Temporal Distribution of Rainfall

Depth is only one part of describing a storm for a hydrologic modeling simulation.  The time duration, or temporal distribution of the rainfall event is just as important for hydrologic models.  For example 3 inches of rainfall is a lot, but spread out over a year it is not.  Of course one year is extreme, but even spreading it out over 24 hours as opposed to 2 hours will make a big difference on the amount of runoff and flooding.

The depth of rainfall divided by the duration is the rainfall intensity.  For example a depth of 2 inches in 3 hours results in an intensity of .67 in/hr, whereas a depth of 2 inches in 24 hours results in an intensity of .083 in/hr.  Some models, like the rational method use intensities, in inches/hr, to define the rainfall input, but others like HEC-1 and HMS use a depth with a corresponding temporal distribution, or time varying intensity.  This is because few storms maintain a constant intensity for more than a short time period.  By defining a separate temporal distribution the varying intensity throughout the storm can be represented and modeled and therefore the watershed runoff can be more accurately represented.