Sandhills. In theory and from evidence based on Platte River flows, it is probable that the valley lands and vegetation have historically evaporated and transpired large volumes of stream flow and ground- water (Bentall, 1982). The same evidence and theory also suggest that ET salvage accomplished through cropping and intensive groundwater pumping primari- ly for irrigation has effectively replaced natural withdrawals. Although the consumptive use of water for irrigation is substantial, the net effect since the 1940s appears to be a substitution for some of the losses previously attributable to ET, some lowering of the water table and an increased (although apparently stabilized) annual depletion of stream flow. The flood plain, wet prairies and marshes have largely given away to agriculture in the Platte River drainage basin and this 1-2 has occurred in part due to lowering of the water table by crops and large-yield wells. Any water-management strategy proposing to provide supplemental water supplies will need to consider ET salvage in shallow water-table areas. However, a more detailed considera- tion of ET salvage is beyond the scope of this study. Other potential methods of providing additional water not included in detail in this study are weather modifica- tion and reduction of losses from water surfaces. Modification of weather to increase precipitation has been attempted in the plains states but with unproven success. Also, attempts to reduce evaporation losses from lakes and reservoirs by spreading a suppressant film on the water surface have generally had little success. Only limited experimentation with either method has been done in Nebraska. Effort has been made in Nebraska to provide sup- plemental water supplies as defined in this study. To date, much of the public effort has been directed to the building of large-size reservoirs (those having a normal storage of 50,000 acre-feet or more). In Nebraska these reservoirs have a combined storage capability of about 3 million acre-feet. Their primary purpose is for irriga- tion. In addition, much of the 3 million acre-feet of water stored in five reservoirs in the Wyoming part of the North Platte River basin becomes available for use in Nebraska. The water stored in the Wyoming reservoirs was also originally intended primarily for irrigation, although power generation has been added as another important use. In Nebraska, hydroelectric generation, cooling, flood control, groundwater recharge, recrea- tion and streamflow maintenance are either planned, unplanned, or added uses of water from these large existing reservoirs. In addition to the large reservoirs built primarily for storing irrigation water, a number of medium-size reser- voirs of 1,000 to 50,000 acre-feet of storage have been built for flood control, regulating power production and