Abstract:  To control water pollution, improve water quality and largely reduce phosphorus pollution, it is essential and urgent to define critical source areas of non-point source phosphorus output loss. By using the semi-quantitative empirical catchment scale phosphorus loss risk grading scheme, landuse type as source factor and slope and distance from a river as transfer factors, critical source areas of non-point source phosphorus loss in data-deficient small watershed can be identified as definite as possible. Shehe agricultural watershed was used as an example for the work by means of GIS technology. Results show that about 80% of the Shehe watershed was sorted into the category of “low risk” of phosphorus loss with surface runoff, while 13% of the watershed, consisting mainly of farmlands alongside the river was in the category of “moderate risk”, and 7% of the watershed, consisting of upland areas high in slope gradient and close to the river, in the category of "high risk” . It is, therefore, necessary to intensify land management in this critical source area in the watershed in order to reduce non-point source phosphorus output.

Key words: small watershed, non-point source, phosphorus, critical source area