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 an idea how I can realize that and calculate the Riverrunoff for the points?

• Calculating runoff from precipitation over a catchment basin is a complicated task. There are many additional factors beyond the drainage area and total precipitation: infiltration, slope, evaporation, manning coefficient, etc. You probably should be looking at models such as HEC-HMS. Feb 25 '15 at 7:34
• I dont need to calculate the precipitation over a cathcment basin (calculation without basins) because i use a constant infiltration
– Tim
Feb 25 '15 at 12:09

What about using r.watershed with the flow input parameter?

r.watershed elevation="elevation" flow="runoff" accumulation="flow_accumulation" ...

For the flow input you need to provide a raster map representing amount of overland flow per cell. For a simplified approach assuming no groundwater flow etc. you can use your generated runoff map (precipitation-evapotranspiration). The output accumulation map provides than an estimate of the accumulated flow from the entire upstream area for each cell.

Of course you can use other calculation steps before to correct for groundwater infiltration per cell if you have that information available. Moreover with that tool it is also not possible to calculate runtimes (i.e. how long it takes for a flood to move downstream etc.).