# The calculation of the total phosphorus and total nitrogen

Following the EE user mannual, TN = AlgaeRatio(N:C)(CHC + CHG + CHD) + RON + LON + DON + NHX + NOX, TP = AlgaeRatio(P:C) (CHC + CHG + CHD) + ROP + LOP + DOP + P4D. When I extracted all those indictors and calculated TN, TP as above formulas, the result of TN was same as the EEMS10.2. However the result of TP was greater with that in EEMS10.2, as shown in fig1.
Question(1): Did I use the formulas in a wrong way?
Question(2): In page 165 of EFDC_Theory10.2, “A mean ratio for all algal groups APC; is described by an empirical approximation to the trend observed in field data (Cerco and Cole, 1994):”. It is mean that EE use a mean ratio APC for all algae groups. Is (CHC + CHG + CHD) in TP formula means the average of those three algae? Because when I calulated as TP = AlgaeRatio(P:C)*average (CHC + CHG + CHD) + ROP + LOP + DOP + P4D, the result of TP is same as that in EEMS10.2.

Hi Hangq,

You are using the correct formulation to calculate the total phosphorus.
TP = AlgaeRatio(P:C)* (CHC + CHG + CHD) + ROP + LOP + DOP + P4D

For the difference in the results obtained, could you please check again the calculation of APC. It should not be a constant for the whole model domain but varies as a function of PO4 as:
APC = 1/(CP1 + CP2 * exp(-CP3 * PO4))
Could you please give the value of the three constants CP1, CP2, CP3 used in your model, so we can also check it?

Thank you,
Kien Tran

The value of CP1, CP2, and CP3 are defined as 42, 85 and 200. And I caculated the APC as 0.02381 based on the formula in EE Theory 10.2.
But I noticed a difference of the APC function between your reply and EE Theory 10.2 (as shown in fig1). I was confuesd about exp(-CP3prmpo4d) due to “prmpo4d” tagged in subscirpt. Which one is right or I understand the APC formula in a wrong way.
Your anwser indicates that APC is varies value. APC varies in spatial or temporal scale?

Hi Hanq,
We are sorry about the error in the APC formula in EFDC theory 10.2.
We have corrected it in the releasing EFDC theory 10.3 as:

So, APC varies in both space and time with the concentration of PO4.
With the value of CP1, CP2 and CP3 defined in your model and the PO4 concentration calculated, the APC is about 0.007874.
Sincerely,
Kien

Hi Kien Tran
It is appreciate for you support. I have correct the formula and get same result as EE.
Thank you.
Han Quan