Use drainage area ratios to calculate the flow in the model

I have seen in many papers and reports that drainage area ratios has been used to find out the inflow in the system. I am not quite sure how to use this concept. Would somebody explain to me this concept using the example ? Also could you please explain this sentence “Balance of inflows computed from mass balance then distributed by drainage area ratios.”



Though a simplification of the hydrologic system, you may calculate an unknown inflow (Q1) by using the ratio of a known drainage area (A2) and corresponding inflow (Q2) in that system ie Q1/A1=Q2/A2.

The sentence you are asking about means: carry out a mass balance, ie inflows and outflows should be equal, and then use the ratios of known drainage areas and inflows to estimate any unknown inflows from their known drainage areas.