Seasonal flow, nutrient concentrations and loading patterns in stream flowdraining an agricultural hill-land watershed

The effects of seasonality on nutrient patterns and export in streamflow we re determined for a 7.3 km(2) agricultural hill-land watershed in Pennsylva nia for a 12-year period, 1984-96. Dissolved phosphorus (DP) concentrations were highest in stormflow for all seasons (0.030 mg l(-1)), especially sum mer (0.039 mg l(-1)) when the flow was the least. About two-thirds of the D P export was in stormflow, with two-thirds of this export occurring during winter and spring when five of the seven largest stormflow events within a year occur. For larger stormflows, DP concentrations were positively correl ated with the flow rate, which contributed to storm dominance of DP export. Export of NO3-N, and to a lesser extent DP, by flow component and season w ere controlled by flow rate rather than concentration. Summer was least imp ortant, contributing only 7-8% of the annual export of water, DP, and NO3-N . The NO3-N concentrations were the lowest for the baseflow (5.36 mg l(-1)) and the highest for the elevated baseflow (7.12 mg l(-1)) across seasons. Mon of the NO3-N export was in non-stormflow than stormflow and occurred mo stly in winter and spring. One 50-year return period storm event generated a substantial portion (9%) of the DP exported for the 12-year period of the record, but had much less impact on water (2%) and NO3-N (1%) export. P ma nagement and control decisions for watersheds need to be developed in a sto rm-based, source-area framework, whereas N management and control decisions depend more on managing and balancing N use over the watershed. Published by Elsevier Science B.V.