Exceedance probability of normal and logarithmic distribution by matlab I have estimated the parameters i.e. mean and standard deviation of a normal and lognormal distribution for my 30 years river discharge data using command NORMFIT and LOGNFIT. Now I want to determine the magnitude of peak discharge value that will be exceeded with a probability of 0.01(the 100-year flood) and the probability of exceeding of mean discharge. If anyone give me the command that I can use in Matlab to solve this problem, it will be really helpful for me.

You want the quantile function, i.e. the inverse cdf. You can do this in matlab with norminv(.99,mu,sigma) and logninv(.99,mu,sigma) where mu and sigma are your fit parameters for the respective cases. Take care to read the documentation to make sure the parameterizations are the same between your fit functions and the inverse cdfs. It certainly will be for the normal but I'm not positive for the lognormal.