National Nutrient Contribution Dynamics in New Zealand Rivers

River water quality degradation poses significant challenges for catchment nutrient management. Effective catchment-scale nutrient management requires a clear understanding of how different contaminants are transported along different flowpaths. Here we investigated nutrient transport pathways and their relative contributions for two prevalent contaminants (total phosphorus: TP; and nitrate-nitrite nitrogen: NNN) at 58 river water quality sites across New Zealand, by applying the Bayesian, chemistry-assisted hydrograph separation approach, in which river flow is partitioned into three components: near-surface event flow (fast flow), seasonal shallow groundwater discharge (medium flow) and long-term groundwater discharge (slow flow). After excluding 15 sites that were impacted by dam/lake outflows or model convergence, results from 43 sites revealed that on an annual basis, medium flow contributes over 50% of annual streamflow at 19 sites and fast flow contribution exceeds 50% at 9 sites. Regarding TP load, TP is primarily transported via fast flow (26 sites), followed by medium flow (14 sites). Concerning NNN loads, most NNN originates from medium flow (25 sites). Correlation analysis with upstream catchment characteristics indicated that annual flow is most highly correlated with precipitation and potential evapotranspiration, followed by geomorphologic factors (e.g., slope) and livestock density, whereas TP loads are most strongly correlated with the number of days with high rainfall, catchment elevation and dairy cow density, and NNN loads are most correlated with annual temperature, geomorphology and geology factors (e.g., slope), as well as land cover (e.g., pastoral) and livestock density which serve as sources of NNN. These findings provide valuable insights for both surface and subsurface transport pathways in New Zealand. The approach offers a practical framework for similar assessments in other regions, to mitigate water quality degradation.