The two most important factors in the mass balance equation are Precipitation and Infiltration. At some point the soil will be saturated and the rain will all be contributed to runoff.
Stage is the level, in feet or meters, about some reference point.
The picture below shows the Stages for the 1993 Mississippi River flood at St Louis. The factors in the mass balance equation are not constant with time. This leads to another important concept in floods, the time lag between the precipitation and the peak discharge. The time lag is important for flood hazard mitigation. The longer the time lag the more time to prepare.