Calculate River Runoff Volume (Q) with GRASS

I want to calculate Hydropower between points at a River. I already created a Rivernetwork and the Accumulationmap (without weighting) with r.watershed. My next Step was to set equivalent points along the River and find out their elevation -> done. So now I want to calculate the Rivervolume for each point (along the River). For this purpose I downloaded Precipitation data. My Idea was to multiply the Accumulationmap with the Cell Size and the Precipitationmap (Previously, the card was processed that only the outflow remains on each cell (Runoff=Precipitation-Evapotranspiration)). But I´m not sure if that makes sense because the Accumulation says only how much Cells flow in but not which one (and there are Different Runoffs from different cells -> some also negative). Do you have a Idea how I can realize that and calculate the Riverrunoff for the points? Thanks in advance.