Trend analysis of streamflow and groundwater quality in Nebraska

In this research, spatial and temporal trends of streamflow and groundwater quality in Nebraska were analyzed. In the first section, fifty years of streamflow data from 110 gauging stations in eight major river basins were examined for streamflow depletion. Temporal trends of streamflow showed a spatial tendency of decreasing streamflow mostly in the west. For the Republican River basin, the Local Weighted Regression model was used to generate precipitation-adjusted stream discharges to evaluate the baseflow which was associated with the groundwater level and groundwater withdrawal. The analysis suggested that the baseflow (approximately) from 15 of the 22 stations had depletion since the 1950s. This decrease plausibly matches the pattern of an increasing number of irrigation wells and declines of groundwater levels. It is most likely that groundwater withdrawal was a primary factor in streamflow depletion. This new application is able to quantify the impact of groundwater withdrawal on streamflow because the adjusted-runoff streamflows reflect the levels of groundwater discharge to streams. The 2001 Nebraska Legislature passed LB329 requiring the Nebraska Department of Environmental Quality to conduct a regular groundwater quality trend analysis for the state and determine whether or not groundwater quality is degrading or improving. In the second section, geostatistical approaches were utilized to evaluate the groundwater nitrate-N contamination in Dawson, Buffalo and Hall counties, Nebraska. A mixed approach of Mann-Kendall test on cokriging estimates was proposed to analyze temporal trends of nitrate-N levels at 465 sampled wells with a non-even time interval sampling from 1974 to 2005. Thirty-seven wells were identified as having an increasing trend and twenty-nine had a decreasing trend. It revealed that the groundwater quality in some areas, such as the middle area of Buffalo, was increasing, but was decreasing in the Central Platte River Valley, such as the middle part of Hall and the south-eastern corner of Buffalo County. Ordinary and indicator krigings were also used for the trend analyses. This proposed approach shows a capability to evaluate groundwater quality trends over time