Baseflow ratio (bfr) calculation

[Alexander N.]

Hello together,

I am currently working on getting soft calibration to work and am wondering how one of the ratios, the base flow ratio (bfr), is calculated. In file "calibration_data_module.f90" it just states the following regarding BFR:

  real :: bfr = 0.    !- or m3        |base flow ratio - base flow/precip - lat+prec+tile

Can anyone makes any sense out of this?

Best regards
Alex

[Natalja C.]

Hello,

There might be a typo: prec --> perc

base flow ratio (Base flow ratio) =

base flow/  precip            - lat                 + perc              + tile

Base flow/ Precipitation - Lateral flow + Percolation + Tile flow.

Does this answer your question? :)

I'll need to check the formula in the code.

Best,

Natalja

[Alexander N.]

Thanks for your reply Natalja!
That is what I was thinking as well and what would make the most sense. However I would appreciate if you could have a look in the source code if you are able to find an exact formula on how it is calculated, since me and my supervisor were not able to find anything about it besides the cited part above, which is just a comment at the variable decleration :) And base flow in this case is the percolation value right or where do I get the base flow value from/how is base flow calculated?

Best regards

Alex

[Alexander N.]

Oops, I reread what you wrote and I am not thinking the same.

I think base flow ratio (Base flow ratio) =

base flow/  precip            - lat                 + surq              + tile

Base flow/ Precipitation - Lateral flow + Surface runoff + Tile flow. This way it would be an ratio of the percolation (which then turns into groundwater flow and base flow) and precipitation.

Considering the water yield ratio (WYR), which is also used in the soft calibration and is described (in the old SWAT IO-Documentation) as WYLD = SURQ + LATQ + GW_Q - TLOSS – pond abstractions with 

SURQ = Surface runoff contribution to streamflow in the main channel during time step (mm H20)

LATQ = Lateral flow contribution to streamflow (mm H20). Water flowing laterally within the soil profile that enters the main channel during time step.

GW_Q = Groundwater contribution to streamflow (mm H20). Water from the shallow aquifer that enters the main channel during the time step. Groundwater flow is also referred to as baseflow.

TLOSS = Transmission losses (mm H20). Water lost from tributary channels in the HRU via transmission through the bed. This water becomes recharge for the shallow aquifer during the time step. Net surface runoff contribution to the main channel streamflow is calculated by substracting TLOSS from SURQ

it would make sense that the water yield ratio tells us what proportion of the precipitation finds its way to the channel and makes the total stream flow and that the base flow ratio tells us which part of precipitation turns into base flow.

 Everything else is not really plausible for me, but as I said, I have yet to find comfirmation :D

[Natalja C.]

I really cannot assist at the moment, I'll need to do a more thorough check. But scanning the source code you can find all the instances of "bfr" or "baseflow" or "base flow". Most of the bfr are associated with soft cal routines. 

I'll come back to this discussion once I dig up the answer :)

Best,

Natalja

[Alexander N.]

Thanks for the image! I did the same and had a look at all these files, however I did not find anything specific to baseflow and how it is calculated in SWAT. But maybe I am just overlooking/not understanding things, since I am not that familiar with the SWAT source code and fortran. I would therefore really appreciate it if you could take a look at it in time or forward my request :)

[Natalja C.]

Hello,

I have the confirmed with Jeff that: 

Baseflow here is defined as: lateral flow + percolation + tile flow. 

There are two methods in the lcu_read_softcal: to convert the baseflow ratio from a fraction of water yield or a fraction of precip. The code is not tidy yet, so it might cause some confusion :)

Hope this helps sort things out!

Best,

Natalja

[Alexander N.]

Hello,
great, now I can continue with my Master's thesis! Thank you very much, I really appreciate your help :) I wish you a good one!

Best regards

Alex