Excessive sediment yield from HRUs with lowermost SOL_K = 0

[Jim Almendinger]

Just a cautionary tale:

If the lowermost SOL_K = 0 in some of your HRUs (as might be presumed to be the case when bedrock is the lowest soil layer), that HRU can produce huge amounts of sediment during wet periods.  Setting SOL_K to almost anything >0 (like, 1 or 2...) will greatly reduce overland runoff and soil erosion.  I would argue that most shallow bedrock has enough fractures to have at least some finite effective permeability. 

I found this out when my current model results were telling me that deciduous forest was the dominant source of sediment, in a landscape dominated by agriculture, which is crazy.  I narrowed it down to one soil type, in one steep subbasin, where annual sediment yields from forest were over 100 t/ha (10-year averages).  I changed the lowermost SOL_K value to match similar shallow soils over bedrock, and the results settled down to something more reasonable.  Sediment yields are still significant -- it is shallow soil over bedrock -- but not crazy-high. 

Cheers,

-- Jim

--

James E. Almendinger, PhD

Director, St. Croix Watershed Research Station

Tel: 651-433-5953 x 11

Email: jalmen...@smm.org

[lei qiao]

Hi Jim,

Nice point. Why increase sol_k works? Because it increase infiltration and percolation? Which runoff method are you using? If it is CN number, it seems decreasing CN works more directly. Interesting point. 

Thanks again,

Lei 

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[alexio...@gmail.com]

Dear Swat users,
following your  conversation concerning the sol_k parameter... I have a small watershed, where there is a huge amount of sediment coming  from HRUs that were not expected to produce this amount of sediment (erosion should be zero according to  fieldwork) . What parameters should I alter, in order to have zero erosion? I have already tried the Usle_k set to 0.001... it did not work.

Thank you  for your help.

Best regards

Simoni