**RATIONALE**
Every catchment or basin of a given drainage area A has a **unit hydrograph** (1 cm of runoff) for a given
[unit] effective rainfall of duration t_{r} (i.e., the unit hydrograph duration is t_{r}).
The shape of the unit hydrograph is characterized by its peak discharge and **catchment lag**.
These properties of the unit hydrograph are a
function of the amount of **runoff diffusion** in the catchment or basin.
In turn, runoff diffusion depends on the diffusivity characteristics of the basin.
Steeper basins have less diffusion; consequently, they have higher peaks and shorter time lags.
Milder basins have more diffusion; therefore, lesser peaks and longer time lags.

In practice, steeper basins are modeled with higher values of Courant number C = t_{r}/K and lower values of N.
The largest practical value of C is 2; the minimum value of N is 1.
Milder basins are modeled with a lower C and a higher N.
For steeper basins, C > 1 and N < 3. For milder basins, C < 0.4 and N > 7.

Considerable
experience is required in order to estimate the values of C and N
for a given application. Unit hydrograph shapes for various values of C and N are given by **Ponce (1980)**.

For a gaged stream, where a measured t_{r}-hr unit hydrograph may be available, **online_uh_cascade** can be used to calibrate the parameters C and N.
The resulting cascade unit hydrograph can be **convoluted** with the effective storm pattern
to calculate the composite flood hydrograph.