Getting the right answer for wrong reasons

Horton’s and Green-Ampt’s equations are based on the notion that runoff generation occurs when rainfall intensity exceeds the potential infiltration rate, a process often referred to as the infiltration-excess runoff generation mechanism.

In reality, infiltration-excess runoff is quite rare. Runoff is generally generated due to saturation excess or subsurface storm-flow mechanisms. Yet, Horton’s and Green-Ampt’s equations often provide very good runoff estimates. How?

Regardless of which runoff-generation mechanism dominates a landscape, the overall runoff process appears to be captured by either of these equations. Yes, you have got the right answer, but for the wrong reasons. Although I have used runoff generation as an example, you are likely to encounter such situations quite often.