Synthetic Unit Hydrographs

The problem with using a derived unit hydrograph is that if we already have enough measured information (runoff and rainfall) to derive the unit hydrograph then statistical methods of estimating a peak runoff are likely to be better anyway.  Most often we wish to perform a hydrologic analysis in ungaged locations where it is not possible to derive the true unit hydrograph.  For this reason synthetic unit hydrographs, such as the SCS, Clark, or Snyder, are used in deterministic models.

A synthetic unit hydrograph is developed based on two important elements; the shape and the time to peak.  The most basic shape is the triangular, rational, unit hydrograph. 

In order to make the shape more natural the SCS modified it to look like a typical measured hydrograph by elongating the falling limb and fitting a higher order shape through the basic triangle. 

The Clark method is another commonly used synthetic unit hydrograph.  This method uses a time area mass curve to develop the shape of the watershed.  A time area curve defines the percentage of the watershed contributing flow at the outlet from the beginning of the storm runoff to the time of concentration, or time when the entire watershed is in equilibrium and contributing to flow at the outlet. 

Further it uses a storage coefficient to account for the fact that there is a delayed response to runoff as the watershed "absorbs" and then "releases" water for runoff much like a sponge.  With a storage coefficient the peak can be reduced by "spreading" or "squeezing" the hydrograph and decreasing or increasing the peak according to the storage properties of the watershed being modeled.  This second degree of freedom in the hydrograph makes it easier to match both peak and volume of a measured event during calibration.