Commonly a simulation will take 2000 to 4000 simulation years to reach a state at which the soil organic C pools are approximately at steady state. Examine particular output variables for equilibrium conditions. We recommend looking at SOM2C and/or SOMTC. The fast C pools and the N pools often change too quickly and may oscillate, and are more sensitive to precipitation and temperature. At equilibrium the value of the representative output variable should change little with time.
When selecting the output variable to examine for equilibrium, consider your time frame and management schedule. For example, a natural grassland without disturbance should show steady state pools amounts on an annual output interval. If your management schedule has events on a multi-year cycle, e.g., alternating crop and fallow with regular cycles of fertilization and harvest, then monthly and annual intervals will reflect the effects of management. In this case, examine output variables over multiple management periods for an approximate steady state.
Usually the main purpose of running a simulation to equilibrium is to achieve soil pool values that will be used for a new site file as the basis of your simulation experiments. Obtaining an equilibrium state for a natural grassland, for instance, provides the basis for examining the response of the system to disturbance. Century5 can create a new site file at the end of a simulation using the -sf command-line option. See Running Century and CMI with Command-Line Options for details.
To run a simulation such that model reaches equilibrium, consider the following factors which will influence the model state.
Different weather sources can influence the simulation results. The actual variation in results will be approximately proportional to the degree of variation in the weather sets.
Using the site parameter's mean weather should allow equilibrium to be reached faster, since the weather input is the same from year to year. Equilibrium is usually reached by 2K-3K years using site means. Also, since the weather is static, the simulation results are reproducible.
Using the stochastic variation from the site means as the weather source will prevent a cyclical pattern from forming which can happen with a repeatedly used weather data set. If the weather data is approximately random from year to year, then no cyclical pattern may be evident in the simulation results over a long simulation time.
If the equilibrium value for SOM2C or SOMTC does not approximate the expected value for the site the first thing to check is the NPP value (variable CPRODA ). If NPP is too high or too low none of the other output variables for the simulation can be expected to be representative of the site. The reasons for too much or too little NPP include
The temperature function used in the growth equations may be parameterized incorrectly for the site (i.e. trying to grow a C3 grass using a C4 temperature curve).
Nitrogen deposition for the site may be too high or too low (check the values for the WDFX output variable).
Precipitation and/or temperature for the site is incorrect (both production and decomposition in Century5 are sensitive to soil water content and soil temperature). Check values and units. If site weather values are used as well as a weather file, do the site statistics represent the weather data?
The soil texture is incorrect (both production and decomposition are sensitive to soil texture).
Invalid site management (e.g., a cultivation event scheduled prior to a harvest event).
Running Century and CMI with Command-Line Options
Editing Site Parameters
Site Parameters Validation