# 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:

${\displaystyle \Delta {T}=(T_{final}-T_{start})\cdot {\frac {y_{y}-y_{start}}{y_{final}-y_{start}}}\cdot f_{st}}$

Delta micro activity is calculated as:

${\displaystyle \Delta {MA}=Q_{10}^{\Delta {T}/10.0}-1.0}$

Delta subsidence is calculated as:

${\displaystyle \Delta {s_{sub}}=\Delta {MA}\cdot f_{ox}}$

The new parameter a value for year y is calculated as:

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

where

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