[SWAT-user:2220] Water balance equation

[Jonathan Mota]

Dear swat users.                                                                                             

It seems to me that the water balance equation (  SW_t = SW_o +  SUM (R_day +  Q_surf - E_a - W_seep - Q_gw  )  ) from SWAT 2005 Theoretical Documentation (SWAT 2005 theory final.pdf) is incorrect, as it includes  Q_gw and  W_seep.

According to the manual Q_gw is obtained from the water in the shallow aquifer and this in turn is obtained from W_seep.
I think that W_seep includes Q_gw so its presence in the balance equation may be incorrect. Am I right?

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[Yuzhou Luo]

Hi Jonathan,

I also noticed and investigated this problem. My quick answer is, this equation is not correct or not clear, but the algorithm used in the SWAT coding is correct.
SWAT calculates water mass balance based on layers, such as soil horizons, shallow aquifer, and deep aquifer.
For each layer, the mass balance equations are slightly different because different processes are involved.
The equation shown on the manual is a general form of those layer-based equations. This equation may not be directly applied for actual simulation, but provide general ideas of how SWAT deals with the water mass balance.

Hope this helps.

=====
Yuzhou Luo, Ph.D., EIT
Research Follow
Department of Land, Air,  and Water Resources
University of California, Davis, CA 95616

[lei qiao]

Hi Yuzhou and others,

I think the Qgw is acutually is the lateral discharge from soil. beside, -Qsurf, not +Qsurf.

--
QIAO Lei (Charlie)
3642 Lindell BLVD
Earth and Atmospheric sciences
Saint Louis University
Saint Louis, MO 63108

[Eva Mia Siska]

Dear SWAT Users,

I wish to put back the discussion of water balance equation in SWAT Theoretical Document 2009.

SWt=SW0+∑(Rday-Qsurf-Ea-wseep-Qgw)

I wonder if Qgw here supposed to be Qlat?

Should the focus is the change of soil water as the case in this equation (SWt-SW0), then the remaining part of the equation should be difference between flux-in and flux-out from soil layer which does not include Qgw rather Qlat.
It would be nice if someone can clarify if my understanding is wrong.

Best,
Eva