The APSIM SYSBAL module checks that simulation mass balance is maintained. It does this by tracking values of carbon, water, nitrogen and phosphorus variables from modules in the simulation, ensuring that there aren’t unexpected gains or losses or mass, Water, N and P.

SYSTEM CALCULATIONS

loss_system =  loss_soil + loss_crop + loss_surface

gain_system =  gain_soil + gain_crop + gain_surface

state_system = state_surface + state_soil + state_crop
dlt_system = state_system – state_system_yest

error_system =  dlt_system + loss_system – gain_system

if (state_system_yest >= 0.00001) then
cum_error_system = cum_error_system + error_system

else
error_system = 0.0
endif

dlt_soil    =  state_soil    – state_soil_yest
dlt_surface =  state_surface – state_surface_yest
dlt_crop    =  state_crop    – state_crop_yest

state_system_yest  = state_system
state_soil_yest    = state_soil
state_surface_yest = state_surface
state_crop_yest    = state_crop

Carbon Balance

System states

state_crop = dm_green    * gm2kg/sm2ha * 0.4     :           + GreenWt     * gm2kg/sm2ha * 0.4
:           + dm_senesced * gm2kg/sm2ha * 0.4
:           + SenescedWt  * gm2kg/sm2ha * 0.4
:           + dm_dead     * gm2kg/sm2ha * 0.4

state_soil    = carbon_tot()
state_surface = surfaceom_c

System losses

loss_soil    = dlt_fom_c_atm()
+ dlt_hum_c_atm()
+ dlt_biom_c_atm()

loss_crop    = dlt_dm_oil_conv_retrans * gm2kg/sm2ha * 0.4

loss_surface = dlt_res_c_atm

System gains

gain_soil = 0.0
gain_crop = dlt_dm_green * gm2kg/sm2ha * 0.4
:          + GrowthWt     * gm2kg/sm2ha * 0.4
gain_surface = 0.0

Dry matter balance

System states

state_crop = dm_green    * gm2kg/sm2ha
:           + GreenWt     * gm2kg/sm2ha
:           + dm_senesced * gm2kg/sm2ha
:           + SenescedWt  * gm2kg/sm2ha
:           + dm_dead     * gm2kg/sm2ha

state_soil    = carbon_tot() / 0.4
state_surface = surfaceom_wt

System losses
loss_soil = dlt_fom_c_atm() / 0.4
:          + dlt_hum_c_atm() / 0.4
:          + dlt_biom_c_atm() / 0.4

loss_crop    = dlt_dm_oil_conv_retrans * gm2kg/sm2ha

loss_surface = dlt_res_c_atm / 0.4

System gains
gain_soil = 0.0
gain_crop = dlt_dm_green * gm2kg/sm2ha
:          + GrowthWt     * gm2kg/sm2ha
gain_surface = 0.0

Soil water balance

System states
state_crop    = 0.0
state_soil    = sw_dep()
state_surface = pond

System losses
loss_soil    = es
:             + drain
loss_crop    = ep
loss_surface = runoff

System gains
gain_soil    = 0.0
gain_crop    = 0.0
gain_surface = rain

Nitrogen Balance

System states

state_crop = n_green    * gm2kg/sm2ha
:           + GreenN     * gm2kg/sm2ha
:           + n_senesced * gm2kg/sm2ha
:           + SenescedN  * gm2kg/sm2ha
:           + n_dead     * gm2kg/sm2ha

state_soil    = nit_tot()
state_surface = surfaceom_n
:              + surfaceom_no3
:              + surfaceom_nh4

System losses

loss_soil = dlt_no3_dnit()
:          + leach_NO3
:          + leach_NH4

loss_crop    = 0.0
loss_surface = 0.0

System gains

gain_soil    = 0.0
gain_crop    = dlt_n_fixed * gm2kg/sm2ha
gain_surface = 0.0

Phosphorus Balance

System states

state_crop = p_green    * gm2kg/sm2ha
:           + GreenP     * gm2kg/sm2ha
:           + p_senesced * gm2kg/sm2ha
:           + SenescedP  * gm2kg/sm2ha
:           + p_dead     * gm2kg/sm2ha

state_soil = fom_p()
:           + hum_p()
:           + biom_p()
:           + rock_p()
:           + unavail_p()
:           + labile_p()
:           + banded_p()

state_surface = surfaceom_p
:              + surfaceom_labile_p

System losses
loss_soil    = 0.0
loss_crop    = 0.0
loss_surface = 0.0

System gains
gain_soil    = 0.0
gain_crop    = 0.0
gain_surface = 0.0