Submodels for simulation of the effect of soil water regime (GLOBAL) and grain yield of maize (ZeaMays, L.) CORNY can be modified and then used simultaneously as a model CORNWAY to estimate relative plant yields. The first model (GLOBAL) is a mathematical deterministic model which simulates water transport in a soil profile during the vegetation period of crops. Model CORNY is semiempirical model for calculating the continuous reduction of the potential yield of maize crop due to soil water deficit during the vegetation period. The most important factor, among others, is the water suply of canopy. Version of CORNY submodel as used here is focused on the role of water in yield reduction. It is supposed that other factors did not limit this process.