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WWRC 94-07
Platte River Wetland Hydrology Study

Executive Summary

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:

1. River stage is most often the dominant influence on the groundwater level.
2. Precipitation is usually the next most dominant influence on groundwater levels.
3. Evapotranspiration from the water table does not become important until May, and is usually insignificant again by late September.
4. Groundwater withdrawals for adjacent irrigation has little or no direct affect on the groundwater levels at the three study sites. The cumulative effect on the study-site groundwater levels from groundwater withdrawals throughout the Platte River Valley was not evaluated, however.
5. Plants in the wettest areas of Crane Meadows appear able to remove water directly from the water table up to about 3 ft below the surface. The water table seldom drops below 3 ft in these areas. Plants in the drier areas of Crane Meadows appear able to remove water directly from the water table up to 5-6 ft below the surface. The water table in these drier areas seldom drops below this limit.
6. Crane Meadows has more area with shallower groundwater levels than either Rowe Sanctuary or Elm Creek.
7. Median groundwater levels typically peak by March and then gradually declined through September. Recharge begins in October and varies between a gradual recharge over the winter at the drier sites, to a relatively rapid recharge at the wetter sites following plant senescence.
8. The groundwater gradient is primarily down-valley at each site. Large precipitation events, however, direct this gradient toward the River at each site and evapotranspiration apparently directs this gradient toward the center of the island at Crane Meadows during the summer.
9. The groundwater gradient adjacent to the Platte River at Elm Creek is relatively steep toward the River from the south, and relatively moderate toward the River from the north. Interpretation of the groundwater gradient next to Rowe Sanctuary and Crane Meadows is confounded because the cross-valley transects are not oriented perpendicular to the Platte River.
10. The relationship between river stage and river flow at Rowe Sanctuary and Crane Meadows was good, but the Elm Creek relationship was poor because the river channel for the Elm Creek gage was fed primarily by a groundwater drain.
11. Depth-to-groundwater duration curves are useful for determining maximum and minimum levels, as well as the percent of time a particular level was equaled or maintained above that level. If this duration information was combined with the response of selected plant species to produce depth-to-groundwater suitability curves, then it may be possible to predict plant response to future water management practices along the Platte River.

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