The Befani-Gopchenko model is suggested to calculate the maximum water expense in the absence of hydrological observations. The model is a combination of reduction, dimensional and critical intensity methods used in normative practice in the majority of world countries. Statistical analysis of time-series observations is applied, and the methods to define parameters and coefficients in the model are suggested. Their spatial distribution corresponds to general geographical properties of hydrographic characteristics in the mountains and is offered to calculate the maximum flood runoff. The applied method contains useful information to help prepare regional normative acts to define major constructive components of hydrological estimations of maximum water runoff.