Land-to-water transfer of nutrients: What knowledge can be gained by combined analysis of river water quality and flow records?

Abstract

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. Therefore, twenty-year series of monthly water quality samples were analysed 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 flowpaths in transporting nitrogen and non-dissolved phosphorus species from the land surface 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.