Can you explain what "25% PMF" means ?  Is this the flow that has a 25 percent chance of being  equaled or exceeded in any one year ?  If so you could, calculate the 2 year, 5 year, 10 year, 25 year, 50 year and 100 year flows using the National Flood Frequency program NFF and then interpolate or extraoplate to other recurrence intervals.

The NFF program is available free from USGS.

Russ Faust
Salem, OR, USA

a 25% PMF is simply 25% of the full PMF.  In other words, take each ordinate of the hydrograph and divide by 4.  25%and 50% PMF are commonly used for design of dams and spillways that are small or have a lower hazard rating.  Higher hazard dams (such as those within an urban setting, or very large dams) are usually designed for the full PMF.

Thank you CVG for the information.  From your reply, it sounds like you've answered your own question.  Or, do I still misunderstand what you are asking ?  If you have a hydrograph for the PMF it is simple to develop the 25%PMF hydrograph.  But perhaps, that's not what you're asking ?

Russ Faust
Salem, OR, USA

To calculate the 25% PMF, use 25% or 1/4 of the PMP (Probable Maximum Precipitation) as the rainfall inut in your hydrograph model.  The resulting peak discharge will be the 25% PMF.

Using 25% of the PMP will give slightly less runoff in your hydrograph than 25% of the PMF.  This is due to the fact that in calculating the PMF, you are taking the total rainfall (PMP) minus the losses to give an effective rainfall.  Then you take 25% of this number.

By taking 25% of the total rainfall and then subtracting the losses, you will end up with less effective rainfall.  

In my experience, the PMF is calculated and then design may be based on a fraction of this number.  This is generally a requirement of the jurisdictional agency responsible for dam safety.

So why not take 25% of the losses as well ?   

That's precisely what I stated in my original reply to kjspe!  But in response to nealt I was suggesting that his approach is not commonly done (at least I have not seen it done that way ever) and unless you did use a fraction of the losses, you would end up with less runoff.