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19 <br />• The relationship between the daily change in ground water surface elevation and <br />precipitation was at least as significant as the one between ground water surface <br />elevation and river gauge height. <br />• Correlations among the water surface elevation of all of the wells indicated that the <br />water surface elevation of the ground water is moving in concert over most of the <br />study area. <br />The time of rises in most wells correlates better with the rises in river gauge height at <br />the farthest downstream station (Grand Island). Because water takes 2 days to travel <br />from Overton to Grand Island, the almost simultaneous rise in water surface <br />elevations in wells at Overton and the river gauge at Grand Island indicates that the <br />entire hydrological system (river and aquifer) is responding to a common <br />influence — namely precipitation. <br />The following conclusions were based on the lagged correlation analysis of the well <br />water surface elevation and river gauge height at the three Platte River gauges and <br />between precipitation and well water surface elevation. <br />Lagged data are developed by looking at one day's values as they relate to values for the <br />previous day (or days). This lag time provides time for water to travel down the river or <br />be recharged. For example, there is an estimated one -day travel time between each of the <br />river gauges. Because of the travel time, the data for a downstream river gauge should <br />correlate better with the previous day's flow at the next upstream river gauge than they <br />would with data for any other time -step. <br />• Almost all (over 90 percent) of the observation wells varied in similar ways, even <br />though they are in different regions of the aquifer. <br />• Recharge from local precipitation is rapid, 1 day or less in most cases. <br />The following conclusions were based on the regression analysis of groundwater <br />flooding potential. Regression analyses develop formulas to show how changes in river <br />gauge height affect the ground water surface elevation. If the relationship between river <br />gauge height and ground water surface elevation is not strong, then the results of the <br />regression prediction are not accurate. To make acceptable predications, regressions <br />should explain at least 75 percent of the changes. <br />• Regressions in this relationship explain 95 to 96 percent of the variations between <br />ground water surface elevation and river gauge height at 50 to 100 feet from the <br />river. <br />Ground Water and River Flow Analyses <br />