HYDROLOGIC CONTROLS ON PHOSPHORUS LOSS FROM UPLAND AGRICULTURAL WATERSHEDS

Development of strategies for controlling P loss from upland agricultu ral watersheds requires an ability to identify specific source areas o f P at field and farm scales, and to predict their resultant effects a t the watershed scale. Key to identification of P source areas is defi ning 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 spe cific zones of runoff and erosion within a watershed, that is, source areas for the P transport mechanisms. Research results from a series o f studies within a small, upland agricultural watershed in fast-centra l Pennsylvania show that the zones of runoff production, and consequen tly the areas ultimately controlling most P transport, are often a lim ited and identifiable portion of the landscape. Quantifying the hydrol ogic controls on P transport within and from a watershed in this way a llows us to focus management options on small and definable portions o f the total watershed that contribute most P export.