RE: sediment yield vs sediment loss in SWAT

[Kerry Mapes]

Hello, maybe someone can provide me with some insight here. I am interested in assessing watershed-level sediment loss due to erosion in my study area for a pollution risk assessment. Many papers I have read are considering sediment yield only, though in my case I am not sure it is the best variable to look at. The reason I think so is that in my study area, pollutants will often sorb to soil particles, which may in turn settle from the water column but are still a potential source of pollution; sediment pollution itself is not really a concern because a lot of our soils are very sandy and will not remain suspended or do not have a lot of erosion potential in the first place. I guess I would like a little more insight into sediment yield (SEDYLD) and sediment loss (USLE) and what the benefits or drawbacks would be to using each. USLE would estimate a long-term annual soil loss, and this is what I think I am leaning towards as the purpose for this model is to guide watershed managers in management decisions and conservation practices rather than quantify a specific amount of sediment pollution in the water column. Thank you for your expert advice,

Kerry

[Kerry Mapes]

Also, is there any difference between SED YIELD (watershed scale sediment yield) and SYLD (HRU scale sediment yield) other than the scale that they are calculated at?

[Kerry Mapes]

Also when I said watershed, I should clarify that I want the amount of sediment transported to the channel for each sub-watershed of the whole area that I am modeling.

[Natalja C.]

Hello Kerry,

I think what you are looking for can be found in the swat theoretical documentation, Section 4 (Erosion). These will give you the formals and the theoretical background on how the soils are eroding and how the sediment load is calculated. The associated parameters are also found there.

In Section 7, chapter 7:2 is the sediment routing (which is not of interest to you, as I understood).

The reason, why you don't find much literature is (my guess) that it is hard to validate the results due to lack of observations. In my case, there are observations on the TSS in the rivers (organics included! which is not modeled by SWAT, so you have to be careful here), but we have no data on sediment load to the channel. I just asked my colleague, who is working with sediment modeling in shallow waters, and she said that, as far as she knows, there are no tools to directly measure the sediment load to the river channel, only to estimate based on assumptions, knowledge on the settling and re-suspension, and some measurement experiments.

The difference between SED YIELD and SYLD is described in the SWAT I/O documentation. Because they are printed out in several places, it is hard for me to comment which one exactly (in which file and what temporal scale) did you mean.

Hope this helps! All the best,

Natalja

[alexio...@gmail.com]

Dear Natalia,
Could you please inform me, how to change usle_c parameter?

Best regards

Simoni

[Nancy B. Sammons]

Alexio:  This variable is read from the crop database file.  You can change the USLE_C values by opening up your projects’ master database file and in the ‘crop’ table.  Just as a kind warning, I must comment that you should be very careful in changing values in this database.  It has been tested and sometimes changing one value will cause a problem in the simulation.  I hope this helps.  Natalja may also have welcomed suggestions.

Nancy

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