Capturing temporal variability for estimates of annual hydrochemical export from a first‐order agricultural catchment in southern Ontario, Canada

This 2‐year study (2000, 2001) reports annual nutrient (phosphorus, nitrate) export from a first‐order agricultural watershed in southern Ontario based on an intensive monitoring programme. The importance of storm and melt events in annual export estimates is demonstrated and the temporal variability in nutrient loading during events is related to processes occurring within the catchment. The feasibility of predicting event‐related nutrient export from hydrometric data is explored. The importance of sampling frequency throughout events is also shown. Export of total phosphorus (TP), soluble reactive phosphorus (SRP) and nitrate ( NO₃⁻ ) for 2000 and 2001 averaged 0·35 kg ha −1 year −1 , 0·09 kg ha −1 year −1 , and 35 kg ha −1 year −1 (as N) respectively. Approximately 75% of annual TP export, 80% of annual SRP export and 70% of annual NO₃⁻ export occurred during 28 events per year. A small number of large‐magnitude events (>34 mm) accounted for 18–42% of annual TP export, 0–61% of annual SRP export and 13–33% of annual NO NO₃⁻ export. Our results show that temporal variability in nutrient export is largely governed by discharge in this basin, and NO₃⁻ export can be predicted from discharge. SRP and TP export can also be predicted from discharge, but only for events that are not large in magnitude. The sampling interval throughout events is important in obtaining precise estimates of nutrient export, as infrequent sampling intervals may over‐ or under‐estimate nutrient export by ± 45% per event for P. This study improves our understanding of NO₃⁻ and P export patterns and our ability to predict or model them by relating temporal variability in event nutrient export to discharge and processes occurring within the basin, and also by exploring the significance of sampling interval in the context of the importance of individual events, season and temporal variability during events.