A Water Balance Evaluation of the Effects of Climate Variability and Human Modification on the Flow Regime of the Mississippi River, 1932-1988.

Human activities affecting runoff and streamflow within the Mississippi River drainage basin have been and continue to be significant. Since the disastrous flood of 1927, the U.S. Army Corps of Engineers has established an extensive regulatory system for flood control purposes and navigational benefits. Along with these river system modifications, significant landscape alterations have occurred within the basin as a result of cultural and economical development. In light of the dramatic Midwest flood of 1993, it becomes increasingly necessary to understand the comprehensive relationship between streamflow, climatic variability, and human-induced change within the overall drainage basin. A water balance methodology provides a framework for evaluating variations in measured discharge over time by separating the effects of land use change and river management from variations in streamflow caused by geographical and temporal variability in climate. Therefore, it is possible to evaluate the collective results of human modification within the watershed. This study utilizes Box-Jenkins statistical time series analysis to identify changes in the annual flow regime of the Mississippi River for water years 1932 to 1988. For comparative purposes, the Missouri and Ohio-Tennessee Rivers are also analyzed. The water balance model explains as much as 96% of the variability in annual discharge within the Mississippi basin and the Missouri and Ohio-Tennessee subbasins. There is no evidence of statistically significant changes or long-term trends in annual precipitation, modelled runoff, and discharge in the Mississippi basin and the Missouri and Ohio-Tennessee subbasins, but the annual discharge fractions generated by the Missouri and Ohio-Tennessee subbasins have changed significantly. This can be attributed to the fact that the size of the overall drainage basin is so great that even large-scale atmospheric circulation patterns cannot affect the entire drainage basin at the same time. The relationship between annual discharge and modelled runoff has not changed in the Mississippi and Missouri basins; however, the relationship between annual discharge and modelled runoff has changed significantly in the Ohio-Tennessee subbasin. This change can possibly be attributed to river management and land-use changes within the subbasin