Synthetic unit hydrographs are commonly used to estimate runoff from rainfall events in a hydrologic analysis. A key parameter required as part of any hydrologic analysis using a synthetic unit hydrograph is time of concentration or lag time. Generally, equations used to compute time of concentration or lag time are empirically derived from basin parameters such as area, slope, and a specific flow path length. A more realistic method for determination of flow path travel time is to divide the flow path according to different hydraulic conditions such as sheet flow, shallow concentrated flow and open channel flow as specified in the NRCS method using TR55. Such equations are all based on flow length and the slope of the flow path, two parameters that are easily calculated from GIS vector objects. A method is presented that uses GIS vector objects with equations assigned for the calculation of time of concentration or lag time for use in hydrologic analysis and design.

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