Effects of precipitation and air temperature on phosphorus fluxes from Rhode River watersheds

We studied fluxes of total P, total phosphate, and total organic P from seven contiguous small watersheds on the Atlantic Coastal Plain in Maryland for up to 25 yr. These watersheds have perched aquifers so all groundwater discharges as well as surface runoff were measured at V-notch weirs equipped with volume-integrating, flow-proportional samplers. Interannnal variations in annual and seasonal precipitation during this study spanned approximately the range of 160-yr weather records in the region. Annual total-P area yields from the overall watershed varied 28-fold, correlations of all P-species fluxes with precipitation were highly significant, and power function regressions of precipitation vs. P-flux explained from 42 to 55% of the variance in the latter. Phosphorus fluxes from a cropland watershed mere much higher and more variable with volume of precipitation, while fluxes from a forested watershed were much lower and primarily composed of organic P. Correlations of P fluxes with precipitation were higher in the spring. Annual and seasonal P concentrations also often increased significantly with precipitation. Variations in seasonal mean air temperature sometimes explained significant amounts of variance in P fluxes, especially phosphate from cropland. A regression model was used to construct graphical and tabular summaries.