Platte River Wetland Hydrology Study

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EXECUTIVE SUND4ARY While the importance of wetlands for native plants and wildlife has been recognized, rolouth_at maintains these areas_ is poorly understood. To address this knowledge gap sect Team began a stu v in 1988 to examine the seasonal relationships between wet irrigation. Threesites along the Platte River between Lexington and Grand slann ; 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 bo_;,x„plot groundwater and river -stage h drora h�s; 3) Developed depth -to- groundwater and river stage duration curves; 4) Determined the relationship between river stage and river flow; 5) Present -M- ro s'- 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 Qroun water levels. Continuous hydrologic and soil temperatures were plotted by water year to show the 'real -time' relationships. Boxplot hydrographs and depth -to- groundwater duration curve were generated from the contmuou�ifv mean groundwater deaths and river stage_ The be curves are cumulative ITeguen clistriputions that snow the percent of time a particular depth or stage Vas equaled or above that level for a perio Zdt spec ie . L—iffe-ar regression was use to determine relationship een 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 ii