Nitrate dynamics in a rural headwater catchment: measurements and modelling

This study was designed to improve our understanding of, and mechanistically simulate, nitrate (NO₃) 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 NO₃ 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 NO₃ concentrations started to be delivered to the stream via preferential subsurface flow within two days of the storm commencing. Groundwater NO₃-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 NO₃ mainly laterally from surface soil during and following each storm. The model accurately accounted for NO₃ 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 NO₃ dynamics at a small-catchment scale with high spatial and temporal resolution.