Hydrologic controls on phosphorus loss from upland agricultural watersheds

Development of strategies for controlling P loss from upland agricultural watersheds requires an ability to identify specific source areas of P at field and farm scales, and to predict their resultant effects at the watershed scale. Key to identification of P source areas is defining the interaction between P availability over the landscape and its potential for movement to the watershed outlet by runoff and erosion, A current weakness in quantifying this interaction is determining specific zones of runoff and erosion within a watershed, that is, source areas for the P transport mechanisms. Research results from a series of studies within a small, upland agricultural watershed in fast-central Pennsylvania show that the zones of runoff production, and consequently the areas ultimately controlling most P transport, are often a limited and identifiable portion of the landscape. Quantifying the hydrologic controls on P transport within and from a watershed in this way allows us to focus management options on small and definable portions of the total watershed that contribute most P export.