Evaluating Nutrient Retention Services

Hi everyone, I am now to the InVest plateform and I have a question regarding the output of the Nutrient Delivery Ratio model. In the user guide they mention the following:

`> The NDR model does not directly quantify the amount of nutrient retained on the landscape. However, if you have scenarios that are being compared with current conditions, the nutrient retention service may be estimated by taking the difference in nutrient export between the scenario and current conditions. This quantifies the difference in nutrient reaching a stream, based on the changes in land cover/climate/etc present in the scenario, which provides a way of evaluating impacts to downstream uses such as drinking water.

To calculate per pixel nitrogen retention services within a single scenario, we recommend subtracting n_total_export.tif from the modified_load_n.tif result located in the intermediate output folder. Similarly, per pixel phosphorus retention services can be calculated by subtracting p_surface_export.tif from modified_load_p.tif. Use the .gpkg output to quantifty watershed scale nutrient retention services by subtracting the n_total_export result from (n_surface_load + n_subsurface_load) for nitrogen and p_surface_export from p_surface_load for phosphorus.`

What I am not sure it that if I want to calculate the amount of phosporus that is retained by my different scenarios it mention in the user guide to use p_surface_export.tif - modified_load_p.tif. So, I understand that I need to use the p_surface_export.tif generated by my scenarios , but should I use the modified_load_p.tif found in my scenarios intermediate folder or the one found in my baseline ? Or maybe both are the same thing? I am confuse about this so any clarification would help.

Hello @CathD -

1/ If you want to calculate the amount of retention for each individual scenario or baseline, then you will use p_surface_export.tif and modified_load_p.tif both from that individual scenario or baseline model run.

2/ If you want to calculate the change in retention between baseline and scenarios, then you could still calculate retention for each individual scenario as described in #1, then calculate (baseline_retention - scenario_retention) to see the change. We have also simply taken the difference in nutrient export between baseline and scenario as a proxy for a change in retention due to the scenario.

~ Stacie

Thank you so much for your reply it really helps!