# Parameter a recalculation formula (Subsidence Overlay)

The a parameter is recalculated yearly, to account for a changing climate. The new value for the a parameter is obtained by multiplying the original Parameter a is with a factor. This factor is calculated based on an interpolated average temperature for that year, a soil temperature factor, micro activity and an oxidation factor.

The value of the a parameter is updated each year as follows:

Delta temperature is calculated as:

$\Delta {T}=(T_{final}-T_{start})\cdot {\frac {y_{y}-y_{start}}{y_{final}-y_{start}}}\cdot f_{st}$ Delta micro activity is calculated as:

$\Delta {MA}=Q_{10}^{\Delta {T}/10.0}-1.0$ Delta subsidence is calculated as:

$\Delta {s_{sub}}=\Delta {MA}\cdot f_{ox}$ The new parameter a value for year y is calculated as:

$a_{y}=a_{0}\cdot (1.0+\Delta {s_{sub}})$ where

$a_{y}$ = The a parameter for the current year.
$a_{0}$ = The a parameter value at the start of the calculation, defined by the model attribute A.
$y_{y}$ = The current year. The first year is defined by START_YEAR.
$y_{start}$ = The start year of the simulation, defined by CLIMATE_START_YEAR.
$y_{end}$ = The final year of the simulation, defined by the addition of the start year and YEARS.
$T_{start}$ = The average temperature of the start year, defined by CLIMATE_START_TEMP.
$T_{final}$ = The average temperature of the final year, defined by CLIMATE_FINAL_TEMP.
$Q_{10}$ = The Q10 factor, configured as 3. This value is currently not adjustable.
$f_{st}$ = Soil temperature factor, defined by CLIMATE_SOIL_TEMP_FACTOR.
$f_{ox}$ = Oxidation factor, defined by CLIMATE_OXIDATION.