Antecedent flow conditions and nitrate concentrations in the Mississippi River basin

摘要

The relationship between antecedent flow conditions and nitrateconcentrations was explored at eight sites in the 2.9 million square kilometers(km2) Mississippi River basin, USA. Antecedent flow conditions werequantified as the ratio between the mean daily flow of the previous year andthe mean daily flow from the period of record (Qratio), and the Qratio wasstatistically related to nitrate anomalies (the unexplained variability innitrate concentration after filtering out season, long-term trend, andcontemporaneous flow effects) at each site. Nitrate anomaly and Qratio werenegatively related at three of the four major tributary sites and upstreamin the Mississippi River, indicating that when mean daily streamflow duringthe previous year was lower than average, nitrate concentrations were higherthan expected. The strength of these relationships increased when data weresubdivided by contemporaneous flow conditions. Five of the eight sites hadsignificant negative relationships (p ≤ 0.05) at high or moderatelyhigh contemporaneous flows, suggesting nitrate that accumulates in thesebasins during a drought is flushed during subsequent high flows. At half ofthe sites, when mean daily flow during the previous year was 50 percentlower than average, nitrate concentration can be from 9 to 27 percenthigher than nitrate concentrations that follow a year with average meandaily flow. Conversely, nitrate concentration can be from 8 to 21 percentlower than expected when flow during the previous year was 50 percent higherthan average. Previously documented for small, relatively homogenous basins,our results suggest that relationships between antecedent flows and nitrateconcentrations are also observable at a regional scale. Relationships werenot observed (using all contemporaneous flow data together) for basinslarger than 1 million km2, suggesting that above this limit the overallsize and diversity within these basins may necessitate the use of morecomplicated statistical approaches or that there may be no discerniblebasin-wide relationship with antecedent flow. The relationships betweennitrate concentration and Qratio identified in this study serve as the basisfor future studies that can better define specific hydrologic processesoccurring during and after a drought (or high flow period) which influencenitrate concentration, such as the duration or magnitude of low flows, andthe timing of low and high flows.