Ground Water and River Flow Analysis

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18 • The first rainfall after a long dry period replenished soil moisture in the dry root zone. Subsequent rains of more than 1/2 inch generally produced a rise in the water table, while rains of less than 1/2 inch affected water table levels only when they came within a day or two of a larger event. • For some wells, the rise in well water levels was considerably greater than would be expected from the amount of the rainfall. This is thought to be the result of water ponding at the end of farm fields or in road ditches close to the well. • Wells farthest from the river undergo fluctuations that cannot be attributed to the river or to precipitation. In these cases, it was assumed that fluctuations result from irrigation well drawdown or local water table recovery. RESULTS OF STATISTICAL ANALYSIS To determine the causes and effects behind the river gauge heights and well elevations, Reclamation performed a statistical analysis of 28 observation wells next to the Platte River. Generally, well elevations and river gauge heights move in the same direction. All of the wells were significantly The river levels control groundwater within 100 feet of the river and influence levels within 700 feet of the river. Other influences become more important than river levels beyond 1,000 feet of the river. The entire hydrological system (river and aquifer) is responding to a common influence namely precipitation. correlated with river levels. However, correlations can indicate that the events move in concert with one another as well as moving in response to each other. Correlations do not necessarily imply cause and effect relationships. Significant conclusions that can be drawn from the statistical analysis of surface water and ground water elevations, changes in elevation, and precipitation are listed below. The complete set of results from this statistical analysis is presented in Appendix C. • There is a relationship between the daily change in well water surface elevations and precipitation in the vast majority (79%) of observation wells in the study area. • Correlations between the well water surface elevations and river gauge heights were higher for wells near the river. Ground Water and River Flow Analyses River levels have an influence on ground water levels near the river. Several thousand feet from the river, the water table elevation is generally several feet higher than the river and thus does not react to river levels. e t I� C' u u