Flood and drought-enhanced variations in streamwater nitrate flux in an agricultural watershed, Clear Creek, Iowa

Background/Question/Methods

Nitrogen (N) fertilization is a cornerstone of modern agriculture, but the practice also leads to eutrophication, hypoxia, and harmful algal blooms in both inland and coastal waters.  Several studies identify Iowa, Illinois and Indiana as major source areas of N discharged by the Mississippi River to the Gulf of Mexico where large-scale hypoxia develops annually. Continental-scale management of nitrogen requires a comprehensive understanding of watershed-specific hydrologic dynamics and their consequences for nitrate flushing from agricultural landscapes. Spatiotemporal variation in nitrate fluxes is inherently complex due to the broad range of physicochemical and hydraulic properties that influence N movement through soils, groundwater, and rivers. Amplification of drought cycles due to climate change will introduce increased variability on stream flow and nutrient delivery to sensitive downstream ecosystems. However, the influence of pronounced wet/dry cycles on N export from agricultural landscapes is poorly understood. To address this need, we monitored nitrate flux with high temporal resolution from an agricultural watershed in Iowa (Clear Creek). Our objective was to quantify differences in storm-related nitrate fluxes in two years with contrasting antecedent moisture conditions. Specifically, we contrast the wet conditions of 2009 (following the 500-year flood of 2008) to the 2012 drought conditions. 

Results/Conclusions

Temporal nitrate concentrations show distinct short-term differences in response to storm events, with a net decrease in concentration (dilution) observed following storm events in 2009 (wet year), and increases in concentration observed in 2012 (dry year). While the magnitude of event-driven nitrate fluxes is highly variable, the annual baseflow flux in 2009 was an order of magnitude greater than observed in 2012. Analysis of nitrate flux vs. stream discharge further distinguishes the differing flow regimes and the impact on resulting nitrate export. While contrasts in this relationship have previously been observed by comparing different watersheds, we show that differences in nitrate flux-flow relationships also occur within the same watershed under differing antecedent moisture conditions. The results of this study show the extreme variations in nitrate flux associated with pronounced wet/dry cycles, and the rapid shift in hydrologic connectivity within three years.  Continuing analysis aims to identify thresholds across which agricultural streams, such as Clear Creek, switch from periods when rain-induced runoff increases stream nitrate concentrations to periods when nitrate concentrations are diluted.  Information connecting storm events, antecedent environmental conditions and nutrient dynamics is critical for improving our predictions of nitrate loading to riverine networks under increased climatic variation.