I showed the outputs of the model to an agronomist and he asked me why the areas with high values of soil loss (usle.tif) also have high sediment retention values (sed_retention.tif). According to him, the most intuitive would be that areas with high soil loss had low sediment retention.
I am wondering why the two rasters are so correlated. Do you know? I have run the SDR model to different regions and the positive correlation are the same.
Maybe areas with high sediment loss have more sediment being retained simply because there are more sediments being generated… but not sure about it.
I am evaluating the impact of loss of native vegetation using a baseline scenario (land use map of 2020) and a deforestation scenario. I am compering usle.tif and sed_retention.tif between these scenarios to evaluate the impact of land use on soil loss and sediment retention, respectively.
Please refer to the User Guide for help interpreting InVEST model outputs. We also recommend searching this Forum for other posts related to SDR results.
If you are interested in understanding which places on the landscape are retaining sediment, we recommend using the sed_deposition_[SUFFIX].tif output of the model, not “sed_retention_[SUFFIX].tif” . “sed_retention_[SUFFIX].tif” compares the current landscape to a landscape that is entirely bare ground and does not quantify retention from upslope.
“sed_deposition_[SUFFIX].tif” quantifies the amount of sediment deposited on the landscape as it moves downslope toward streams. Another approach to evaluating the retention service is to compare “sed_export_[SUFFIX].tif” between your baseline and your deforestation scenario. The difference between “sed_export_[SUFFIX].tif” outputs reveals how much more/less erosion is retained by the scenario landscape relative to the baseline. Lastly, if you’re looking at watershed or sub-watershed scales, check out the ratio between “sed_export” and “usle_tot” in the “watershed_results_sdr_[SUFFIX].shp” output shapefile to see what percentage of erosion is being retained in each watershed.