# Parameter Descriptions

## Parameters

ModVege makes use of a sizable number of different variables. Some of them are used as inputs to the simulation, some tune model behaviour and others are the actual state variables of the simulated grassland system.

The following section attempt to provide an as comprehensive as possible list of all of these. For a thorough understanding, the reader is referred to the original publication by [@jouven2006ModelPredictingDynamics].

### State Variables

These values evolve during a model run and are stored in the respective fields in the [ModvegeSite] instance that was run.

- `BM` Standing biomass in kg DM per ha.
- `BMG` Standing green biomass (kg DM / ha).
- `cBM` Cumulativeley grown biomass (kg DM / ha).
- `dBM` Daily grown biomass (kg DM / ha).
- `hvBM` Cumulative harvested biomass (kg DM / ha).
- `OMD` Organic matter digestibility (kg / kg).
- `OMDG` OMD of green matter (kg / kg).
- `ST` Temperature sum in degree Celsius days.
- `REP` Reproductive function. Gives the fraction of growth that is assigned to reproductive growth. The remainder goes into vegetative growth. Dimensionless.
- `PGRO` Potential growth in kg DM / ha.
- `GRO` Effective growth in kg DM / ha.
- `LAI` Leaf area index, accounting for the proportion of light intercepted by the sward. Dimensionless.
- `LAIGV` LAI of green vegetative biomass. Dimensionless.
- `AET` Actual evapotranspiration in mm.
- `WR` Water reserves in mm.
- `ENV` Function representing environmental effects on growth. Acts as a multiplicative factor. Dimensionless.
- `ENVfPAR` Part of ENV due to strength of incident radiation. Dimensionless.
- `ENVfT` Part of ENV due to temperature. Dimensionless.
- `ENVfW` Part of ENV due to water limitation. Dimensionless.

### Initial conditions

- `AgeGV` Age of green vegetative matter in degree Celsius days.

- `AgeGR` Age of green reproductive matter in degree Celsius days.

- `AgeDV` Age of dead vegetative matter in degree Celsius days.

- `AgeDR` Age of dead reproductive matter in degree Celsius days.

- `BMGV` biomass of GV (kg DM per ha).

- `BMGR` biomass of GR (kg DM per ha).

- `BMDV` biomass of DV (kg DM per ha).

- `BMDR` biomass of DR (kg DM per ha).

- `BMDR` biomass of DR (kg DM per ha).

- `SENG` senescence of GV (kg DM per ha).

- `SENG` senescence of GR (kg DM per ha).

- `ABSG` abscission of DV (kg DM per ha).

- `ABSG` abscission of DR (kg DM per ha).

- `ST` thermal time (degree days).

- `cBM` cumulative total biomass (kg per ha).

### Model parameters

- `LON` geographic longitude of site in degree.

- `LAT` geographic latitude of site in degree.

- `ELV` geographic elevation of site in m.a.s.l.

- `WHC` water-holding capacity of site in mm.

- `NI` site nutritional index (dimensionless).

- `RUEmax` maximum radiuation use efficiency in g DM per MJ.

- `w_FGA` relative weight of functional group A.

- `w_FGB` relative weight of functional group B.

- `w_FGC` relative weight of functional group C.

- `w_FGD` relative weight of functional group D.

- `sigmaGV` rate of GV respirative biomass loss (dimensionless).

- `sigmaGR` rate of GR respirative biomass loss (dimensionless).

- `T0` photosynthesis activation temperature (degree C).

- `T1` photosynthesis plateau temperature (degree C).

- `T2` photosynthesis max temperature (degree C).

- `KGV` basic senescence rate GV (dimensionless).

- `KGR` basic senescence rate GR (dimensionless).

- `KlGV` basic abscission rate GV (dimensionless).

- `KlGR` basic abscission rate GR (dimensionless).

- `maxOMDDV` organic matter digestibility in gram per gram DV.

- `minOMDDR` organic matter digestibility in gram per gram DR.

- `CO2_growth_factor` strength of effect of CO2 concentration on growth. See parameter b in [fCO2_growth_mod()].

- `crop_coefficient` multiplicative factor Kc by which reference evapotranspiration ET0 has to be multiplied to get the crop evapotranspiration ETc: ETc = Kc ET0

- `senescence_cap` fraction cs of GRO to which SEN is limited: SENimax = cs GROi for i in GV, GR. Makes it less likely for grass population to die out. Can be set to large values in order to effectively disable senescence capping.

- `stubble_height` float. Minimum height the grass can assume. The biomass will not fall below that height. This effectively presents a simple model of plant reserves.

- `SGS_method` string. Choice of method to determine the start of the growing season. Can be either `"MTD"` for the multicriterial thermal definition (see [start_of_growing_season_mtd()]) or `"simple"` for a commonly used approach as described in [start_of_growing_season()]).

### Functional group parameters

- `SLA` Specific Leaf Area in m2 per g.

- `pcLAM` Percentage of laminae (number between 0 and 1).

- `ST1` Temperature sum in degree Celsiues days after which the seasonality function `SEA` starts to decrease from its maximum plateau. See also [SEA()].

- `ST2` Temperature sum in degree Celsiues days after which the seasonality function `SEA` has decreased back to its minimum value. See also [SEA()].

- `maxSEA` Maximum value of the seasonality function [SEA()]

- `minSEA` Minimum value of the seasonality function [SEA()]. Usually, `minSEA = 1 - (maxSEA - 1)`.

- `maxOMDGV` Maximum organic matter digestability for green vegetative matter in arbitrary units.

- `minOMDGV` Minimum organic matter digestability for green vegetative matter in arbitrary units.

- `maxOMDGR` Maximum organic matter digestability for green reproductive matter in arbitrary units.

- `minOMDGR` Minimum organic matter digestability for green reproductive matter in arbitrary units.

- `BDGV` Bulk density of green vegetative dry matter in g per m3.

- `BDDV` Bulk density of dead vegetative dry matter in g per m3.

- `BDGR` Bulk density of green reproductive dry matter in g per m3.

- `BDDR` Bulk density of dead reproductive dry matter in g per m3.

- `fg_parameter_names` Vector of strings of the variable names of all vegetation parameters governed by functional group composition.

### Weather variables

- `DOY` day of year in given year

- `Ta` average temperature of given day (Celsius).

- `precip` precipitation in millimeter per day.

- `PAR` photosynthetically active radiation in MJ/m2. Can be calculated from average sunlight irradiance `SRad` in J/s/m2 as: `PAR = SRad * 0.47 * 24 * 60 * 60 / 1e6`

- `ET0` evapotranspiration in mm.