Executive Summary While the importance of wetlands for native plants and wildlife has been recognized, the hydrology that maintains these areas is poorly understood. To address this knowledge gap the Project Team began a study in 1988 to examine the seasonal relationships between wet meadow groundwater elevations and river stage, precipitation, evapotranspiration, and adjacent irrigation. Three study sites along the Platte River between Lexington and Grand Island, Nebraska, were selected for the study. This research was initially funded by U.S. Fish and Wildlife Service, with continuation funding provided by the U.S. Bureau of Reclamation and parts of the analysis funded by the Nebraska Game and Parks Commission.
To accomplish our analysis we: 1) Described the continuous hydrologic and soil temperatures; 2) Developed box-plot groundwater and river-stage hydrographs; 3) Developed depth-to-groundwater and river stage duration curves; 4) Determined the relationship between river stage and river flow; 5) Presented cross-valley groundwater-level transects; 6) Developed groundwater-level contours for each site; 7) Developed depth-to-groundwater maps for a representative area at each site; 8) Described the apparent effective rooting depth for plants affecting the water table level; 9) Examined the effect of adjacent groundwater withdrawal for irrigation; and 10) Separated the influence of river stage, precipitation, and evapotranspiration on wet-meadow groundwater levels.
Continuous hydrologic and soil temperatures were plotted by water year to show the 'real-time' relationships. Box-plot hydrographs and depth-to-groundwater duration curves were generated from the continuous daily mean groundwater depths and river stage. The box-plot hydrographs summarize the variation within and between months, while the duration curves are cumulative frequency distributions that show the percent of time a particular depth or stage was equaled or above that level for the period specified. Linear regression was used to determine the relationship between the river stage at a site and the river flow at an adjacent USGS gaging station. Groundwater levels for dates selected to represent the lowest and highest periodic measurements, and a median level for spring and summer, were used for the cross-valley transects, groundwater-level contours at each site, and the depth-to-groundwater maps for selected areas at each site. The effective plant rooting depth was determined by plotting the observed evapotranspiration from the water table versus the depth to the water table. Continuous plots of groundwater, precipitation, and periods of pumping for adjacent irrigation were examined for possible groundwater fluctuations caused by pumping. Correlation analysis was used to separate the effects of river stage, precipitation, and evapotranspiration on the groundwater level.
The continuous data and the box-plot hydrographs showed that the median groundwater levels typically peaked by March, and then declined through September. Recharge began in October and varied between a gradual recharge over the winter for the drier wells, to a relatively rapid recharge following plant senescence in the fall at the wetter sites. The duration curves showed that the water table was within 0.5 ft of the surface 0%, < 1 %, and 56-95 % of the time for Elm Creek, Rowe Sanctuary, and for the two wettest wells at Crane Meadows, respectively, for February through April. Reasonably good relationships were developed between the river stage and river flow for Rowe Sanctuary and Crane Meadows, but the relationship was poor at Elm Creek because the river only enters the Elm Creek channel at high flow. Interpretation of the groundwater profile next to Rowe Sanctuary and Crane Meadows was confounded because the cross-valley transects were not oriented perpendicular to the Platte River. The Elm Creek transect was almost perpendicular to the River, however, and there was a steep gradient toward the River from the south and a moderate gradient toward the River from the north. Groundwater contours at each site showed a predominant gradient down-valley, but precipitation directed this gradient toward the River at each site and evapotranspiration apparently directed this gradient toward the center of the island at Crane Meadows. The depth-to-groundwater maps showed that Elm Creek had the deepest groundwater levels, Rowe Sanctuary had intermediate groundwater levels, and Crane Meadows had the highest groundwater levels. Up to 77% of the representative area at Crane Meadows had groundwater levels within 1 ft or above the surface for the median spring level. The effective rooting depth for plants located in the wettest areas of Crane Meadows was about 3 ft below the surface, and 5 to 6 ft below the surface for the drier areas. Groundwater withdrawals for adjacent irrigation had little or no direct affect on the groundwater levels at the three study sites. This was probably because most irrigation wells were at least a half mile from the nearest groundwater-level recorder. River stage, precipitation, and evapotranspiration were nearly always highly correlated with the groundwater level, with river stage usually the most highly correlated.
The conclusions from this study are:
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