Effects of precipitation and air temperature on nitrogen discharges from Rhode River watersheds

We studied discharges of total-N, nitrate, ammonium, and total organic-N fr om seven contiguous small watersheds on the Atlantic Coastal Plain in Maryl and for up to 25 yr. These watersheds have perched aquifers so all groundwa ter discharges as well as surface runoff were measured at V-notch weirs whi ch included volume-integrating flow-proportional samplers. Interannual vari ations in annual and seasonal precipitation during this study spanned appro ximately the range of 160 yr weather records in the region. Annual total-N area yields from the overall watershed varied nine-fold, correlations of al l N-parameter discharges with precipitation were highly significant, and po wer function regressions of precipitation vs N-discharge explained from 36 to 59% of the variance. Nitrogen fluxes from a cropland watershed were much higher and more variable with volume of precipitation, while fluxes from a forested watershed were much lower and were primarily composed of organic- N. Correlations of N-fluxes with precipitation were higher in the winter an d spring. Annual and seasonal N-concentrations also often increased signifi cantly with precipitation. Variations in seasonal air temperature sometimes explained significant amounts of variance in N-discharges, especially ammo nium. A model composed of regressions was used to construct graphical and t abular summaries.