Nitrate transport and supply limitations quantified using high-frequency stream monitoring and turning point analysis

Agricultural landscapes often leak inorganic nitrogen to the stream network, usually in the form of nitrate-nitrite (NOx-N), degrading downstream water quality on both the local and regional scales. While the spatial distribution of nitrate sources has been delineated in many watersheds, less is known about the complicated temporal dynamics that drive stream NOx-N because traditional methods of stream grab sampling are often conducted at a low frequency. Deployment of accurate real-time, continuous measurement devices that have been developed in recent years enables high-frequency sampling that provides detailed information on the concentration-discharge relation and the timing of NOx-N delivery to streams. We aggregated 15-min interval NOx-N and discharge data over a nine-year period into daily averages and then used robust statistical methods to identify how the discharge regime within an artificially-drained agricultural watershed reflected catchment hydrology and NOx-N delivery pathways. We then quantified how transport and supply limitations varied from year-to-year and how dependence of these limitations varied with climate, especially drought. Our results show NOx-N concentrations increased linearly with discharge up to an average turning point of 1.42 mm of area-normalized discharge, after which concentrations decline with increasing discharge. We estimate transport and supply limitations to govern 57 and 43 percent, respectively, of the NOx-N flux over the nine-year period. Drought effects on the NOx-N flux linger for multiple years and this is reflected in a greater tendency toward supply limitations in the.three years following drought. How the turning point varies with climate may aid in prediction of NOx-N loading in future climate regimes. (C) 2017 Elsevier B.V. All rights reserved.