Conversion of the subbasin output SURQ from mm/day to m3/3

[Heou Maleki]

Dear All,

I am trying to convert the surface runoff at a specific outlet within a catchment and would like to ask for your support for clarification. For simplicity, I have taken the figure from the QSWAT tutorial that shows a catchment with 9 subbasins as follow because I have many in my catchment:

Assuming the blue symbol is where my outlet is and refers to subbasin 9. I want to have the surface runoff contrinution to the streamflow at that point.

There are simulated flows in m3/s in the output.rch file and I can extract the flows of the outlet that correspond to reach 9. However, the surface runoff (SURQ) is provided in mm/day in the output.sub file for each subbasin. For the conversion, I need to use the the area.

From my understanding:

- the SURQ for subbasin 9 is the surface runoff contribution to streamflow only for the subbasin 9. So if I use the area of the subbasin 9, I will get the surface runoff in m3/s for subbasin 9 and not for the whole catchment.

- the flow in m3/s in the reach 9 includes all the outflows from upstream subbasins which in turn include the surface runoff for each individual subbasin (each subbasin flow in m3/s includes the subbasin's surface runoff).

- in order to get the surface runoff contribution to the simulated flow at the outlet, I need to convert all the surface runoff of each individual subbasin using their respective areas and then sum them up all to get the surface runoff contrinution of streamflow in m3/s within the catchment. If there is another gauging station inside the catchment, I will do the same by considering all upstream subbasins.

Please could anybody confirm if I am right with this. Otherwise, I would be very grateful if you can advice.

Thanks in advance.

Heou Maleki

[Natalja C.]

Hello,

Not entirely sure I understood you, but:

point 1. Yes, but keep in mind the temporal scale. You have SURQ in mm/day, and you want m3/s.

point 2 - yes. 

point 3 - not sure. I think the surface runoff is already computed at the hru level, Anyway, you try that experiment and see if the numbers make sense :) 

Best,

Natalja

[Heou Maleki BADJANA]

Hi Natalija,

Many thanks for your reply.

For the conversion, I use the following formulae:

SURQ(m3/s) = SURQ(mm)*AREA(converted in m2)/(86400*1000)

I have tried summing up all the converted SURQ for upstream subbasins and realised that the results in m3/s is completely higher than the FLOW_OUT of the corresponding reach extracted from output.rch.

I tried the SURQ_CTNmm from output.hru. I calculated the sum of all the HRUs of the upstream subbasin and realised that here again, the results of surface runoff contribution to streamflow is higher than the simulated flow. 

Moreover, I have tried to look at the output.std file which provides the surface runoff at the entire watershed level in mm. And there, the SURQmm for the entire watershed corresponds just to the SURQmm of the most downstream subbasin in the output.sub file and NOT the sum of SURQmm of all the upstream subbasins. 

I am not sure if I am misunderstanding something. Any further clarification or suggestion would be much appreciated.

Thanks.

Heou Maleki

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[Jim Almendinger]

Just a few comments to add to Nahalja's comments:

-- Flow out of the reach could be reduced by "transmission loss" (seepage out of the stream channel), so it's possible that your flow out could be less than surface Q inputs.  But this is highly unlikely (unless you have unrealistically high channel K values). 

-- The basin-wide SURQmm value would be the average of the SURQmm of the contributing subbasins (weighted by the area of each subbasin) -- it would not be the sum.  It's entirely possible that the basin-wide SURQmm happened to be similar to the SURQmm of the lowermost subbasin. 

-- Just to confirm: you are dealing with daily output, right?  Not monthly or annual?  When you get a SURQmm in the output, I'm just not sure how many time units were averaged to get that value.  

Cheers,

-- Jim

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[Heou Maleki]

Hi Jim,

Thanks very much for your continuous support.

- For your second point point, I hope you mean computing the average (weighted by the area of each subbasin) like when using the Thiesssen polygon method? I did it and got exactly the same values of SURQmm as those in the output.std file for the basin wide. 

Based on this, I converted the SURQmm for each subbasin into m3/s first and then computed the average (weighted by the area of each subbasin) and it is now less than the FLOW_OUT in m3/s in the output.rch file.

Yes I can confirm that I am dealing with daily output.

Again, thanks so much.

Cheers,

Heou Maleki

[Jim Almendinger]

So, that's good, right?  The total SURQ should be less than the FLOW_OUT, because the FLOW_OUT also includes lateral Q and groundwater Q (and tile Q, if you have tile). 

So does that answer the concern?  Or was there something else?

-- Jim

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[Heou Maleki BADJANA]

Hi Jim,

Yes, that's right. That's the solution to my concern. Nothing else.

Thank you so much!

Kind regards

Heou Maleki