Monthly precipitation-runoff polygons and mean runoff coefficients

The runoff coefficient, the ratio of total streamflow volume to the total precipitation over a certain area and time, plays a fundamental role in the planning, design and operation of water resources in a catchment. Most often its long-term global value, especially on an annual basis, is presented either in a tabular form as a function of vegetation cover, degree of urbanization, climatological features and geological setting of the catchment area, or in the form of empirical formulations. However, in this paper, monthly runoff coefficient changes within an annual period are represented through a simple polygon diagram concept obtained from monthly precipitation and runoff data. This method provides not only variations in the monthly runoff coefficients but also, qualitatively, many useful interpretations concerning the rainfall-runoff transformation in a catchment. The application of the polygon method is presented for some catchments around the city of Istanbul, Turkey.