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feeling misunderstand: Slope Length Factor in SDR intermediate results

Dear, 
I have been running the SDR Model and get good results, but this time I am challenged with Governmental agency the credibility of InVEST model to estimate soil loss. The two projects were running in parallel using almost the same input but both models (GIS) are predicting different the soil loss (here I am defending InVEST). Thus we are aligning inputs and see how final results are generated in each model ( Both model quoted LS calculated based on Desmet 1996). As InVEST generated intermediate results, I am looking at the Slope length raster from intermediate results. I found a raster with LS, which I am assuming to be the LS factor. But when compared with the one they calculated in ArcGIS model, It seems to be too different and InVEST seems to over-estimate this LS factor. I am doubting my knowledge on intermediate results of SDR whether this raster (LS) is representing real LS in RUSLE equation of this LS (raster) is only Slope length not combined with steepness. 

I have tried to calculated LS using their approach (GIS) and I have found almost similar results (LS) with their LS so, your assistance is highly needed to be able to defend InVEST model algorithm behind the SDR model.

Thanks 
Evariste 
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