An agrophysical concept for estimating the crop water regime is offered. Two biophysical indices are introduced: for energy levels of the soil moisture and for their degree of appearance during the vegetation period. An energy level classification is developed that takes into account the biophysical relationship between the photosynthetic activity and the soil moisture potential, and the essential features of the potential changes of water in plants and soils. Equivalent energy levels were defined for natural crop water regimes during the whole vegetation period and its separate stages. A simple dependence of maize yield on the index of the energy levels was experimentally established when the soil moisture is the only limiting factor.