High peak flows and unexpected percolation behavior

[Gopal Penny]

Hi All,

I'm trying to calibrate my SWAT+ model and having a difficult time reducing peak streamflow events and increasing baseflow. Others seem to have had a similar problem where streamflow peaks are too high and there is little-to-no baseflow (see https://groups.google.com/g/swatplus/c/i5fKwTL-yHk/m/0gGTDVskAwAJ).

In trying to figure out where the problem is, I selected a particular HRU (141) and ran through the calculations in SWAT 2009 Theoretical Documentation. Given the soil parameters in HRU 141 (2 soil layers), there should be a maximum daily percolation of about 125 mm from the top soil layer (top 30 cm) and 276 mm from the bottom soil layer (next 100 cm). The soil parameters and derivative values (i.e., FC, porosity - n) are below: 

     Dp      Bd   Awc  porosity      k  clay     WP     FC   sw_max       TT perc_fraction max_perc

   [mm] [t/m^3]   [1]       [1] [mm/h]   [1]    [1]    [1]     [mm]      [h]         <dbl>     [mm]

1   300     1.0  0.11 0.6226415  35.65    24 0.0960 0.2060 186.7925  3.50610         0.999 124.8594

2  1000     1.1  0.11 0.5849057  21.84    26 0.1144 0.2244 584.9057 16.50667         0.766 276.2758

In the simulation, SWAT+ outputs a maximum daily percolation much lower -- around 2.2 mm (green line, middle panel of attached figure). Such low percolation causes the soil layer to fill up (purple), and curve numbers (red) are approaching 100 (bottom panel of figure) when this happens.

In order to test the sensitivity of percolation, I set soil_k to be 1000 mm/hr for all soil types using the calibration page of the SWAT+ Editor (i.e., calibration.cal). I expected percolation to increase, but it does not. However, water yield for the HRU seems to have decreased noticeably (magenta line in the top panel, comparing the two figures).

Note that one figure is with the original soil_k values, while another is with soil_k set to 1000 mm/hr, indicated in the filename as "_k1000".

Any help with this would be appreciated! Thank you,

Gopal

[hydjber]

Hello,

I have the same problem (much lower percolation and increased soil water). But in my case this happens when using tile drainage:

https://groups.google.com/g/swatplus/c/hGh1vPvDKIA

Does anyone have a solution to this?

Thanks,

Jenny

[Gopal Penny]

My solution to this problem was to set the PERCO parameter to 1. I’m not sure what the intended purpose of this parameter is, but it seems to generate a nonlinear response with respect to percolation.

https://groups.google.com/g/swatplus/c/g16CVsPdZak/m/2Y3E8FbTAAAJ