Vast grasslands and fertile soils drew Native Americans to hunt and farm in the area that became the State of Nebraska. These same qualities appealed to settlers moving west during the 19th century in search of affordable land. Cattle ranchers found room to sustain their herds in the Sandhills, where grass roots have all but stopped the progression of sand dunes. The development of surface- and ground-water irrigation led to dependable harvests of corn, alfalfa, soybeans, and sorghum. Today the region is still an important agricultural production center for the Nation.
The Central Nebraska Basins Study Unit includes about 30,000 mi2 (square miles) of the Platte River Basin from the confluence of the North and South Platte Rivers near North Platte, Nebraska, to its mouth near Omaha, Nebraska [1]. The environmental settings of the Central Nebraska Basins are characterized by differences in soils, precipitation, and land use and are divided into the Sandhills, Loess Hills, Glaciated Area, and Platte Valley subunits.
In the northwest part of the Study Unit, the Sandhills subunit has sandy soils, is semiarid, and is used as rangeland. Nearly all the precipitation in the Sandhills subunit infiltrates directly to ground water with very little runoff. Progressing eastward, precipitation increases, soil textures are finer, and runoff increases. The Loess Hills subunit, in central Nebraska, is characterized by a mixture of rangeland and cropland, whereas the Glaciated Area subunit, in the eastern part of the Study Unit, is nearly all cropland. Along the southern part of the Study Unit is the Platte Valley subunit, where sandy soils and a shallow water table provide favorable conditions for crop production.
Permeable soils of the Sandhills result in stable streamflow sustained by ground-water discharge. |
Greater rainfall and relief, less permeable soils, and no regulation result in highly variable streamflow. |
The Platte River is regulated by a series of upstream dams producing similar patterns of streamflow year after year. |
Hydrologic conditions affect water quality because the processes by which many chemicals enter water bodies are functions of precipitation running off the land surface and entering streams or aquifers. For example, herbicides commonly are applied to row-cropped areas in the spring. If this application is followed by intense rainfall, herbicides can be transported overland to streams [2] or downward to ground water. Therefore, any description of current water-quality conditions should consider present and typical hydrologic conditions. In the accompanying figures, the blue-shaded area represents the range of streamflows that occurred 25 to 75 percent of the time from 1954 to 1991 (a period of data available for all sites shown).
Generally, streamflows throughout the Study Unit were greater than normal during 1992 to 1995. This was particularly noticeable in the eastern part of Nebraska where streamflow has a strong seasonal pattern. In contrast, the Dismal River, in the Nebraska Sandhills, had a fairly uniform streamflow and was slightly above the normal range from 1992 to 1995. Streamflow in the Platte River is regulated by releases from dams outside of the Study Unit and by diversions for irrigation.
Streamflow in the eastern part of the Study Unit was much greater than normal during this study. |
The sharp declines each year are the result of irrigation withdrawals. Wet conditions have recently increased water levels. |
Runoff from the Central Nebraska Basins generally is unregulated by dams and thus shows substantial variability. |
Comparisons of streamflow between the Platte River at Brady and the Platte River near Louisville show the effect of precipitation and streamflow regulation in the Central Nebraska Basins. Because flow and runoff were greater than normal for streams sampled during the course of this study, constituent loads associated with nonpoint sources, such as nutrients and pesticides, might be expected to be larger than would be found at normal flows.
Ground-water withdrawals for irrigation result in substantial declines in ground-water levels during each year. Ground water may be susceptible to contamination from nonpoint sources where unconfined aquifers are shallow and overlying soil texture is coarse, such as in the Platte Valley. Generally, ground-water levels in the Platte Valley declined from the 1950s to the early 1980s. The larger than normal precipitation during 1992 to 1995 increased water levels, and increased nutrient and pesticide concentrations could be expected.
