Clarification on River TN Variable and Units in SWAT+ Toolbox v3.0.9

[Fahad Hasan]

Hi Everyone,

I am using SWAT+ Toolbox v3.0.9 and have a question about River TN.

My channel_sd_month output contains orgn_out, no3_out, nh3_out, and no2_out (all in kgN), but there is no tn_out variable. I manually calculate simulated TN as:

TN = orgn_out + no3_out + nh3_out + no2_out

For observations, I have monthly TN as both mg/L (TKN + NO₃+NO₂) and kg N/month (using observed USGS flow).

However, when I import either the mg/L or kg N/month observations into SWAT+ Toolbox as River TN, the plotted simulated values do not match my manually calculated TN, and the graph looks completely different.

Could someone clarify:

1. How is River TN calculated internally in SWAT+ Toolbox?
2. Which output variables are used?
3. What units should the observation file for River TN be in (mg/L, kg/day, kg/month, or something else)?

Thank you!