A hydrograph is a representation of flow for a particular stream and is used to evaluate the watershed runoff volumes and the peak flow rates from a single storm or set of storms. This information is then used in the design of flood control facilities. Watershed runoff is a function of several things, including rainfall intensity storm duration type of vegetation in the watershed area size and shape of the watershed and many other factors The synthetic hydrograph approach was initially developed in 1932 by L.K. Sherman and has been fine tuned by several others since then. Effective rainfall is the total rainfall less infiltration, evaporation, transpiration, absortion and detention. The next step is to subtract the base flow from the total runoff.
Base flow is defined as the normal flow of the stream without the influence of storm-runoff.
To determine the actual flow from runoff it is necessaary to subtract the base flow from the total runoff. To find base flow, draw a line from the minimum beginning value of the hydrograph curve to the minimum ending value. The base flow for each time unit can then be read off the graph. In this example, the base flow is constant at 50 cfs. The dotted line is the base flow and the dashed line is the total direct flow from the storm.