Design Storms

In summary, rainfall can be defined as an intensity, generally a constant intensity as is the case for the rational method, or as a hyetograph where the depth is associated with a temporal distribution.  The Natural Resource Conservation Service (NRCS), formerly the SCS, has defined some typical design storms for various regions of the United States.  These distributions are dimensionless in depth, but generally 24 hours in length.  They indicate the differences among typical storms where 1) rainfall begins with a shorter period of higher intensity towards the middle and tapers off, 2) a relatively constant intensity throughout, or 3) a high intensity storm where most of the rainfall occurs over a relatively small duration.  The animations below illustrate these differences.

The temporal distribution along with a computed depth from gages, a continuous map, or other estimate provides the basis for defining rainfall in a HEC-1 or HMS (or similar deterministic) model.  Because of interception and infiltration in the watershed not all of the rainfall will be converted to runoff.  Before the rainfall depth can be converted to a runoff hydrograph, the effective rainfall must be determined by subtracting losses from initial abstractions and infiltration.