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()
andwater_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