HEC HMS : Loss method and Impervious Area

[Maraligaf]

Hi,

I am a new user of HEC-HMS and I am having some problems with the choice of the loss methods.
I have tried several of them (Initial and Constant Rate, DEficit and Constant Rate, SCS Curve Number, Smith Parlange, Green Ampt). And for all of them, the only excess precipitation I get is from the impervious area.

I have changed the amount of precipitation, and the parameters (from 100 times smaller to 5 times bigger) and there are no difference, except for the SCS Curve Number method.
If I try with "Impervious Area = 0%", I don't have any excess precipitation at all.
I don't think it is normal.
Could someone help me ?

Magali

 

[angel112]

hi Maraligaf,

According to your saying , you are not getting precipitation excess, for that check your meteorological model that you have set. If you have not defined Subbasin with appropriate method in meteorological model , you won't get precipitation excess. Like if you are applying inverse distance method, you have to specify the rainfall station location or centroid location defining it .

Best wishes !

 

[Maraligaf]

Hi,

Thank you for your answer but I don't think this is the problem. I have used two precipitation methods and the problem remains.

First I used the "Gage Weights" method. Because it is a small watershed, I only used 1 gage point.
Then for each of my subbasins, I said "yes" to "Use Gage" for this gage, and "1" for the "depth weight" and "1" for the 2Time Weight".

I also tried with a hypohetical storm (SCS Storm with "Type 1").

Did I do something wrong or the meteorologial model is not the problem ?

 

[Maraligaf]

I understood my problem. I have changed the parameters of my loss method (again but much more), for instance the constant rate for the Initial and Constant rate model, and I now have "real" excess precipitation.

I just don't understand why this rate has to be so low (16 mm/h for a 24h-30mm precipitation) while the technical manual suggests that it should be estimated by the saturated hydraulic conductivity. I'll see when i will have observed data...

Thnak you for your help anyway !

maraligaf