The method is recommended for rural watersheds where observations indicate a long hydrograph recession limb. The Williams formula for Tp is not recommended in Ontario as it has been shown to give significant errors.
WILLIAMS IUH
As is predecessor (INTERHYMO / OTTHYMO.89), Visual OTTHYMO does not recommend default values for K and Tp in the Williams command, since it is considered that this IUH requires calibration.
COMPARISON OF WILLIAMS AND NASH IUH