Skip to contents

24 Hour FAO 56 and ASCE-EWRI Reference Evapotranspiration ETo

Source: R/ETo.R
ETo.Rd

24 Hour FAO 56 and ASCE-EWRI Reference Evapotranspiration ETo

Usage

ETo(
  Tmax,
  Tmin,
  DOY,
  latitude,
  longitude = NULL,
  z = NULL,
  uz = 2,
  altitude = NULL,
  RHmax = NULL,
  RHmin = NULL,
  n = NULL,
  Krs = 0.16,
  albedo = 0.23,
  as = 0.25,
  bs = 0.5,
  Rs = NULL,
  Rn = NULL,
  G = 0,
  EF = NULL,
  wmo = NULL,
  airport = NULL,
  scale = "sine function",
  Kc = NULL,
  surface = "grass"
)

# Default S3 method
ETo(
  Tmax,
  Tmin,
  DOY,
  latitude,
  longitude = NULL,
  z = NULL,
  uz = 2,
  altitude = NULL,
  RHmax = NULL,
  RHmin = NULL,
  n = NULL,
  Krs = 0.16,
  albedo = 0.23,
  as = 0.25,
  bs = 0.5,
  Rs = NULL,
  Rn = NULL,
  G = 0,
  EF = NULL,
  wmo = NULL,
  airport = NULL,
  scale = "evaporative fraction",
  Kc = NULL,
  surface = "grass"
)

Arguments

Tmax

Numeric. Maximum air Temperature in degree Celsius

Tmin

Numeric. Minimum air Temperature in degree Celsius

DOY

Numeric or Date [YYYY-mm-dd]. Day of the Year. If you give data in the form of date [YYYY-mm-dd], it will be converted to DOY

latitude

geographical coordinates in decimal degrees. It should be negative for southern hemisphere

longitude

The longitude of the measurement site i.e. geographical coordinates in decimal degrees for the weather station. It should be negative for West and positive for East.

z

The height above surface where wind speed was measured, in metres.

uz

The wind speed in m/s

altitude

The elevation of the weather station, in metres

RHmax

Maximum Relative Humidity in percent

RHmin

Minimum Relative Humidity in percent

n

Sunshine hours

Krs

Relationship between the fraction of extraterrestrial radiation that reaches the earth's surface, Rs/Ra, and the air temperature difference Tmax - Tmin for interior (Krs = 0.16) and coastal (Krs = 0.19) regions

albedo

albedo values. For grass it is 0.23[dimensionless]

as

Regression constant, intercept, expressing the fraction of extra-terrestrial radiation reaching the earth on overcast days (n = 0); either a calibrated value or as = 0.25 can be used.

bs

regression constant, slope, expressing the fraction of extraterrestrial radiation reaching the earth on overcast days (n = 0); either a calibrated value or bs = 0.5 can be used.

Rs

shortwave incoming radiation [MJ/m2/day]

Rn

Net daily radiation in MJ/m2/day. If it is NULL, it will be computed

G

Daily Soil heat flux in W/m2

EF

Spatial. ET fraction [-] from sebal or tseb sebs. It is advisable to link it to any of the two functions.

wmo

numeric. World Meteorological Organization weather station code. You can use the Worldwide Station List at https://www.wunderground.com/ [Accessed on 2016-5-6] or https://www.wetterzentrale.de/ [Accessed on 2016-5-6]

airport

numeric. IATA or ICAO code. They are alphabetically listed at https://en.wikipedia.org/wiki/List_of_airports_by_IATA_code:_A [Accessed on 2016-5-6]

scale

character The type of scale models to upscale instantaneous ET to daily ET. it takes "Evaporative Fraction" (Bastiaanssen et al 1998); "Sine Function" (Jackson et al 1983); "Solar Radiation" (French et al 2005). Remember the blank spaces between the parameters. You can also respectively use "evaporative" or "solar" or "sine".

Kc

Crop coefficient

surface

character. It is either "grass" or "alfalfa"

Value

ETo:

24 hour reference ETo [mm/day]

ETa:

24 hour actual ET. It is not NULL if EF has a numeric value

ETc:

24 hour crop ET. It is not NULL if Kc has a numeric value

Rn:

Net daily Radiation [W/m2]

Rn_mj_m2_day:

Net daily Radiation [MJ/m2/day]

y:

psychrometric constant [kPa/degree Celsius]

slope:

slope of saturation vapour pressure curve [kPa/degree Celsius]

vpd:

Vapour pressure deficit[kPa]

References

ALLEN, R. G., PEREIRA, L. S., RAES, D., & SMITH, M. 1996. Crop Evapotranspiration (guidelines for computing crop water requirements) FAO Irrigation and Drainage Paper No. 56: FAO.

See also

ETohr

Author

George Owusu

Examples

# \donttest{
#with minimal data
PET=ETo(Tmax=31,Tmin=28,latitude=5.6,DOY="6-2-2001")
#> Computing ETr24 or ETo24
PET$ETo
#> [1] 2.373057
#with enough data as in FAO56 EXAMPLE 18 on 6 July in Uccle (Brussels, Belgium) 
#located at 50 degrees 48 minutes N and at 100 m above sea level
PET24=ETo(Tmax=21.5,Tmin=12.3,z=10,uz=2.78,altitude=100,
RHmax=84,RHmin=63,n=9.25,DOY=187,latitude=50.8)
#> Computing ETr24 or ETo24
PET24$ETo
#> [1] 3.880216
# }