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# Sources on Area relationship between watersheds

## Sources on Area relationship between watersheds

(OP)
Hey there,

I am working on an update for a hydrologic study of a big catchment, which was originally studied in the 90s. In the original studies, the mean daily flows for some subcatchments (e.g between river junctions or between dam site alternatives along the main river) are calculated using simple area relationships. in the style:

Qsubcatchment2 = Qsubcatchment1 / Asubcatchment1 * Asubcatchment2

I guess there is some kind of law or source where this propotionality between catchments and subcatchments is referenced or explained. Can someone recommend a source?

Anybody around here familiar with this kind od calculations? or did these guys just simplify the calculations a bit too much?

Thanks for any help!

Replies continue below

### RE: Sources on Area relationship between watersheds

Assuming that each area has the same Curve Number, the runoff volume will be proportional to the area. But getting a full hydrograph (or even the peak flow) is not a simple ratio, and will depend on other factors, especially the time-of concentration. As you suggest, they simplified the calculations a bit too much!

Peter Smart

### RE: Sources on Area relationship between watersheds

runoff volume and peak flows are also related to length of flow path, time of concentration, elevation and to aerial reductions among other things, so this method is only suitable for very rough / feasibility level analysis. It might be suitable to develop an envelope curve for minimum and maximum runoff as a function of area. These sorts of envelopes do exist, for specific regions and are useful for checking how reasonable the runoff estimate is. For instance:

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