Continuous hydrograph synthesis with an API‐type hydrologic model

The U. S. ESSA Weather Bureau Hydrologic Research and Development laboratory has developed a complete hydrologie model utilizing an antecedent precipitation index ( API ) type rainfall‐runoff relation to compute surface runoff. With increasing demand for continuous river forecasts as well as flood forecasts, it is necessary to have a model that will predict all components of flow as functions of observable independent parameters on a continuous basis. To formulate the model, existing and proved techniques were used where possible and new techniques developed as necessary. The model consists of four basic parts: a relation for computing ground‐water recession, a method of computing the ground‐water flow hydrograph as a function of the direct runoff hydrograph, an API ‐type rainfall‐runoff relation, and a unit hydrograph. The rainfall‐runoff relation is of the incremental type, yielding a runoff computation for each 6‐hour period rather than computing the total storm runoff. This has been accomplished through the inclusion of a new parameter, retention index. Two important features of the model are the ease of adjusting parameters to observed flow and the sequential development of the four basic parts with a minimum of interaction.