The fixed parameters for the Century Model are not really fixed, rather they vary with the general type of biome to be simulated. Crop, grass, forest will have different fixed parameter sets. Several versions of these parameter sets are provided with the Century distribution files, described in the following list:
file name  biome 

arcfix.100  arctic grassland 
borfix.100  boreal forest 
dryffix.100  dry temperate forest 
drygfix.100  dry grassland 
drytrpfix.100  dry tropical forests 
ffix.100  temperate forest 
gfix.100  temperate grassland 
trpfix.100  wet tropical forest 
Parameter  Description  Units  Valid Values  Default Values 
ADEP(1) through ADEP(10)  Default depths of soil layers (only NLAYER values used).  cm  > 0  
AGPPA  Crop/grass systems: Intercept parameter in the equation estimating potential aboveground biomass production for calculation of root/shoot ratio (g/m2/y) (used only if FRTC(1) = 0).  
AGPPB  Crop/grass systems: Slope parameter in the equation estimating potential aboveground biomass production for calculation of root/shoot ratio (g/m2/y/cm) (used only if FRTC(1) = 0). NOTE  AGPPB is multiplied by annual precipitation (cm).  
ANEREF(1)  Ratio of rain/potential evapotranspiration below which there is no negative impact of soil anaerobic conditions on decomposition.  
ANEREF(2)  Ratio of rain/potential evapotranspiration above which there is maximum negative impact of soil anaerobic conditions on decomposition.  
ANEREF(3)  Minimum value of the impact of soil anaerobic conditions on decomposition; functions as a multiplier for the maximum decomposition rate.  
ANIMPT  Slope term used to vary the impact of soil anaerobic conditions on decomposition flows to the passive soil organic matter pool.  
AWTL(1) through AWTL(10) 
Factor for transpiration loss for each soil layer (only NLAYER values
used). Indicates which fraction of the water available for transpiration
can be extracted by the roots, and is proportional to the root density in
the layer. T(i) = AWTL(i) / AWTL_{total} * PT_{total} where T = transpiration, PT = potential transpiration. 

BGPPA  Intercept parameter in the equation estimating potential belowground biomass production for calculation of root/shoot ratio (g/m2/y) (used only if FRTC(1) = 0 (g/m2/y).  
BGPPB  Slope parameter in the equation estimating potential belowground biomass production for calculation of root/shoot ratio (g/m2/y) (used only if FRTC(1) = 0). NOTE  BGPPB is multiplied by annual precipitation (cm).  
CO2PPM(1)  Initial parts per million for CO_{2} effect.  ppm  
CO2PPM(2)  Final parts per million for CO_{2} effect.  ppm  
CO2RMP 
Flag indicating whether CO_{2} effect should be: 0 = step function 1 = ramp function 
0 or 1  
DAMR(1,1), DAMR(1,2), DAMR(1,3)  Fraction of surface N, P, and S, respectively, absorbed by residue.  0.0 to 1.0  
DAMR(2,1), DAMR(2,2), DAMR(2,3)  Fraction of soil N, P, and S, respectively, absorbed by residue.  0.0 to 1.0  
DAMRMN(1), DAMRMN(2), DAMRMN(3)  Minimum ratio of C to N, P, and S, respectively, allowed in residue after direct absorption.  
DEC1(1) , DEC1(2)  Structural decomposition rate for surface and soil, respectively; the fraction of the pool that turns over each year.  0.0 to ?  
DEC2(1) , DEC2(2)  Metabolic decomposition rate for surface and soil, respectively; the fraction of the pool that turns over each year.  0.0 to ?  
DEC3(1) , DEC3(2)  Decomposition rate of surface and soil organic matter, respectively, with active turnover; the fraction of the pool that turns over each year.  0.0 to ?  
DEC4  Decomposition rate of soil organic matter with slow turnover, the fraction of the pool that turns over each year.  0.0 to ?  
DEC5  Decomposition rate of soil organic matter with intermediate turnover, the fraction of the pool that turns over each year.  0.0 to ?  
DECK5  Available soil water content at which shoot and root death rates are half maximum.  cm  
DLIGDF  Difference in delta 13C for lignin compared to whole plant delta 13C.  
DRESP  Discrimination factor for 13C during decomposition of organic matter due to microbial respiration.  
EDEPTH  Initial and minimum depth of the simulation soil layer where C , N , P , and S dynamics are calculated. The actual simulation soil depth can change with erosion and deposition.  cm  20  20 
ELITST  Effect of litter on soil temperature relative to live and standing dead biomass.  0.4  
ENRICH  The enrichment factor for SOM losses due to erosion. This factor reflects the variation in SOM with depth through the simulation layer. It is common for SOM density (g/cm3) to decrease with depth below the surface organic horizons.  1.0  
FAVAIL(1), FAVAIL(3)  Fraction of N and S, respectively, available per month to plants. The P fraction, FAVAIL(2), is computed.  0.0 to 1.0  
FAVAIL(4), FAVAIL(5)  Minimum and maximum fraction, respectively, of P available per month to plants.  0.0 to 1.0  
FAVAIL(6)  Mineral N in surface layer corresponding to maximum fraction of P available.  g m^{2}  0.0 to 1.0  
FLEACH(1), FLEACH(2)  Intercept and slope, respectively, for a normal month to compute the fraction of MINERL N, P, and S which will leach to the next layer when there is a saturated water flow. Normal leaching is a function of sand content.  0.6 and 0.4  
FLEACH(3), FLEACH(4) , FLEACH(5)  Leaching fraction multiplier for N, P, and S, respectively, used to compute the fraction of MINERL N, P, and S which will leach to the next layer when there is a saturated water flow. Normal leaching is a function of sand content.  0.0 to 1.0 
(3) = 0.95 (4) = 0.0 (5) = 0.1 

FWLOSS(1)  Scaling factor for interception and evaporation of precipitation by live and standing dead biomass.  0.0 to 1.0  1.0  
FWLOSS(2)  Scaling factor for bare soil evaporation of precipitation.  0.0 to 1.0  1.0  
FWLOSS(4)  Scaling factor for potential evapotranspiration.  0.8  
FXMCA , FXMCB  Intercept and slope, respectively, for effect of biomass on nonsymbiotic soil N fixation. Used only when NSNFIX = 1.  0.125 and 0.005  
FXMXS  Maximum monthly nonsymbiotic soil N fixation rate (reduced by effect of N : P ratio, used when NSNFIX = 1).  0.35  
FXNPB  N / P control for N fixation based on availability of top soil layer (used when NSNFIX = 1).  7.0  
GREMB  Grazing effect multiplier for grazing effects types 4, 5, 6 (crop/grass grazing parameter GRZEFF).  
IDEF 
Flag for method of estimating the effect of soil water content on
the rate of decomposition of SOM:

Century5: 1 or 2 DayCent5: 1, 2, or 3 

LHZF(13) 
The fraction of the active, slow, and passive soil C pool
that the respective lower soil horizon C pool will be initialized to. Index = 1 affects SOM1CI(2,*); index = 2 affects SOM2CI(*); index = 3 affects SOM3CI(*). 
0.2, 0.4, 0.8 respectively.  
MINLCH  Maximum water flow applied to leaching of minerals (cm of H_{2}O leached from the bottommost soil layer). 
Century5: cm month^{1} DayCent5: cm day^{1} 
> 0 
Century5: 18.0 DayCent5: 0.1 
NSNFIX  Equals 1 if nonsymbiotic N fixation should be based on N : P ratio in mineral pool, otherwise nonsymbiotic N fixation is based on annual precipitation.  0 or 1  
NTSPM  Number of times per month (Century5) or day (DayCent5) to run the decomposition submodel. (Do not change this value!) Currently this parameter is ignored since it is set internally in the models. 
Century5: 4 DayCent5: 1 

OMLECH(1), OMLECH(2) 
OMLECH(1) and OMLECH(2) are the intercept and slope, respectively, in the linear influence of sand content upon the amount of organic C leaching from active SOM in the simulation layer. This effect of sand upon leaching is calculated as: effect = OMLECH(1) + OMLECH(2) * sand fraction 
(1) = 0.03 (2) = 0.12 

OMLECH(3)  Specifies the maximum amount of water flowing out of the simulation layer that can leach organic C. Water flow above this amount has no effect on leaching of organic C. 
Century5: cm month^{1} DayCent5: cm day^{1} 
Century5: 60.0 DayCent5: 1.9 

P1CO2A(1), P1CO2A(2)  Intercept parameter which controls flow from surface and soil organic matter, respectively, with fast turnover to CO_{2} (fraction of organic C lost to CO_{2} when there is no sand in the soil).  
P1CO2B(1), P1CO2B(2)  Slope parameter which controls flow from surface and soil organic matter, respectively, with fast turnover to CO_{2} (slope is multiplied by the fraction sand content of the soil).  
P2CO2  Controls flow from soil organic matter with intermediate turnover to CO_{2} (fraction of organic C lost as CO_{2} during decomposition).  
P3CO2  Controls flow from soil organic matter with slow turnover rate to CO_{2} (fraction of C lost as CO_{2} during decomposition).  
PABRES  Amount of residue which will give maximum direct absorption of N (g C /m2).  
PCEMIC(X,E)  Maximum (X = 1) or minimum (X = 2) C/E ratio for surface microbial pool. Values for E are: N = 1, P = 2, S = 3.  
PCEMIC(3,E)  Minimum E content of decomposing aboveground material, above which the C / E ratio of the surface microbes equals PCEMIC(2,E). Values for E are: N = 1, P = 2, S = 3.  
PEFTXA , PEFTXB  Intercept and slope, respectively, for the linear regression equation to compute the effect of soil texture on the microbial decomposition rate. The intercept is the effect of texture when there is no sand in the soil.  
PHESP(1)  Minimum pH for determining the effect of pH on the solubility of secondary P (flow of secondary P to mineral P ) (for TEXESP(2) = m * (pH input) + b, m and b calculated using these PHESP values).  
PHESP(2)  Value of TEXESP(2), the solubility of secondary P, corresponding to minimum pH.  yr^{1}  
PHESP(3)  Maximum pH for determining effect on solubility of secondary P (flow of secondary P to mineral P) (for TEXESP(2) = m * (pH input) + b, m and b calculated using these PHESP values).  
PHESP(4)  Value TEXESP(2), the solubility of secondary P , corresponding to maximum pH.  yr^{1}  
PLIGST(1)  Effect of lignin on surface structural or fine branch and large wood decomposition.  
PLIGST(2)  Effect of lignin on soil structural or coarse root decomposition.  
PMCO2(1)  Controls flow from surface metabolic to CO_{2} (fraction of C lost as CO_{2} during decomposition).  
PMCO2(2)  Controls flow from soil metabolic to CO_{2} (fraction of C lost as CO_{2} during decomposition).  
PMNSEC(1)  Slope for N; controls the flow from mineral to secondary N.  yr^{1}  
PMNSEC(2)  Slope for P; controls the flow from mineral to secondary P.  yr^{1}  
PMNSEC(3)  Slope for S; controls the flow from mineral to secondary S.  yr^{1}  
PMNTMP  Effect of biomass on minimum surface temperature. A typical value is 0.004.  
PMXBIO  Maximum aboveground nonwood biomass (live + standing dead + 10% litter) level for soil temperature calculation and for calculation of the potential negative effect on plant growth of physical obstruction by standing dead and litter.  g C m^{2}  
PMXTMP  Effect of biomass on maximum surface temperature. A typical value is 0.0035.  
PPARMN(E)  Controls the flow from parent material to mineral compartment (fraction of parent material that flows to mineral N, P, and S). Values for E are: N = 1, P = 2, S = 3.  
PS1CO2(12)  Controls amount of CO_{2} loss when surface and soil structural organic matter decomposes to SOM1. Index = 1 is the surface pool; index = 2 is the soil pool.  
PS1S3(12)  Intercept and slope for flow from soil organic matter with fast turnover to soil organic matter with slow turnover (fraction of C from SOM1C to SOM3C). Index = 1 is intercept; index = 2 is slope.  
PS2S3(12)  Intercept and slope value which controls flow from soil organic matter with intermediate turnover to soil organic matter with slow turnover (fraction of C from SOM2C to SOM3C). Index = 1 is intercept; index = 2 is slope.  
PSECMN(13)  Controls the flow from secondary to mineral N/P/S. Index 1 = N, 2 = P, 3 = S.  
PSECOC  Controls the flow from secondary to occluded P.  
RAD1P(13,1)  Controls on the addition term for C/N ratio of slow SOM formed from surface active pool. Index 1 = intercept; index 2 = slope; index 3 = minimum ratio.  
RAD1P(13,2)  Controls on the addition term for C/P ratio of slow SOM formed from surface active pool. Index 1 = intercept; index 2 = slope; index 3 = minimum ratio.  
RAD1P(13,3)  Controls on the addition term for C/S ratio of slow SOM formed from surface active pool. Index 1 = intercept; index 2 = slope; index 3 = minimum ratio.  
RCESTR(13)  C/E ratio for structural material. Index 1 = C/N, 2 = C/P, 3 = C/S.  
RICTRL  Root impact control term used for calculating the impact of root biomass on nutrient availability. As root biomass decreases,the availability of nutrients also decreases nonlinearly.  0.0 to 1.0  0.015  
RIINT  Root impact intercept used for calculating the impact of root biomass on nutrient availability. Minimum root impact = 1  RIINT.  0.0 to 1.0  0.80  
RSPLIG  Fraction of lignin flow (in structural decomposition) lost as CO_{2}.  0.0 to 1.0  
SEED  Random number generator seed value.  
SPL(1), SPL(2)  Determines the metabolic vs. structural split (fraction metabolic is a function of lignin to N ratio). (1) = Intercept parameter. (2) = Slope parameter.  0.0 to 1.0 for both  
STRMAX(1), STRMAX(2)  Maximum amount of structural material that will decompose. (1) = surface layer, (2) = belowground.  g C m^{2}  
TEFF() 
Parameters of the equation for the
temperature component of
DEFAC,
the decomposition factor.
Century5 and DayCent5 use different equations.
See this page of graphs
for the behavior of the equations with various parameter values.
Monthly Century:
Daily Century:

Monthly: TEFF(1) = 0 TEFF(2) = 0.125 TEFF(3) = 0.07 Daily: TEFF(1) = 15.40 TEFF(2) = 11.75 TEFF(3) = 29.70 TEFF(4) = 0.031 

TEXEPP(1) 
Texture effect on parent P mineralization: 0 = use PPARMN(2) in the weathering equation. 1 = include the effect of texture using the remaining TEXEPP values with the arctangent function. 
0 or 1  
TEXEPP(2)  TEXEPP(5) 
Texture effect on parent P mineralization: 

TEXESP(1) 
Texture effect on secondary P flow to mineral P: Value 0 = to use PSECMN(2) in the weathering equation Value 1 = include the effect of pH and sand content using the equation specified by TEXESP(2) (a function of pH and PHESP(14)) and TEXESP(3). 
0 or 1  
TEXESP(3)  Texture effect on secondary P flow to mineral P: Slope value used in determining effect of sand content.  
TMELT(1) 
Minimum temperature above which at least some snow will melt. The monthly default value is approximately converted from the daily default value for when the latter is summed over 30 days. (See Singh and Singh, 2001.) Melt (cm) = TMELT(2) * ( Air_Temp  TMELT(1) ) 
deg. C 
Century5: 8.0 DayCent5: 0.0 

TMELT(2) 
Slope of the snow melting function: Centimeters of snow per month or day, that will melt per degree above the TMELT(1) temperature. The monthly default value is approximately converted from the daily default value for when the latter is summed over 30 days. (See Singh and Singh, 2001.) 
Century5: cm month (deg. C)^{1} DayCent5: cm day (deg. C)^{1} 
Century5: 4.0 DayCent5: 0.14 

VARAT1(1,E)  Maximum C/E ratio for material entering SOM1. Values for E are: N = 1, P = 2, S = 3.  
VARAT1(2,E)  Minimum C/E ratio for material entering SOM1. Values for E are: N = 1, P = 2, S = 3.  
VARAT1(3,E)  Amount E present when minimum C/E ratio applies to SOM1. Values for E are: N = 1, P = 2, S = 3.  
VARAT2(1,E)  Maximum C/E ratio for material entering SOM2. Values for E are: N = 1, P = 2, S = 3.  
VARAT2(2,E)  Minimum C/E ratio for material entering SOM2. Values for E are: N = 1, P = 2, S = 3.  
VARAT2(3,E)  Amount E present when minimum C/E ratio applies to SOM2. Values for E are: N = 1, P = 2, S = 3.  
VARAT3(1,E)  Maximum C/E ratio for material entering SOM3. Values for E are: N = 1, P = 2, S = 3.  
VARAT3(2,E)  Minimum C/E ratio for material entering SOM3. Values for E are: N = 1, P = 2, S = 3.  
VARAT3(3,E)  Amount E present when minimum ratio applies to SOM3. Values for E are: N = 1, P = 2, S = 3.  
VLOSSE  Monthly Century only. Fraction per year of excess N (i.e. N left in the soil after nutrient uptake by the plant) which is volatilized.  0.0 to 1.0  0.02  
VLOSSG  Monthly Century only. Fraction per month of gross N mineralization which is volatilized.  0.0 to 1.0  0.01 
Obsolete Model (Fixed) Parameters
About the Model Internal Parameterization