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Actual evapotranspiration (AET, in mm day^-1) is distinguished into four components:

• interception loss from the vegetation canopies;
• transpiration through plants;
• evaporation directly from the soil;
• sublimation of snow.

The main drivers of all these components are the potential evapotranspiration (see petpar), the fraction of surface covered by vegetation (incl. phenology status), and the soilwater status. The main components are described in Gerten [1] et al. (2004), while more details on transpiration are given in Gerten [2] et al. (2007); sublimation was implemented by Schaphoff [3] et al. (2013).

Note that computing total AET requires a weighting of the individual components with their areal fraction (daily cover and phenology), accounting for the modified PT coefficient for the ET fraction, see Issue #885.

Interception loss ()E_I) is the amount of water, either rain or snow, that is stored in (i.e. intercepted by) the vegetation canopy and evaporated back to the atmosphere the same day (theoretically the water can be stored over longer or shorter intervals, but LPJmL resolves this process only at daily time step). Following Kergoat [4] (1998), canopy water storage (S_I) is assumed to be the product of daily precipitation (P), leaf area index (LAI), and a PFT- and CFT-specific parameter (i) that approximates the leaf form of the PFTs and the precipitation regime (rainfall intensity) where they typically grow. The parameter i is tabulated in Gerten et al. (2004); CFTs are parameterized as grass, woody bioenergy plants parameterized broadleaved trees. E_I is then given as the product of PET and the fraction of the day that the canopy is wet, the latter being the ratio of S_I and PET (with a maximum value of 1). The full equation is:

E_I = PET • Min(((P • LAI • i) / PET), 1)

The actual transpiration (mm d^-1; aet in LPJmL code) is calculated at a daily time step as the minimum of a supply function S (or supply) and a non-water stressed evapotranspiration rate D (or demand) (Haxeltine and Prentice, 1996; Gerten et al., 2004; Gerten et al., 2007):

aet=min(supply,demand)

demand

Transpiration is driven by photosynthetic rate, since $demand$ is a function of the canopy conductance (g):

Where wet is the fraction of day-time when the canopy is wet (and thus only interception occurs?), thus (1-wet) is the remaining day-time canopy-available energy (Gerten et al., 2004); pet is the daily total equilibrium evapotranspiration calculated from latitude, temperature and sunshine hours (see Petpar); (1.391, dimensionless) is the maximum Priestley-Taylor coefficient; g_p (mm s^-1) is the non-water stressed potential canopy conductance, predicted by the photosynthesis model, using a simple planetary boundary layer parametrization adapted from Monteith (1995); gm (3.26 mm s^-1) is a conductance scaling parameter (parameters values are derived from Monteith (1995); Huntingford and Monteith (1998)). The potential canopy conductance (g_p) is a function of photosynthetic rate (source:trunk/src/lpj/photosynthesis.c), thus linking growth rate and transpiration, and is given by (source:trunk/src/lpj/gp.c):

Where adtmm is the daytime net photosynthesis; co2 (bar) is the CO₂ ambient mole faction; is the stomata-controlled ratio between intercellular and ambient CO₂ partial pressure in the absence of water limitation.
Note: Compared with what reported in Gerten et al. (2004, 2007), in LPJmL code the parameter gmin (0.3-0.5 mm s^-1) is missing (or maybe included somewhere else in the code?). This parameter would be an additional term in the g_p equation and would account for plant water loss not associated with photosynthesis.

supply

Transpirational supply is determined by a function with the following general form:

where emax (mm d^-1) the maximum transpiration rate that can be sustained under well-watered conditions, which declines linearly with wr, the relative soil moisture (ratio between current soil water content and plant-available water capacity (wmax), the texture-dependent difference between field capacity and wilting point).

Soil evaporation (E_S) occurs only from the upper 30 cm of the soil column that is uncovered by vegetation; see soilwater routine.

Sublimation is set to 1.0 mm per day (also see permafrost).

source:trunk/src/lpj/interception.c
source:trunk/src/lpj/water_stressed.c
source:trunk/src/lpj/gp_sum.c
source:trunk/src/soil/waterbalance.c

Dieter Gerten, Sibyll Schaphoff

soilwater, photosynthesis, petpar

1. Gerten, D., S. Schaphoff, U. Haberlandt, W. Lucht, and S. Sitch (2004): Terrestrial vegetation and water balance—hydrological evaluation of a dynamic global vegetation model, Journal of Hydrology, 286, 1–4,249-270, doi: 10.1016/j.jhydrol.2003.09.029.

2. Gerten, D., S. Schaphoff, and W. Lucht (2007): Potential future changes in water limitations of the terrestrial biosphere, Climatic Change, 80, 3-4, 277-299, doi: 10.1007/s10584-006-9104-8.

3. Schaphoff, S., Heyder, U., Ostberg, S., Gerten D., Heinke, J. and Lucht, W. (2013): Contribution of permafrost soils to the global carbon budget. Environmental Research Letter, 8, 1, doi: 10.1088/1748-9326/8/1/014026

4. Kergoat, L. (1998): A model for hydrological equilibrium of leaf area index on a global scale. Journal of Hydrology, 212–213, 268–286, doi: 10.1016/S0022-1694(98)00211-X