Flow Stratification of River Water Quality Data to Elucidate Nutrient Transfer Pathways in Mesoscale Catchments

While analysis of river water quality time series data alone allows observation of means, variances, trends, and seasonality, it cannot elucidate the catchment mechanisms responsible for these observations. Incorporating river flow data into the analysis allows additional insight to be gained into the mechanisms driving water quality change. Twenty-year series of monthly water quality samples were analyzed alongside high-resolution flow records in 26 catchments across the agriculturally dominated Waikato region of New Zealand. Concentration-discharge relationships indicated the importance of near-surface flow paths in transporting nitrogen and non-dissolved phosphorus species from the land into rivers. Dissolved phosphorus, on the other hand, appears to be discharged primarily in deeper groundwater carrying higher concentrations of geogenic origin. Subsequent data stratification was able to explain the origin of nitrate or phosphorus trends in some catchments as being due to either historical or recent land use changes. These results highlight the value of combined analysis of water quality data with river flow records.