In Visual OTTHYMO, the response of a watershed to the effective rainfall is obtained by convolution of a short duration unit hydrograph (UH) derived from the theory of conceptual “instantaneous unit hydrographs” or IUH. The characteristics of these unit hydrographs are not dependent on rainfall duration. However, depending on the size of the area being simulated, their use usually requires short computational time steps (1 to 15 minutes).

Visual OTTHYMO has three types of IUH’s which have a common parameter, the time to peak, Tp. Another parameter, K, is related to the hydrographs’ recession limb. K is also called a ‘storage coefficient’ and has different values in each IUH.

Another option in Visual OTTHYMO is the SCS non-dimensional UH, which is a specific NASH IUH defined only by Tp (see IUH Relations).

For Tp equal to the computational time step, the STANDARD IUH is identical to the single linear reservoir IUH from the URBHYD command in OTTHYMO 83.