The shape of the SCS UH is obtained from the NASH relation, with N=5. This value is greater than the one determined from studies in Ontario, Switzerland, and the United Kingdom. It is, however, conservative if the time to peak is correct.
The SCS non-dimensional unit hydrograph is used by SCS abstraction methods for both rural and urban areas. Comparisons with measurements show that even if Tp, Ia, and CN* are calibrated, the proper shape of the hydrograph is not always generated.
The 1986 SCS TR-55 publication indicates the following limitations:
- Hydrographs obtained by this method are not developed for comparisons with measure-ments.
- The method should only be used in cases where runoff is greater than 12.5 mm.
- The lag formula given in previous SCS publications is no longer recommended (it may un-derestimate the peak flow).
The SCS methods apply the non-dimensional UH in conjunction the SCS CN method with the as-sumption that Ia = 0.2 x S. Although this may overestimate the rainfall losses, it was maintained in the SCS command for special agency requests.
It is recommended the to determine Tc with the velocity method: