Dear Ender,

This is the common rational formula : 

Q =  (C *  i * A)/3.6, with Q the flow (m3/s), C the runoff coefficient of the watershed (0

the Rational method let you to estimate the peak runoff (cubic m/sec) but not  volume (cubic m). To compute the volume you should use an hydrograph (flow vs time). After Q estimation with Rational, a very rough formula you could adapt to small and impervious basins is: V=Q*3Tc*0.5 (it is based on a triangular hydrograph with peak=Q and time = 3*Time of concentration).

I think NRCS methodology or SWMM could be more suitable to have a better estimation, have a look to the links!

http://www.nrcs.usda.gov/wps/portal/nrcs/detailfull/national/water/?cid=stelprdb1042901

http://www2.epa.gov/water-research/storm-water-management-model-swmm

Anuga 2d modelling system allows to simulate floods processes using DEM information. As result you will have a pretty good estimation of flood areas an volumes. Please see the link for more information.

http://sourceforge.net/projects/anuga/

http://en.wikipedia.org/wiki/ANUGA_Hydro

http://anuga.anu.edu.au/wiki/InstallViewer

You can compute the excess rainfall and multiply it by the basin area.

Excess rainfall can be computed by SCS-CN method, or by the runoff coefficient

First of all, thanks to Carlo, Efrain Antonio and Yannis Ferro for their participation and answers.

1) Like Efrain mentioned above rational methods outcome is only the peak runoff. If i use peak discharge as a multiplier of Tb(base length of the triangle hydrograph) than the volume would be unrealistic. The volume would be too much because if i dont devide the TbxQp with a constant the hydrograph will not be triangle no more it would be a rectengular. 

2) At this point the ration (0.5) Efrain has mentioned turns the rectengular hydrograph into a triangular hydrograph. My real problem here is to determine the length of the base of the triangular.( I will use 3xTc and evaluate the outcome). In the formule that i am using the Tb=Tcx100/14. And than with the result we multply the qp. In this method we assume that the hydrograph consists 1196,5 unit square

3) Mr . Carlo we are using SCS-CN method too of course but we are having problem with only Rational Method.

THE ABOVE DISCUSSION IS OK. BUT AS INTENSITY OF RAINFALL VARIES BOTH TEMPORALLY AND SPATIALLY, THE FORMULA SHOULD BE USED WITH CAUTION LIMITING IT FOR WATERSHEDS WITH AN AREA NOT EXCEEDING 1300 HECTARE (I.E 13 SQ. KM). THANKS, PROF ARNAB SARMA 

HEC-HMS (Hydrologic Engineering Corps - Hydrologic Modeling System) is free and a very good, stable and updated package from the USA Corps.

The software include all the methods listed in the previous message (SCS-CN, triangular hydrograph and so on).

This is the standard in the world today.

http://www.hec.usace.army.mil/software/hec-hms/

I recommend that you work with HEC-HMS. I think that the better option for you.

Regards !

Dear Ender Dagdelen; Here are few links may be useful to you.

Thanks

http://agris.fao.org/agris-search/search.do?recordID=SO2005100383

http://link.springer.com/article/10.1007/BF01543085

http://www.sciencedirect.com/science/article/pii/S0022169401004887

All above discussion is useful. To make things easy, divide your watershed into small subbasins to be able to use the Rational Formula. Route the triangular  flood hydrograph from one outlet to another outlet downstream (e.g., using Muskingum method). The resulting hydrograph shall be trapezoidal shape. Give the time base and shape of hydrograph you can calculate the volume.

Remember to include any other  abstractions if your storm duration is long (greater than 24 hrs)