Are there some basic methods how to calculate surface discharge?

Yes, one of the most basic is so called SCS CN method. The method was developed in US by their Soil Conservation services but recently serves as one of the widest used methods over the word. The method could be characterized as the empirical model of precipitation - run-off process of small watersheds. The abbreviation CN means curve number and every CN represent specific characteristics of the watershed. With the knowledge of real values of relevant watershed characteristics, the CN is obtained and used for surface discharge calculation.