Latent Heat Penman-Monteith-Type Method — latent

Calculates latent heat flux using a simplified Penman-Monteith-type combination equation. Positive latent heat flux signifies flux away from the surface. Negative latent heat flux indicates flux toward the surface or a condensation-like direction, depending on context.

Usage

latent_penman(...)

# Default S3 method
latent_penman(
  datetime,
  v,
  temp,
  rh,
  z = 2,
  rad_bal,
  elev,
  lat,
  lon,
  soil_flux,
  obs_height,
  surface_type,
  ...
)

# S3 method for class 'weather_station'
latent_penman(weather_station, ...)

Arguments

...: Additional arguments.
datetime: POSIXt object (POSIXct, POSIXlt). See base::as.POSIXlt and base::strptime for conversion.
v: Wind velocity in m/s.
temp: Air temperature in degrees C.
rh: Relative humidity in %.
z: Height of measurement for temperature and wind speed in m.
rad_bal: Net radiation balance in W m-2.
elev: Elevation above sea level in m.
lat: Latitude in decimal degrees.
lon: Longitude in decimal degrees.
soil_flux: Soil heat flux in W m-2.
obs_height: Observation height in m. Used for calculating aerodynamic resistance.
surface_type: Surface type for determining surface resistance. Options: `r surface_resistance$surface_type“.
weather_station: A weather_station object.

Value

Latent heat flux in W m-2.

Details

The latent heat flux ($Q_e$) using the simplified Penman-Monteith-type method is calculated as: $$Q_e = \frac{\Delta (R_n - G) + \gamma \frac{c_p \rho}{r_a} (e_s - e_a)}{\Delta + \gamma (1 + \frac{r_s}{r_a})}$$ where: $\Delta$ is the slope of the saturation vapor pressure curve, $\gamma$ is the psychrometric constant, $R_n$ is the net radiation, $G$ is the soil heat flux, $c_p$ is the specific heat of air, $\rho$ is the air density, $r_a$ is the aerodynamic resistance, $r_s$ is the surface resistance, $e_s$ is the saturation vapor pressure, and $e_a$ is the actual vapor pressure.

pres_sat_vapor_p() and pres_vapor_p() return pressure in hPa. latent_penman() converts $e_s$ and $e_a$ internally to kPa before computing the aerodynamic vapour-pressure-deficit term. $\Delta$ and $\gamma$ are handled on the same kPa scale.

The aerodynamic resistance ($r_a$) is calculated based on wind speed, observation height, and surface roughness. The surface resistance ($r_s$) is selected based on the specified surface type. The resistance table and fieldClim surface aliases are package parameters/mappings used with this Penman-Monteith-type implementation; they are not documented here as a one-to-one source table. The function returns latent heat flux only; it does not return evaporation depth or a paired sensible heat flux and is not forced to close $R_n - G$.

For weather-station objects, the method uses hum1 as relative humidity when present and otherwise uses rh. Both fields are interpreted as relative humidity in percent for this calculation.

Available Surface Types:

Temperate grassland
Coniferous forest
Temperate deciduous forest
Tropical rain forest
Cereal crops
Broadleaved herbaceous crops

surface_type may either be a Penman resistance class (Temperate grassland, Cereal crops, ...), or a mapped fieldClim surface class such as field, lawn, agriculture, coniferous forest, deciduous forest, mixed forest, or shrub. For example, field is mapped to Temperate grassland.

References

Monteith, John L., Mike H. Unsworth, and Ann Webb. "Principles of environmental physics." Quarterly Journal of the Royal Meteorological Society 120.520 (1994): 1699.

Foken 2016, method background for Penman-type combination methods.

Examples

# Calculate latent heat flux using the Penman-Monteith-type method
datetime <- as.POSIXlt("2022-07-15 12:00:00")
latent_penman(
  datetime = datetime, v = 2, temp = 25, rh = 60, z = 2, rad_bal = 200,
  elev = 100, lat = 50, lon = 8,
  soil_flux = 50, obs_height = 10,
  surface_type = "Temperate grassland"
)
#> Warning: latent_penman: invalid aerodynamic resistance for some values; returning NA there.
#> [1] NA