I have one hydrograph close to the watershed outlet from which I can extract a baseflow value. But what values would I assign to all the other subbasins?
The HEC-HMS user manual says the following: " When there are multiple subbasins above the observed flow location, a ratio can be applied to the observed flow to calculate the baseflow initial condition for each subbasin. One approach to determing the ratio is to consider the subbasin area and the drainage area at the observed flow location. "
Can someone explain to me how to calculate this ratio exactely, maybe with an example?
I've solved this the following way (correct me if I'm wrong): I divided the baseflow value that I have by the drainage area of this point and then used the resulting value for all subbasins as initial discharge per area.
Yes, that is what you would do to use this approach. The baseflow per unit area is in many cases at its highest near the outlet, but factors as evapotranspiration, irrigation, beaver dams, diversions, coarse hyporheic substrates can influence. Baseflow is not a static value, so when the soils are relative wet after a high rainfall, runoff event, baseflow is going to be higher, than after an extended dry period. It’s too bad modelers use this term and it may be important to review their instructions and definitions of how they are intending it to be applied. it might have been better if they used an example employing a flow duration curve, perhaps suggesting a baseflow desired was a value exceeded for example 50% of the time. If you are separating the baseflow value from a hydrograph, there are methods to do this using the curve flattening inflection point.
But using the suggested method, if you had a watershed or basin outlet that is known to have baseflow of 100 cfs (cubic feet per second), using storm hydrograph separation, and the drainage area is 10 square miles, your unit area baseflow is 10 CSM (cubic foot per second per square mile). This value can then be applied to any other sub basin or subwatershed, for example 16 sq miles would have a baseflow of 160 cfs. This approach could just as easily be used with International units.
At some small drainage area size, for example ephemeral streams, there is basically no baseflow. A substantial portion of any drainage, watershed or basin has ephemeral flow or only surface runoff or no runoff. But I would stick with the unit area approach as described unless you intend to work at small ephemeral catchment scales, where baseflow should be 0.
I suggest you to multiply those with a factor considering the drainage area relative to the catchment you have the observed hydrograph
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