Hydrographs

Unlike the Rational Method (which is linear) the SCS runoff equation is non-linear. Furthermore, no runoff will occur until the initial abstraction has been satisfied.

From the web site:

Why don't I get any runoff until about 10 hours?
According to the SCS runoff equation, there will be no runoff until the rainfall exceeds the initial abstraction, which may not happen until 8-10 hours into the storm. Until that point, the runoff will be zero. This effect is most pronounced for low CN values and low rainfall depths, such as water quality events. For example, a CN value of 80 has an initial abstraction of 1/2 inch, which isn't satisfied until 11.2 hours for a 2-inch 24-hour Type II storm, and significant runoff does not occur until about 11.7 hours.

Peter Smart
HydroCAD Software

Regarding the pond outflow, an abrupt increase can occur if the pond is retaining the initial inflow, and then overflowing via a weir, which is characterized by a sharp increase in discharge vs. head. If you want the pond to discharge at an earlier time, then you need to have a low-flow outlet near the bottom of the pond.


Peter Smart
HydroCAD Software

I do have a low flow (5" orifice) outlet at the very bottom of my pond. Do you think my hydrograph seems typical?

My coworker thinks this doesn't make sense, and I have a setting "off".

The CN of my drainage area is 83, and I have a pond with a 24" outlet through a control structure with a low-flow orifice and weir.

I don't see anything wrong with your hydrograph. It appears completely consistent with the situation you have described.

Peter Smart
HydroCAD Software

Thank you Peter!