Critical source area controls on water quality in an agricultural watershed located in the Chesapeake Basin

The importance of agricultural land use activities for supplying nutrients (N, P) to the Chesapeake Bay is examined and nutrient sources for a typical agricultural hill-land watershed within the Chesapeake Basin are identified and assessed. Based on up to 30 years of experimental and monitoring data, the outflow, N, and P exported from this Pennsylvania watershed is examined in terms of critical source areas. Most of the surface runoff and P export occurs from areas near the stream. About 90% of the algal-available P exported in outflow was generated during the largest 7 storms/year. In contrast, nearly all the nitrate (NO₃) exported originated as subsurface flow entering the soil or ground water some distance from the stream, and mostly occurred during nonstorm flow periods. The NO₃ export observed over the long term corresponds to the N excess computed by N balance obtained by farmer survey for agricultural land. By combining land use, hydrologic processes, watershed position, soil P status, and N balance information for agricultural land, the major source areas for P and N are predictable and identifiable. We apply these ideas and techniques to our research watershed and present the results as an example of this approach.