Evaluating nitrate-N !uxes from agricultural landscapes is inherently complex due to the wide range of intrinsic and dynamic controlling variables. In this study, we investigate the in!uence of contrasting antecedent moisture conditions on nitrate-N !ux magnitude and dynamics in a single agricultural watershed on intra-annual and rainfall-event temporal scales. High temporal resolution discharge and nitrate concentration data were collected to evaluate nitrate-N !ux magnitude associated with wet (2009) and dry (2012) conditions. Analysis of individual rainfall events revealed a marked and consistent difference in nitrate-N !ux response attributed to wet/dry cycles. Large magnitude dilutions (up to 10 mg N L1) persisted during the wet antecedent conditions (2009), consistent with a dominant base!ow contribution and excess groundwater release in relation to precipitation volume (discharge > > precipitation). Smaller-magnitude concentrations (<7 mg N L1) were observed during the drought conditions of 2012, consistent with a quick!ow-dominated response to rain events and in#ltration/
storage of precipitation resulting in discharge < precipitation. Nitrate-N loads and yields from the watershed were much higher (up to an order of magnitude) in the wet year vs. the dry year. Our results suggest that the response of nitrate-N loading to rain events is highly dependent on intra-annual antecedent moisture conditions and subsurface hydrologic connectivity, which together dictate the dominant hydrologic pathways for stream recharge. Additionally, the results of our study indicate that continued pronounced wet/dry cycles may become more dominant as the short-term driver of future nitrate-N exports.
Generalized conceptual diagram summarizing the hydrologic and nitrate N observations from this study. PDI, Palmer Drought Index.
Davis. C.A., Ward, A.S., Burgin, A.J., Loecke, T.D., Riveros-Iregui, D.A., Schnoebelen, D.J., Just, C.L., Thomas, S.A., Weber, L.J., St. Clair, M.A. (2014): Antecedent Moisture Controls on Stream Nitrate Flux in an Agricultural Watershed. Journal of Environmental Quality. DOI: 10.2134/jeq2013.11.0438
This Paper/Book acknowledges NSF CZO grant support.