Nitrate dynamics in a rural headwater catchment: measurements and modelling

This study was designed to improve our understanding of, and mechanistically simulate, nitrate (NO 3 ) dynamics in a steep 9.8 ha rural headwater catchment, including its production in soil and delivery to a stream via surface and subsurface processes. A two‐dimensional modelling approach was evaluated for (1) integrating these processes at a hillslope scale annually and within storms, (2) estimating denitrification, and (3) running virtual experiments to generate insights and hypotheses about using trees in streamside management zones (SMZs) to mitigate NO 3 delivery to streams. Total flow was mathematically separated into quick‐ and slow‐flow components; the latter was routed through the HYDRUS software with a nitrogen module designed for constructed wetlands. Flow was monitored for two years. High surface‐soil NO 3 concentrations started to be delivered to the stream via preferential subsurface flow within two days of the storm commencing. Groundwater NO 3 ‐N concentrations decreased from 1.0 to less than 0.1 mg l −1 from up‐slope to down‐slope water tables, respectively, which was attributed to denitrification. Measurements were consistent with the flushing of NO 3 mainly laterally from surface soil during and following each storm. The model accurately accounted for NO 3 turnover, leading to the hypotheses that denitrification was a minor flux (<3 kg N ha −1 ) compared to uptake (98 − 127 kg N ha −1 ), and that SMZ trees would reduce denitrification if they lowered the water table. This research provides an example of the measurement and modelling of NO 3 dynamics at a small‐catchment scale with high spatial and temporal resolution. Copyright © 2013 John Wiley & Sons, Ltd.