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发布日期: Jul 19, 2017
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邹丽雪 (Jul 28, 2017)

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Dynamics of nitrate concentration‐discharge patterns in an urban watershed

Abstract.
Concentration-discharge (c-Q) relations have been used to infer watershed-scale processes governing solute fluxes. Prior studies have documented inconsistent concentration-discharge patterns at the storm event scale driven by changes in end-member concentrations. Other studies have evaluated c-Q data from all periods in a composite fashion to quantify chemostasis (relatively invariant changes in concentration over several orders of magnitude variation in streamflow). Here we examine three-years of high-frequency nitrate and discharge data (49,861 data points) to complement 14 years of weekly data (699 data points) for an urban stream in Baltimore, MD USA to quantify c-Q relationships. We show that these relationships are variable through time and depend on the temporal scale at which they are investigated. On a storm-event scale, the sensor data exhibit a watershed-specific dQ/Q threshold when storms switch from counter-clockwise to clockwise c-Q behavior. On a seasonal scale, we show the influence of hydrologic variability and in-stream metabolism as controls on stream nitrate concentrations and fluxes. On a composite scale, we evaluate the c-Q data for chemostasis using analysis of both c-Q slopes and CVc/CVQ, as a function of time. The slopes of c-Q data for both long-term weekly and high frequency data sets are in close agreement on an annual basis and vary between dry and wet years; the CVc/CVQ analysis is less sensitive to hydroclimate variability. This work highlights the value of both long-term and high-frequency c-Q data collection for calculating and analyzing solute fluxes.