What does water yield mean? It is not similar to "total flow" or "stream flow"?

[Oscar Silva]

I can't figure out what "water yield" means in the SWAT results. In the "basin_wb" files, the water yield is the sum of surface runoff, lateral flow, and tile drainage flow. My question is: why is baseflow not considered as part of the water yield? Another question: which variable corresponds to the base stream in the "basin_wb" files. Or is it in another file? On the other hand: In the hydrology summary in SWAT Check, "stream flow" or "total flow" is the sum of surface runoff and base flow. So it seems that water yield is not similar to "stream flow" or "total flow". However, when water yield is measured at a point in the channel, all water flows (both surface and subsurface) are taken into account. And I have a problem: when I compare the simulated "water yield" with the total flow records in the channel (from a gauging station), the fit is very good. This seems like a contradiction, since "water yield" according to the "Basin_wb" files does not consider the base flow, but the gauging records do. That means that if I add to "water yield" (basin_wb file) the eventual base flow, it would be a much larger value than the value measured in the gauges. I will be very grateful for your help and considerations.

Best regards,

Oscar Silva.

Universidad Central de Venezuela.

[Sushant Mehan]

Hi Oscar, 

Water yield is the net amount of water that leaves the subbasin and contributes to the streamflow in the reach during the time step. Generally, calculated analytically as 

WYLD = Surface runoff+Lateral Discharge+Ground water Discharge+Tile Flow-Transmission losses-Abstractions like one from a pond if that makes sense. 

Thanks,

Sushant

[oscar...@gmail.com]

Hi Sushant.

 

Thanks!

 

Thats clear: wlyld = Qsur + Qlat + Qground + QTile -+ Trans losses.

 

But: Wyld in basin_wb output files appears to be = Qsur + Qlat. Ground water o base flow is not considered into water yield amount or concept.

And: the hydrology summary (SWAT Check) shows a 0.52 ratio BaseFlow/TotalFlow. Why this base flow does not account in the water yield sum?

 

Thanks again!

Best regards.

 

Oscar Silva.

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[Sushant Mehan]

Hi Oscar, 

From the source code, it looks challenging to decipher the groundwater component from wyld, except latq has some associated coefficients associated ith or affecting groundwater flow. Definitely, not straightforward. 

Thanks,

Sushant

[Julia Glenday]

Hi all!

I want to echo and add to Oscar's question about understanding the final channel outflow, vs 'water yield' and other outputs, and see if anyone else out there can assist us both. 

I have a SWAT+ model that is giving me a 'water yield' (basin_wb_day.txt file) that is far far greater than my channel flo_out once I get them both into the same units (convert channel flo_out in m3/s into mm). 

Anyone know how this can be?

I have not added significant channel seepage rates or anything like that.....

I was assuming that the 'water yield' value is what enters the channel from the surface and lateral flows from soils. 

HOWEVER it may just be the simple sum of what leaves the surface and soil of every HRU in the model? 

If I have lateral flow and some surface flow from my upland LSU HRUs entering my riparian LSU HRUs, rather than going straight to the channel, this water could get further delayed and subject to ET or recharging the riparian aquifer. 

If 'water yield' is the simple sum of all HRU outputs individually, and it does not account for the routing of water via the riparian zone, then this may explain why, in my case, the channel flow is a lot lower than the 'water yield' output.

Does anyone know if that is actually what is happening? If so, it would be helpful to clarify that in the documentation.

In answer to Oscar's question on groundwater contributions to streamflow and where to find these, there is a SWAT+ output file aquifer_day.txt which has aquifer to channel flows in it.

However these values are in mm and it is not clear if this is mm as a depth over the entire basin area or mm as a depth over the area of that particular aquifer (in my case upland & riparian).

When there is seepage from the channel, it shows up in the channel_sd_day.txt file, but I don't know if that amount is added to the aquifer and also shows up in the aquifer_day.txt file amounts somewhere (as part of the 'recharge'?) or not... anyone know this?

Thanks,

Julia

[Natalja C.]

Hello all,

There is no need to assume, one has to look at the source code. Feel free to refer to the swat+ code anytime (it is open source after all) such questions occur, as most of the answers are found there.

Wateryield (output_landscape_module.f90, line 11):

real :: wateryld = 0.         !mm H2O        |water yield - sum of surface runoff, lateral soil flow and tile flow

Regarding seepage, I had trouble following the question, but I think this piece of code will give you a hint of what is goin on (hru_control.f90, line 283-290):

 !! compute effective rainfall (amount that percs into soil)
        if (ires > 0) then
          !! for wetland use seepage into soil from ponded water
          inflpcp = hru(j)%water_seep
        else
          !! no wetland (no ponded water)
          inflpcp = precip_eff - surfq(j)
        end if

All the best,

Natalja