SWAT Bug in Reading CHM fileIf you are dealing with more than 6 soil layers, be cautious on the water quality outputs. A bug in SWAT would have impact on the initial N/P in soil and in turn would have impact on water quality in the main channel.
Description
Initial soil N/P could be set in CHM files for up to 10 soil layers. By default, NO3, organic N and organic P is set as 0 mg/kg, which means SWAT would calculate the initial concentration based on organic carbon and depth. Soluble P is set as 5 mg/kg by default.
Soil Layer : 1 2 3 4 5 6 7 8 9 10
Soil NO3 [mg/kg] : 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
Soil organic N [mg/kg] : 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
Soil labile P [mg/kg] : 5.00 5.00 5.00 5.00 5.00 5.00 5.00 5.00 5.00 5.00
Soil organic P [mg/kg] : 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
Phosphorus perc coef : 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00 10.00
Following codes are used to read above values in readchm.f, where mlyr is the maximum number of soil layers.
read (106,5100,iostat=eof) (sol_no3(j,ihru), j = 1, mlyr)
read (106,5100,iostat=eof) (sol_orgn(j,ihru), j = 1, mlyr)
read (106,5100,iostat=eof) (sol_solp(j,ihru), j = 1, mlyr)
read (106,5100,iostat=eof) (sol_orgp(j,ihru), j = 1, mlyr)
read (106,5100,iostat=eof) (pperco_sub(j,ihru), j = 1, mlyr)
………………………
5100 format (27x,10f12.2)
The problem is on mlyr. It’s not the actual number of soil layers, which is sol_nly. It’s value could exceed 10 since SWAT adds 4 on top of it in getallo.f shown below.
!! septic change 1-28-09 gsm
mlyr = mlyr + 4
!! septic change 1-28-09 gsm
For my case, the mlyr is 14 and SWAT would try to read 14 values from CHM file for initial soil N/P. As there are only 10 values in each record, SWAT would go to next record to get additional 4 values. Thus, one read function would read 2 data records in CHM file. The values it’s reading are not the values they are set. Organic N would be 5 mg/kg rather than 0 mg/kg. This will cause much less organic N compared to the one calculated from organic carbon.
Solution
The solution would be simple: replace mlyr with sol_nly. sol_nly is the actual number of soil layers and has been used in read soil parameters from SOL files (readsol.f). It never exceeds 10.
Michael Yu 