Evapotranspiration

Compute an estimate of reference (= potential) evapotranspiration from meteorologial data. Evapotranspiration from vegetation includes transpiraction by plants plus evaporation from the soil or other wet surfaces. \(ET_0\) is the reference value assuming no limitation to transpiration due to soil water, similar to potential evapotranspiration (PET). An actual evapotranpiration value \(ET\) can be estimated only if additional information on the plants and soil is available.

Usage

ET_ref(
  temperature,
  water.vp,
  wind.speed,
  net.irradiance,
  nighttime = FALSE,
  atmospheric.pressure = 10.13,
  soil.heat.flux = 0,
  method = "FAO.PM",
  check.range = TRUE
)

ET_ref_day(
  temperature,
  water.vp,
  wind.speed,
  net.radiation,
  atmospheric.pressure = 10.13,
  soil.heat.flux = 0,
  method = "FAO.PM",
  check.range = TRUE
)

Arguments

temperature

numeric vector of air temperatures (C) at 2 m height.

water.vp

numeric vector of water vapour pressure in air (Pa).

wind.speed

numeric Wind speed (m/s) at 2 m height.

net.irradiance

numeric Long wave and short wave balance (W/m2).

nighttime

logical Used only for methods that distinguish between daytime- and nighttime canopy conductances.

atmospheric.pressure

numeric Atmospheric pressure (Pa).

soil.heat.flux

numeric Soil heat flux (W/m2), positive if soil temperature is increasing.

method

character The name of an estimation method.

check.range

logical Flag indicating whether to check or not that arguments for temperature are within range of method. Passed to function calls to water_vp_sat() and water_vp_sat_slope().

net.radiation

numeric Long wave and short wave balance (J/m2/day).

Value

A numeric vector of reference evapotranspiration estimates expressed in mm/h for ET_ref() and ET_PM() and in mm/d for ET_ref_day().

Details

Currently three methods, based on the Penmann-Monteith equation formulated as recommended by FAO56 (Allen et al., 1998) as well as modified in 2005 for tall and short vegetation according to ASCE-EWRI are implemented in function ET_ref(). The computations rely on data measured according WHO standards at 2 m above ground level to estimate reference evapotranspiration (\(ET_0\)). The formulations are those for ET expressed in mm/h, but modified to use as input flux rates in W/m2 and pressures expressed in Pa.

References

Allen R G, Pereira L S, Raes D, Smith M. 1998. Crop evapotranspiration: Guidelines for computing crop water requirements. Rome: FAO. Allen R G, Pruitt W O, Wright J L, Howell T A, Ventura F, Snyder R, Itenfisu D, Steduto P, Berengena J, Yrisarry J, et al. 2006. A recommendation on standardized surface resistance for hourly calculation of reference ETo by the FAO56 Penman-Monteith method. Agricultural Water Management 81.

See also

Other Evapotranspiration and energy balance related functions.: net_irradiance()

Examples

# instantaneous
ET_ref(temperature = 20,
       water.vp = water_RH2vp(relative.humidity = 70,
                              temperature = 20),
       wind.speed = 0,
       net.irradiance = 10)
#> [1] 0.0172792

ET_ref(temperature = c(5, 20, 35),
       water.vp = water_RH2vp(70, c(5, 20, 35)),
       wind.speed = 0,
       net.irradiance = 10)
#> [1] 0.01727809 0.01727920 0.01727963

# Hot and dry air
ET_ref(temperature = 35,
       water.vp = water_RH2vp(10, 35),
       wind.speed = 5,
       net.irradiance = 400)
#> [1] 0.6912254

ET_ref(temperature = 35,
       water.vp = water_RH2vp(10, 35),
       wind.speed = 5,
       net.irradiance = 400,
       method = "FAO.PM")
#> [1] 0.6912254

ET_ref(temperature = 35,
       water.vp = water_RH2vp(10, 35),
       wind.speed = 5,
       net.irradiance = 400,
       method = "ASCE.PM.short")
#> [1] 0.5875579

ET_ref(temperature = 35,
       water.vp = water_RH2vp(10, 35),
       wind.speed = 5,
       net.irradiance = 400,
       method = "ASCE.PM.tall")
#> [1] 0.5876088

# Low temperature and high humidity
ET_ref(temperature = 5,
       water.vp = water_RH2vp(95, 5),
       wind.speed = 0.5,
       net.irradiance = -10,
       nighttime = TRUE,
       method = "ASCE.PM.short")
#> [1] -0.01468528

ET_ref_day(temperature = 35,
           water.vp = water_RH2vp(10, 35),
           wind.speed = 5,
           net.radiation = 35e6) # 35 MJ / d / m2
#> [1] 16.80062