Traditionally torrential rains are considered as the main factor of flood emergence. With the examples of two disastrous floods in 2015 in absolutely different parts of the world, the authors roughly estimate the water balance and suggest an alternative hypothesis. The simplest model, taking into account precipitation, evaporation and soil permeability, clearly points out the significant discrepancy between potentially accumulated and observed water masses. This observation pushes the idea that precipitation is necessary but not sufficient for disastrous flood emergence, so the only other available water source—groundwater—cannot be ignored.