Hydrologic Delineation of Nonpoint Source Contributing Areas

Land‐use regulations for reduction of nonpoint source pollution from surface‐applied contaminants do not usually consider the differing hydrologic responses of land areas based on soil type, slope, proximity to the stream, and expected rainfall. To help overcome this shortcoming, a technique is developed to define the area of a watershed contributing surface runoff directly to the stream as the result of any rain. Using design rainfalls and initial watershed soil moisture as input, a map can be drawn showing the return period with which any part of the watershed contributes surface runoff, and thereby potential nonpoint source pollution, to the stream. Use of the approach is illustrated by application to a small, agricultural watershed in east‐central Pennsylvania. Employing such a technique to define restricted‐use areas on a watershed eliminates the subjective nature of commonly proposed land‐use regulations by accounting for both the watershed characteristics and the probabilities controlling the generation of surface runoff and its direct affects on the stream.