SDR model

Hi, There is a doubt on the results obtained after running the Sediment Delivery Ratio model. SDR model was run to obtain results for ten number of sub-watersheds and out of that for seven number of sub-watersheds, the sediment retention and sediment export values were obtained as zero values.  
I will be very thankful if any comment can be made on whether there is any issue regarding to this results and any reasons regarding to this matter. The results have shown below:

ID,N,10,0 GRIDCODE,N,10,0 ws_id,N,5,0 sed_retent,N,24,15 sed_export,N,24,15 usle_tot,N,24,15
1 9 10 2746743.96792412 5474.4909853687 154440.675036703
2 8 9 3963749.40184605 26213.8036206842 462011.900261286
3 7 8 513417.816145003 954.1657695714 18019.290820431
4 6 7 0 0 620748.306815928
5 5 6 0 0 67181.3941711401
6 1 2 0 0 44373.0021965461
7 2 3 0 0 165970.855639844
8 3 4 0 0 300009.314385469
9 0 1 0 0 693109.551537711
10 4 5 0 0 466055.987595842


  • swolnyswolny Member, NatCap Staff
    Hi @ramonadias -

    In your table, the subwatersheds that are missing sed_export values do have USLE values. I've seen that before when a watershed does not have any streams defined. The model works by first calculating potential erosion from a pixel (USLE) and then calculates export by considering the path between that pixel and the nearest stream (sed_export.) If there is no stream defined, there is nowhere for the potential erosion to flow to, and so sediment export is also undefined.

    So look at the stream.tif output from the model and see if there is a stream defined within the subwatersheds that have 0 values for sed_export and sed_retent. If there is not, then you'll need to think more about the Threshold Flow Accumulation input to the model. If there are streams defined, then it's a different problem that we'll need to figure out.

    ~ Stacie
  • Hi, 
    In the stream.tif output from the model only shows a stream in the area which I have obtained sed_retent and sed_export values. And also in the sed_retention.tif and sed_export.tif output maps only show the watershed areas which I have obtained  sed_retent and sed_export values.

    Therefore what should be done inorder to define streams and to obtain missing  sed_retent and sed_export values.

    Thank You.
  • swolnyswolny Member, NatCap Staff
    As I mentioned before, the Threshold Flow Accumulation (TFA) input to the model is used, along with the DEM, to create the stream network. It defines how many upslope grid cells must flow into any particular cell for it to be considered part of a stream. Higher values of TFA mean that a larger upslope area is required to create a stream, and it will produce a stream network with fewer tributaries. Similarly, lower values of TFA produce a stream network with more tributaries. 

    The "correct" value of TFA is the value that produces a stream network that looks as close as possible to the real-world streams in your watershed. This will be different for each watershed. So find a real-world stream map, and try out different values of TFA until the stream.tif looks as close as possible to your real stream map. You can use the InVEST tool RouteDEM if you want to do this outside of the SDR model.

    Now, even after doing that, it may be that some of your sub-watersheds have a very flat area where streams are not easy to define, and where you would need to use a very small TFA, which produces unrealistic streams in your other sub-watersheds. This is a little harder to work with, and we can talk about it if needed.

    ~ Stacie

  • Hi, Thank you very much for your all  valuable comments made to tackle the issues. All those comment were really helpful to success the work.

    Thank You.
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