Simulation models like HSPF or SWAT can be used to model hydrologic processes over several years. Once this is calibrated to flows, you could use scenarios of extreme in weather and other inputs to study hydrologic extremes at watershed scale.  

Thank you Timoty, would you explain how i can use the the extreme scenarios in modeling? is that you mean i can use extreme events as an input  in  SWAT?

We have done some work where we calibrated a rainfall runoff model to either wet or dry conditions and used these parameterizations as surrogates for extreme conditions. The paper is currently under review here:

http://www.hydrol-earth-syst-sci-discuss.net/hess-2016-7/

Depending on the data availability you can either use observed climate data series (with extremes) as input into your hydrological model (e.g. SWAT) or you might generate synthetic climate series based on statistics of observed climate using a synthetic weather generator. The first approach might be useful when the observed climate series is (much) longer than the observed discharge series used to calibrate and validate the model, the second approach is useful when the observed climate series has a similar length as the observed discharge series and has been used for calibration and validation of the model. Typical examples of synthetic weather generators are alternating renewal models, time series models (e.g. ARMA and Markov chains), point process models, disaggregation models and resampling models.

Thank you dears ,for your valuable comments and ideas are very interesting and important, would you add some reference which help me?

One recent example of a review of stochastic rainfall generators: Chen J, Brissette FP (2014) Stochastic generation of daily precipitation amounts: review and evaluation of different models. Climate Research 59:189-206.

and a somewhat older review: Wilks DS and Wilby RL (1999) The weather generation game: a review of stochastic weather models. Progress in Physical Geography 23: 329–357.

Good luck,

Martijn

Thank you Martijn for your following and valuable comments too.