BOD and organics in phase 6
In the HSPF river simulation, BOD and phytoplankton are simulated as constituents. There is no other
tracking of labile organics so these are considered to be the representation of labile organics in the
stream. There is no phytoplankton input from the EOS simulation, so labile organics must be entered
into the river simulation as BOD. Since the N:P ratio for BOD is set in the river simulation, the N:P ratio
for labile organics are set as well. Refractory N and P are tracked as separate constituents in the HSPF
river simulation.
The molar ratio of N:P is set at 16:1 as the HSPF default. This makes for a fixed ratio of
( 16 mols N / 1 mol P ) * ( 14 g N / mol N ) * ( 31 g P / mol P ) = 7.23
for labile organics
The AGCHEM simulation of phosphorus considers all organic P as subject to mineralization and does not
have a desorption routine or dissolved organic pathway. Organic P was not tracked and was always
calculated as a 1/7.23 of the labile organic N and 1/72.3 of the refractory organic N. Phosphate targets
were generated by subtracting the organic P values based on organic N targets from the total P targets.
This method caused difficulty in BMP tracking since each P BMP also effected N and N BMPs effected P.
In phase 6, we can simulate all constituents independently on the land side. We will have separate
simulations of labile N and P, and refractory N and P. When they get put into the river, we can do the
following:
If LORN < 7.23*LORP
BOD = 22.9 * LORN
RORP = RORP + (LORP-LORN/7.23)
Else
BOD = 22.9 * LORP*7.23
RORN = RORN * (LORN-LORP*7.23)
EndIf
The 22.9 factor in the above comes from the molecular weight of P and N plus the following HSPF
defaults
BPCNTC = percentage of biomass which is carbon = 49
CVBPC = factor to convert P to C
= 106 mols/mol
CVBPN = factor to convert P to N
= 16 mols/mol
CVBO = factor to convert biomass to oxygen = 1.98