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The model considers the soil as one bucket layer in which the water balance is computed on a daily time step considering inputs and outputs. Incoming water flux is constituted by daily precipitation, whereas water leaves the soil through total evapotranspiration, represented by canopy interception and transpiration, as well as soil evaporation.
When dormant period for all species occurs, only the rainfall and soil evaporation are considering in the balance.
Soil evaporation (mm day-1), both in dormant and growing season is computed as:
where PTc is the Priestley-Taylor coefficient, rteta is the relative soil moist ratio (%vol) and ESq is the Potential Soil Evaporation computed as:
where gamma is the psychrometric constant (65 Kpa °C-1), e20 is the rate of change of saturated VPD (mbar) with 20°C of temperature (2.2), lambda is the latent heat of vaporization (2.45 * 10-6 Jkg-1) and Radsoil is the net radiation at the soil level.
Evapotranspiration (mm day-1) is calculated as:
where Etp (mm day-1) is the Potential Evapotranspiration at canopy level, iR is the fraction of daily rainfall intercepted by the canopy and evaporated into the atmoshpere, CC is the canopy cover and P is the daily precipitation.
Potential Evapotranspiration is calculated as:
where roAir is the air density (1.2 kg m −3 ), lambda is the latent heat of vaporization of water (∼2.5 × 10 6 J Kg −1 ), VPDConv (∼6.22 × 10 −4 ) converts VPD to saturation deficit, gc is the canopy conductance (mm s −1 ) computed from maximum canopy conductance gcxx reduced by the physiological modifier (Landsberg and Waring, 1997) and gBx is the canopy boundary layer conductance for the species x respectively. The net radiation Radz is computed as:
where Radzunderstorey is the radiation intercepted by the lower layer, Rad0 is the radiation above the overstorey canopy, CCz overstorey and etazoverstorey are respectively the canopy cover and the percentage of light absorbed by the overstorey canopy.
The fraction iR is calculated as:
where LAIimax is the LAI at which interception reaches its maximum level.
In the growing season, soil water availability (ASW) is computed for each canopy layer as:
DeltaASW is given by the following formula in the growing season:
and it’s calculated as: