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In the model context, this refers to surface material, that is, simulation pools that are on or above the soil surface.
active soil organic matter
soil organic matter with a fast turnover time of weeks to about a year. Century stores active SOM in the SOM1 pool. Also referred to as the fast SOM pool.
Agricultural Research Service, a department within the NRCS.
available water holding capacity
In the model context, this refers to ...

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In the model context, this refers to belowground material, that is, simulation pools that are in the simulation depth of the soil as well as mineralized nutrient pools in the soil below the simulation depth.
A block is a unit of management instructions for the simulation site. A block is defined by the sequence of events which specify the management of the site over one or more years. For example, events for a cropping system might include type of crop, cultivation method, planting month, fertilization event, harvesting month, etc. Block years begin in January and end in December, and events in the block definition are specified using a year and month relative to the start of the block.
bulk density
The unit density of the oven-dry soil in g cm-3 . Bulk density is inversely proportional to both the porosity and the fraction of soil organic matter. Soils generally have a bulk density of between 0.7 and 1.5.

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Organic carbon.
CENTURY is a mass balance nutrient cycling model that simulates the flow of carbon and nitrogen through the soil organic matter pools of slow, intermediate, and fast turnover times, using a monthly time step (Parton et al . 1987, Parton et al . 1988, and Sanford et al . 1991). CENTURY simulates the plant-soil dynamics of carbon, nitrogen, phosphorous, and sulfur of grassland systems, agricultural crop systems, forest systems, and savanna systems.
carbon dioxide.
CO2 effect
The Century model includes the effects of documented changes in atmosphericCO2 and thus predicts the effects on crop productions. First, increasing CO2 has a direct effect on C availability by stimulating photosynthesis and reducing photorespiration. The second effect of increased CO2 concentrations is a decrease in stomatal conductance at high CO2 concentrations, which reduces the transpiration rate per unit leaf area. Reduced transpiration will also increase the leaf temperature which can further increase photosynthesis. The third major effect of increased CO2 is a decrease in the plant N concentration in C3 species. A fourth effect of increased CO2 on plant growth which affects soil organic matter levels is an increase in root growth. Most studies with elevated CO2 with grain crops in which root growth has been measured show very little or no effect on the root to shoot ratio. When selecting the CO2 Effect option for Century it can be implemented with either a constant CO2 concentration or with a linear ramp with annual increments from an initial concentration to a final concentration. The various effects of CO2 described above are controlled by functions of the CO2 concentration and crop/grass or tree-specific parameters. Parameter values are set using reference concentrations of 350 and 700 ppm CO2 for ambient and doubled CO2, respectively.
crop parameters
Parameters used to define a crop. The model can simulate a wide variety of crops and grasslands by altering these parameters. Potential production is a function of a genetic maximum defined for each crop, and 0-1 scalars depending on soil temperature, moisture status, shading by dead vegetation, and seedling growth. For grain crops a harvest index is calculated based on a genetic maximum and moisture stress in the months corresponding to anthesis and grain fill. Moisture stress is calculated as the ratio of actual to potential transpiration in these months. The fractions of aboveground N, P, and S partitioned to grain are crop-specific constants modified by the square root of the moisture stress term, resulting in higher grain nutrient concentrations when moisture stress reduces the harvest index. At harvest a proportion of the aboveground nitrogen is lost to volatilization.
cultivation parameters
Parameters used to define a cultivation method. Cultivation options allow for the transfer of defined fractions of shoots, roots, standing dead, and surface litter into standing dead, surface and soil litter pools as is appropriate. The model can simulate a variety of conventional cultivation methods such as plowing or sweep tillage, thinning operations, or herbicide application. Each cultivation option has parameters for the multiplicative effect of soil disturbance by cultivation on organic matter decomposition rates for the active, slow, and passive pools. The value for these parameters is dependent on the degree of soil stirring and disruption caused by each implement.

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decay rate
decomposition factor
decomposition rate

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Element, which in Century refers to the elements modeled by Century: carbon (C), nitrogen (N), phosphorus (P), and sulfur (S). C and N are always modelled in a Century simulation. To model P, set the site paramater NELEM to 2. To model S and P, set the site paramater NELEM to 3.
equilibrium simulation
When the Century model is initialized for a specific biome/site a simulation of 2000 to 3000 years is done to allow the pools to reach equilibrium. The output from the equilibrium simulation is used as a starting point for user-defined simulation.
erosion file
A file generated by erosion when an erosion file name is specified in the site management. An erosion file can be used as the input deposition file for a DEPO management event.
Actions that direct the management of the simulated site. Events include the crop being simulated, the type of cultivation to use, the planting month for a crop, the harvesting month for a crop, the harvesting method for a crop, the type of fire (intensity) in a forest, the type of forest being simulated, the amount of fertilizer to add during a month, etc. Events are specified in a management block definition, and the time of an event (year and month) is relative to the start of the block (not simulation time.) Some events (e.g., select a crop type) require additional information about the event (e.g., the type of crop.)

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fertilization parameters - Parameters related to fertilization treatments. Fertilizer addition is specified as a fixed amount for each fertilization event.

field capacity - Water is moved down through the soil profile according to the principle of a tipping bucket. Each layer of the soil is conceptualized as a bucket, which can hold only the amount of water represented by the field capacity. When the soil water content in a soil layer reaches field capacity any additional water flowing into the layer is moved to the next deeper layer of the soil profile.

fire parameters - Parameters related to a fire event. The major impact of fire is to increase the root to shoot ratio, increase the C:N ratio of live shoots and roots, remove vegetation, and return nutrients during the years when fire occurs. The effect of different intensities of fire in herbaceous vegetation can be parameterized by specifying the fractions of live shoots, standing dead, and surface litter removed by a fire along with the return of N, P, and S in inorganic forms. Fire can also affect plant growth.

fixed parameters - Parameters that are not normally modified during a simulation. The Century models use biome-specific fixed parameter files. Before using one of these files, rename it to " fix.100 " (Note: UNIX is case-sensitive.) Available fixed parameter files are listed in Fixed Parameters.

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GPSR - Great Plains System Research Unit

Grazing Parameters - Parameters that are related to a grazing event. Grazing removes vegetation, returns nutrients to the soil, alters the root to shoot ratio, and increases the N content of live shoots and roots. The grazing options can be parameterized to remove defined fractions of aboveground live and standing dead plant material each month. The fractional returns of C, N, P, and S are specified, having allowed for losses in animal carcasses and milk, transfer of dung and urine off the area being simulated, volatile loses of N from dung and urine patches, and leaching of N and S under urine patches. The proportion of N, P, and S returned in organic forms is also specified as is the lignin content of the feces. Grazing can have variable effects on plant production.

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harvest index
Typically calculated as the ratio of harvest biomass to aboveground biomass. In CENTURY, for grain crops a harvest index is calculated based upon a genetic maximum and moisture stress in the months corresponding to anthesis and grain fill.
harvest parameters
Parameters that describe crop and grass harvesting methods. At harvest, grain may be removed from the system and live shoots can either be removed or transferred to standing dead and surface residue. The crop harvest routine also allows for the harvest of roots, hay crops, or straw removal after a grain crop. The crop may be killed at harvest, as for cereal grain crops, or a fraction of roots and shoots may be unaffected by harvest operations and growth may continue.

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initial system - The initial system (crop/grass, tree, savana) which you specify in the site management. This system determines the values to which many of Century pools are set prior to the start of the simulation.

instance - In the site management, an instance of a management block is the placement within the simulation at which time the events within a defined in the block will be executed. Instances occur in simulation time, whereas events in a management block are specified relative to the start of the block ("block time").

irrigation parameters - Parameters that are related to methods of irrigation. Irrigation amounts can be either fixed amounts or automatically set according to the soil moisture status. Automatic irrigations are scheduled if the available water stored in the plant root zone falls below a nominated fraction of the available water holding capacity. The amount of water applied by the automatic option allows for the addition of a nominated amount of water or for irrigation up to field capacity, or up to field capacity plus an allowance for potential evapotranspiration.

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labile - Readily or continually undergoing chemical, physical, or biological change or breakdown.

labeled C - An isotope of C used for labelling, specifically, 13C or 14C.

LAI - Leaf Area Index. In Century, this is calculated using ...

lignin - Woody cell walls of plant material which is the cementing material between plant cells.

lower horizon - See lower layer pools.

lower layer pools - Pools of C and E of active, slow, passive, structural and metabolic types. These pools represent the plant-available organic matter present in the soil below the simulation layer.

lower simulation layer - See lower layer pools.

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management - Site management contains the general simulation configuration information, and the sequence of events affecting the site during the simulation. Management events are specified in management blocks.

metabolic organic matter -

microcosm - A microcosm is used to simulate litter bag decomposition and soil incubations at constant temperature and soil moisture. The incubation option will simulate the dynamics of soil organic matter and surface or buried litter under constant soil temperature and soil water conditions. Use this option to simulation changes in C levels and nutrient mineralization in laboratory incubations. To simulate a litter bag simulation you would specify the initial litter level and C:N, C:P, and C:S ratio of the litter. The lignin content of the litter bag would be specified for either aboveground or belowground material depending on the placement of the bag. Incubation of the soil occurs in a similar matter by initializing all of the soil variables. Some microcosm options include fertilization, cultivation (mixing of the soil) and the addition of new labeled or unlabeled plant material during the incubation. Plant growth does not occur during the incubation.

mineral pools - In Century, the mineral pools contain mineralized N, P, and S in g m-2 .

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Nitrogen; always simulated in the CENTURY models.
Network Common Data Form , is a interface for array-oriented data access and a collection of software libraries that provide implementations for the interface. The netCDF libraries define a machine-independent format for representing scientific data. Together, the interface, libraries, and format support the creation, access, and sharing of data. Information on the netCDF format and software can be obtained at
The Natural Resources Conservation Service, and division of the USDA.

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occluded - Taken into and retained by another substance.

OMPC - Organic matter percent in a soil layer. In Century, this value is calculated according to total soil C (SOMSC output variable) and the site parameter SWFLAG.

organic matter addition parameters - Parameters describing a management event to add organic matter to the simulation soil layer. The organic matter options will contain a lignin fraction content and grams of C . These parameter sets are stored in the file omad.100.

output frequency - The simulation time interval used to determine when simulation output is generated. The frequency is specified as a multiple of 1/12 years; the minimum frequency is the Century simulation time step (one month). Some commonly used values are:

output variables - Values calculated during the simulation which are written to output at the end of a simulation time step according to the output frequency specified in the instance of a management block in the site management. Crop and forest production growing season accumulators are reset to zero in the planting month or the first month of growth. Annual accumulators for precipitation, evaporation, respiration, and mineralization are reset at the end of the calendar year.

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P - Phosphorus, which you can simulate in Century by setting the site paramater NELEM to 2.

parameter file - The Century model obtains input values through the parameter files. Each file contains a certain subset of variables; for example, the cultivation file contains the values related to cultivation. Within each file there may be multiple options in which the variables are defined for multiple variations of the event. For example, within the cultivation parameter file there are several cultivation options such as plowing or rod-weeder. For each option, the parameters values are set appropriately to simulate that particular option.

parameter set - a set of parameters used in a simulation. Site parameters and fixed parameters are set once at the start of a simulation. Other parameter sets will change during the simulation according to site management. Management parameter sets include crop and grassland type, cultivation, fertilization, fire, grazing, harvest, irrigation, organic matter addition, tree type, and tree removal. Parameter sets are stored in files, according to their type. The site and fixed parameters are stored with one set per file. The management parameter sets have several sets per file.

parameters - Properties that determine various characteristics of a simulation. Site parameters describe a site for a simulation. Model parameters include plant descriptions, cultivation and harvest events, and more.

passive soil organic matter - soil organic matter with a very long turnover time of about 500 to thousands of years. Century stores passive SOM in the SOM3 pool. Also referred to as the slow SOM pool.

PET - potential evapotranspiration.

plant-available soil water - Water content greater than the wilting point for each soil layer (g H2O cm-3 soil). In Century, this is summed over the number of rooting layers (NLAYPG).

plant material - Plant biomass measured as grams of C, N, P, and S per square meter. Depending upon the pool, this can include above and below ground live and dead plant material, and for trees, live and dead wood.

pool - A Century variable containing the amount a simulated substance, such as C, N, P, S, and soil water content. Pool amounts will vary during a simulation, and will be written to output when requested.

ppm - parts per million

precipitation - Measured in centimeters of H2O per month.

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respired -

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S - Sulfur, which you can simulate in Century by setting the site paramater NELEM to 3.(This will also force P to be simulated.)

senescence - The growth phase in a plant or plant part (such as a leaf) from full maturity to death.

simulation layer depth - Soil depth (cm) in which the Century simulation occurs. This depth is initially at 20 cm, but with deposition can increase to a maximum of 50 cm. If the simulation depth is greater than 20 cm, erosion will decrease the depth to a 20 cm minimum.

site - A set of configuration files for the simulation of an agro-ecosystem with parameters describing the site and management directing the simulation of the site.

site management - See management.

site parameters - Input parameters which are site-specific, including climate. soil and physical controls, external nutrient input, organic matter initial values, forest organic matter initial values, initial mineral N, P, and S, initial relative water content, initial lower soil pools values, and erosion pools' loss factors.

slow soil organic matter - soil organic matter with an intermediate turnover time of about 10 to hundreds of years. Century stores slow SOM in the SOM2 pool.

soil layer - One layer in the soil submodel, specified by its position in the layer sequence, and its thickness and physical characteristics.

soil water content - Volumetric soil water content (g H2O cm-3 soil) per soil layer.

SOM1 - Century pool containing active soil organic matter.

SOM2 - Century pool containing slow soil organic matter.

SOM3 - Century pool containing passive soil organic matter.

soil organic matter - (Acronym: SOM) The soil organic matter pools in Century have units of g m-2 over the simulation depth. Soil organic matter pools are divided into surface and subsurface layers, and each layer has active, slow, and passive sets of pools. Each set of pools contains a pool for C, N, P, and S (depending upon your specification of the NELEM site parameter.)

soil organic matter submodel - This is the core submodel of Century which simulates the flow of C, N, P, and S through plant litter and the inorganic and organic pools in the soil. The major input variables for the model include:

Most of the parameters that control the flow of C in the system are in the fixed parameters file. The model includes three organic matter pools ( active, slow, and passive ) with different potential decomposition rates, aboveground and belowground litter pools, and a surface microbial pool which is associated with decomposing surface litter.

soil texture - Century 5 uses soil texture (the fraction of sand, silt, and clay) as part of the description of each soil layer.

stochastic - Variables are given a random value based on the probability of certain known behaviors. This process can be used to predict behavior, for example, the probability of a precipitation event occurring given historical information about precipitation events.

structural organic matter -

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temperature - Air or soil temperature measured in degrees Celcius.

texture - See soil texture.

time step - Time increment used in a Century simulation. In monthly Century5 there are three time steps: the simulation time step is fixed at one month; the decomposition time step is fixed at 1/4 of the simulation time step; the output time step is set according the output frequency specified in the management scheme.

tree parameters - Parameters used to define a type of tree, and is specified in a TREE event. Trees are divided into leaves, fine roots, fine branches, large wood, and coarse roots with carbon and nutrients allocated to the different plant parts using a fixed allocation scheme. Maximum monthly gross production is calculated as the product of maximum gross production rate, moisture, soil temperature, and live leaf-area-index terms. Some of the important forest specific parameters include the net production rate, the leaf area index to wood biomass relationship parameters, the sapwood to large wood C ratio parameter, and the allocation of C into different plant parts.

tree removal parameters - Parameters used to define a tree removal event. A tree removal event can simulate the impact of different forest harvest practices, fires, and the effect of large scale disturbances such as hurricanes. For each disturbance or harvest event, the fraction of each live plant part lost and the fraction of material that is returned to the soil system is specified. Death of fine and coarse roots are also considered in the removal event along with the removal of dead wood. Another feature is that the nutrient concentration of live leaves that go into surface residue can be elevated above the dead leaf nutrient concentration (e.g. simulating the effect of adding live leaves to surface residue as a result of hurricane disturbance) by specifying the nutrient return fraction of the leaves to be greater than one.

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unlabeled C - The common isotope of C, 12C.

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volatilized - To cause to be passed off as vapor into the atmosphere.

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weather data file - File containing actual weather data for a site. When running Century you can elect to use this actual weather data to drive the simulation or, if there is weather data for at minimum 10 years, this file can be used to create statistical values that are used to drive the stochastic weather generator in Century.

wilting point - Soil water content below which plants are no longer be able transpire water from the soil.

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This page was last modifed on 22 May 2000.