FAO-56 Penman-Monteith grass reference evapotranspiration

Description

FAO Penman-Monteith equations to compute grass reference evapotranspiration from weather data for daily, monthly, hourly or shorter periods.

Usage

ETref(
  x,
  Tmax = NULL,
  Tmin = NULL,
  Rhmax = NULL,
  Rhmin = NULL,
  Tmean = NULL,
  Rhmean = NULL,
  u = NULL,
  Rs = NULL,
  n = NULL,
  P = NULL,
  elev,
  lat.rad = NULL,
  lat.deg = NULL,
  long.deg = NULL,
  tl,
  G = NULL,
  actVP = NULL,
  control = list()
)

Arguments

Details

x:

must be provided as.numeric (1-366) or as a common date-time object (e.g, POSIXct, POSIXlt or Date objects). All formats for which is.timepoint from the lubridate package returns TRUE can be used

control: (see also controlDefaults and constDefaults)

Lz:
longitude of the centre of the local time zone (degrees west of Greenwich)
- 0 for Greenwich
- 345 for Germany
- 330 for Cairo (Egypt)
- 255 for Bangkok (Thailand)
- 75, 90, 105 and 120 for Eastern, Central, Rocky Mountain and Pacific time zones (United States)
Lz is only needed if calculation period is shorter than 1 day.

uz: height of wind measurements (m)

albedo: default 0.23 for the hypothetical grass and alfalfa reference crops used in the FAO-56 PM equations

as: regression constant, expressing fraction of extraterrestrial radiation reaching earth on overcast days (n = 0) (default = 0.25)

bs: as + bs fraction of extraterrestrial radiation reaching earth on clear days (n = N) (default = 0.5)

Tko: reference temperature [degreeC] at elevation z0. Only needed if atmospheric pressure is missing. Often assumed to be 20 degreeC.

z0: elevation at reference level (fefault = 0 [m])

est.ratio.Rs.Rso:
Rs/Rso is used to represent cloud cover. For hourly or shorter periods during the nighttime, the ratio Rs/Rso is set equal to the Rs/Rso calculated for a time period occurring 2-3 hours before sunset. If single values during nighttime are calculated Rs/Rso ration 2-3 hours before sunset can not be calculated and an approximation is needed. Following Allen (1999) one can assume Rs/Rso = 0.4 to 0.6 during nighttime periods in humid and subhumid climates and Rs/Rso = 0.7 to 0.8 in arid and semiarid climates. A value of Rs/Rso = 0.3 presumes total cloud cover.

Value

grass reference evapotranspiration [mm]

Note

eq. 6 from reference for daily and eq. 53 for hourly or shorter periods

Author(s)

Ullrich Dettmann

References

Allen, R. G., Pereira, L. S., Raes, D., & Smith, M. (1998). Crop evapotranspiration-Guidelines for computing crop water requirements-FAO Irrigation and drainage paper 56. FAO, Rome, 300(9).

Examples

# --------------------------------------------
#  Daily Evapotranspiration
# --------------------------------------------

ETref(x = 187, Rs = 22.07, elev = 100, lat.deg = 50.8, Tmax = 21.5, Tmin = 12.3,
      Rhmax = 84, Rhmin = 63,
      u = 2.78, control = list(uz = 10), P = 100.1)

# Calculation with sunshine hour (n) instead of
# global radiation (Rs) (Rs ist estimated from n with estRs):

ETref(x = 187, n = 9.25, elev = 100, lat.deg = 50.8, Tmax = 21.5, Tmin = 12.3,
     Rhmax = 84, Rhmin = 63,
     u = 2.78, control = list(uz = 10), P = 100.1)

# --------------------------------------------
#  Hourly Evapotranspiration
# --------------------------------------------

ETref(x = as.POSIXct(c('2018-10-01 14:30', '2018-10-01 15:30')),
      Tmean = c(38, 37.8), Rhmean = c(52, 52.3), u = c(3.3, 3.2), Rs = c(2.450, 2.5), elev = 8,
      lat.deg = 16.22, long.deg = 343.75, G = c(0.175, 0.178) , P = c(101.21, 101.21) ,
      control = list(Lz = 15))

# If only one time step is calculated tl must be provided (1 for hourly, 0.5 for 30 minute periods):

ETref(x = as.POSIXct('2018-10-01 14:30'), tl = 1,
      Tmean = 38, Rhmean = 52, u = 3.3, Rs = 2.450, elev = 8,
      lat.deg = 16.22, long.deg = 343.75, G = 0.1749218, P = 101.2056,
      control = list(Lz = 15))

# Calculation with missing soil heat flux (G) and atmospheric pressure (P) (G is estimated with estG
# and P with estP)

ETref(x = as.POSIXct('2018-10-01 14:30'), tl = 1,
      Tmean = 38, Rhmean = 52, u = 3.3, Rs = 2.450, elev = 8,
      lat.deg = 16.22, long.deg = 343.75,
      control = list(Lz = 15))
# --------------------------------------------

Extraterrestrial radiation

Description

Extraterrestrial radiation [MJ/(m2 time)] in dependence to time, latitude and longitude.

Usage

Ra(x, lat.rad = NULL, lat.deg = NULL, long.deg, tl, control = list(Lz = 345))

Arguments

Details

x:

must be provided as.numeric (1-366) or as a common date-time object (e.g, POSIXct, POSIXlt or Date objects). All formats for which is.timepoint from the lubridate package returns TRUE can be used

control:

Lz (for periods < 1 day):
longitude of the centre of the local time zone (degrees west of Greenwich)
- 0 for Greenwich
- 345 for Germany
- 330 for Cairo (Egypt)
- 255 for Bangkok (Thailand)
- 75, 90, 105 and 120 for Eastern, Central, Rocky Mountain and Pacific time zones (United States)

Note

eq. 21 (period = 1 day) and eq. 28 (hourly or shorter) of the reference

References

Allen, R. G., Pereira, L. S., Raes, D., & Smith, M. (1998). Crop evapotranspiration-Guidelines for computing crop water requirements-FAO Irrigation and drainage paper 56. FAO, Rome, 300(9).

Examples

# --------------------------------------------
#  Daily period
# --------------------------------------------

Ra(x = 105, lat.deg = 13.73)

# --------------------------------------------
#  Hourly period
# --------------------------------------------

Ra(x = as.POSIXct(c('2018-10-01 14:30', '2018-10-01 15:30')),
   lat.deg = 16.21, long.deg = 343.75, control = list(Lz = 15))

Ra(x = as.POSIXct('2018-10-01 14:30'), tl = 1,
   lat.deg = 16.21, long.deg = 343.75, control = list(Lz = 15))

Net radiation (Rn)

Description

Difference between the incoming net shortwave radiation (Rns) and the outgoing net longwave radiation (Rnl).

Usage

Rn(
  x,
  Tmax = NULL,
  Tmin = NULL,
  Rhmax = NULL,
  Rhmin = NULL,
  Rs = NULL,
  n = NULL,
  elev,
  lat.rad = NULL,
  lat.deg = NULL,
  long.deg = NULL,
  Rhmean = NULL,
  actVP = NULL,
  Tmean = NULL,
  tl,
  control = list()
)

Arguments

Details

for daily and hourly calculations

x:

must be provided as.numeric (1-366) or as a common date-time object (e.g, POSIXct, POSIXlt or Date objects). All formats for which is.timepoint from the lubridate package returns TRUE can be used

control:

albedo: default 0.23 for the hypothetical grass and alfalfa reference crops used in the FAO-56 PM equations

as: regression constant, expressing fraction of extraterrestrial radiation reaching earth on overcast days (n = 0) (default = 0.25)

bs: as + bs fraction of extraterrestrial radiation reaching earth on clear days (n = N) (default = 0.5)

Value

net radiation

Note

eq. 40 of reference

References

Allen, R. G., Pereira, L. S., Raes, D., & Smith, M. (1998). Crop evapotranspiration-Guidelines for computing crop water requirements-FAO Irrigation and drainage paper 56. FAO, Rome, 300(9).

See Also

Rns, Rnl

Examples

# --------------------------------------------
#  Daily period
# --------------------------------------------

Rn(x = 105, n = 8.5, elev = 2, actVP = 2.85, Tmax = 34.8,
   Tmin = 25.6, lat.deg = 13.73)

Rn(x = 135, elev = 1, Rs = 14.5, Tmax = 25.1, Tmin = 19.1,
   lat.deg = -22.9, actVP = 2.1)

# --------------------------------------------
#  Hourly period
# --------------------------------------------

Rn(x = as.POSIXct(c('2018-10-01 14:30', '2018-10-01 15:30')), Tmean = c(38, 37.8),
   Rhmean = c(52, 52.2), Rs = c(2.450, 2.1), elev = 8, lat.deg = 16.2,
   long.deg = 343.75, control = list(Lz = 15))

Rn(x = as.POSIXct('2018-10-01 14:30'), Tmean = 38, Rhmean = 52, tl = 1, Rs = 2.450,
   elev = 8, lat.deg = 16.2, long.deg = 343.75, control = list(Lz = 15))

Net longwave radiation (Rnl)

Description

Net longwave radiation (Rnl).

Usage

Rnl(
  x,
  Tmax = NULL,
  Tmin = NULL,
  Rhmax = NULL,
  Rhmin = NULL,
  Rs = NULL,
  lat.rad = NULL,
  lat.deg = NULL,
  long.deg = NULL,
  elev,
  actVP = NULL,
  tl,
  Tmean = NULL,
  Rhmean = NULL,
  control = list()
)

Arguments

Details

x:

must be provided as.numeric (1-366) or as a common date-time object (e.g, POSIXct, POSIXlt or Date objects). All formats for which is.timepoint from the lubridate package returns TRUE can be used

control:

Lz:
longitude of the centre of the local time zone (degrees west of Greenwich)
- 0 for Greenwich
- 345 for Germany
- 330 for Cairo (Egypt)
- 255 for Bangkok (Thailand)
- 75, 90, 105 and 120 for Eastern, Central, Rocky Mountain and Pacific time zones (United States)
Lz is only needed if calculation period is shorter 1 day.

est.ratio.Rs.Rso:
Rs/Rso is used to represent cloud cover. For hourly or shorter periods during the nighttime, the ratio Rs/Rso is set equal to the Rs/Rso calculated for a time period occurring 2-3 hours before sunset. If single values during nighttime are calculated Rs/Rso ration 2-3 hours before sunset can not be calculated and an approximation is needed. Following Allen (1999) one can assume Rs/Rso = 0.4 to 0.6 during nighttime periods in humid and subhumid climates and Rs/Rso = 0.7 to 0.8 in arid and semiarid climates. A value of Rs/Rso = 0.3 presumes total cloud cover.

Value

Rnl net outgoing longwave radiation [MJ(/m2 time)]

Note

eq. 39 of reference

References

Allen, R. G., Pereira, L. S., Raes, D., & Smith, M. (1998). Crop evapotranspiration-Guidelines for computing crop water requirements-FAO Irrigation and drainage paper 56. FAO, Rome, 300(9).

Net solar or net shortwave radiation (Rns)

Description

Net shortwave radiation is the balance between incoming and reflected solar radiation.

Usage

Rns(Rs, control = list(albedo = 0.23))

Arguments

Details

control:

albedo [-]: 0.23 for hypothetical grass and alfalfa reference crops used in the FAO-56 PM equations

valid for daily and shorter periods

Value

Rns net solar or shortwave radiation [MJ/(m2 time)]

Note

eq. 38 of reference

References

Allen, R. G., Pereira, L. S., Raes, D., & Smith, M. (1998). Crop evapotranspiration-Guidelines for computing crop water requirements-FAO Irrigation and drainage paper 56. FAO, Rome, 300(9).

Examples

Rns(Rs = 22.1)
Rns(Rs = 22.1, control = list(albedo = 0.20))

Clear-sky solar radiation (Rso)

Description

Clear-sky solar radiation for daily and shorter periods.

Usage

Rso(
  x,
  lat.rad = NULL,
  lat.deg = NULL,
  long.deg = NULL,
  elev,
  tl,
  control = list(Lz = 345)
)

Arguments

Details

x:

must be provided as.numeric (1-366) or as a common date-time object (e.g, POSIXct, POSIXlt, and Date objects). All formats for which is.timepoint from the lubridate package returns TRUE can be used

control:

Lz:
longitude of the centre of the local time zone (degrees west of Greenhich)
- 0 for Greenwich
- 345 for Germany
- 330 for Cairo (Egypt)
- 255 for Bangkok (Thailand)
- 75, 90, 105 and 120 for Eastern, Central, Rocky Mountain and Pacific time zones (United States)
Lz is only needed if calculation period is shorter 1 day.

for day, hour and shorter periods

References

eq. 37; Allen, R. G., Pereira, L. S., Raes, D., & Smith, M. (1998). Crop evapotranspiration-Guidelines for computing crop water requirements-FAO Irrigation and drainage paper 56. FAO, Rome, 300(9).

Examples

# --------------------------------------------
#  Daily period
# --------------------------------------------

Rso(x = 135, elev = 1, lat.deg = -22.9)

# --------------------------------------------
#  Hourly period
# --------------------------------------------

Rso(x = as.POSIXct('2018-10-01 12:30'), tl = 1, elev = 8, lat.deg = 16.2,
    long.deg = 343.75, control = list(Lz = 15))

Solar declination

Description

Calculate solar declination for daily and shorther periods.

Usage

SolarDec(x)

Arguments

Details

x:

must be provided as number (1-366) or as a common date-time object (e.g, POSIXct, POSIXlt or Date objects). All formats for which is.timepoint from the lubridate package returns TRUE can be used

Note

eq. 24 of reference

References

Allen, R. G., Pereira, L. S., Raes, D., & Smith, M. (1998). Crop evapotranspiration-Guidelines for computing crop water requirements-FAO Irrigation and drainage paper 56. FAO, Rome, 300(9).

Actual vapor pressure

Description

Calculate actual vapour pressure [kPa] either from psychrometric data or from relative humidity data.

Usage

VP(
  Tmax = NULL,
  Tmin = NULL,
  Rhmax = NULL,
  Rhmin = NULL,
  interval = "day",
  Tmean = NULL,
  Rhmean = NULL,
  Twet = NULL,
  Tdry = NULL,
  apsy = NULL,
  P = NULL
)

Arguments

Details

x:

must be provided as.numeric (1-366) or as a common date-time object (e.g, POSIXct, POSIXlt or Date objects). All formats for which is.timepoint from the lubridate package returns TRUE can be used

interval:

- use hour for periods <= one hour
- for day, week or month the same equations are used

Note

eq. 17 of reference (Determination of actual vapour pressure from relative maximum and minimum humidity)

eq. 15 of reference (Actual vapour pressure derived from psychrometric data) (see psyc_cons)

eq. 19 of reference (used in the absence of RHmax and RHmin)

eq. 54 of reference (for periods shorter than a day)

References

Allen, R. G., Pereira, L. S., Raes, D., & Smith, M. (1998). Crop evapotranspiration-Guidelines for computing crop water requirements-FAO Irrigation and drainage paper 56. FAO, Rome, 300(9).

See Also

e0, satVP

Examples

VP(Tmax = 25, Tmin = 18, Rhmax = 82, Rhmin = 54)
VP(Tmax = 25, Tmin = 18, Rhmean = 68)

Adjust wind speed data

Description

Adjust wind speed data to 2 meter height.

Usage

adj_u2(u, uz)

Arguments

Note

eq. 47 of reference

References

Allen, R. G., Pereira, L. S., Raes, D., & Smith, M. (1998). Crop evapotranspiration-Guidelines for computing crop water requirements-FAO Irrigation and drainage paper 56. FAO, Rome, 300(9).

Examples

adj_u2(3.2, uz = 10)

Constants

Description

Constants required for calculations.

Usage

constDefaults

Format

GSC: solar constant (0.0820 [MJ/(m2 min)])

sigma: Stefan-Boltzmann constant (4.903 10^-9 [MJ/(K4 m2 day)])

a1: constant lapse rate moist air (0.0065 [K/m]) (e.g. in estP)

R: specific gas constant (287 [J/(kg K)]) (e.g. in estP)

g: gravitational acceleration (9.807 [m/s2]) (e.g. in estP)

eps: ratio molecular weight of water vapour/dry air (0.622) (e.g. in psyc_cons)

lambda: latent heat of vaporization (2.45 [MJ/kg]) (e.g. in psyc_cons)

cp: specific heat at constant pressure (1.013 10-3 [MJ/(kg degreeC)]) (e.g. in psyc_cons)

Control default values

Description

Default values for control. If necessary modify with control = list() in function call.

Usage

controlDefaults

Format

albedo: 0.23 [-] for hypothetical grass and alfalfa reference crops used in the FAO-56 PM equations (e.g. in ETref, Rn or estG)

Po: atmospheric pressure at sea level (101.3 [kPa])

z0: elevation at reference level (0 [m]) (e.g. in ETref or estP)

Tko: reference temperature [degreeC] at elevation z0. Often assumed to be 20 degreeC (e.g. in ETref or estP)

uz: height of windspeed measurement above ground surface (2 [m]) (e.g. in ETref or adj_u2)

Lz:
longitude of the centre of the local time zone (degrees west of Greenwich)
- 0 for Greenwich
- 345 for Germany
- 330 for Cairo (Egypt)
- 255 for Bangkok (Thailand)
- 75, 90, 105 and 120 for Eastern, Central, Rocky Mountain and Pacific time zones (United States)
Lz is only needed if calculation period is shorter 1 day.

as: regression constant, expressing fraction of extraterrestrial radiation reaching earth on overcast days (default = 0.25) (e.g. in ETref, Rn or estRs)

bs: as + bs fraction of extraterrestrial radiation reaching earth on clear days (default = 0.5) (e.g. in ETref, Rn or estRs)

est.ratio.Rs.Rso:
Rs/Rso is used to represent cloud cover. For hourly or shorter periods during the nighttime, the ratio Rs/Rso is set equal to the Rs/Rso calculated for a time period occurring 2-3 hours before sunset. If single values during nighttime are calculated Rs/Rso ratio 2-3 hours before sunset can not be calculated and an approximation is needed. Following Allen (1999) one can assume Rs/Rso = 0.4 to 0.6 during nighttime periods in humid and subhumid climates and Rs/Rso = 0.7 to 0.8 in arid and semiarid climates. A value of Rs/Rso = 0.3 presumes total cloud cover.

References

Allen, R. G., Pereira, L. S., Raes, D., & Smith, M. (1998). Crop evapotranspiration-Guidelines for computing crop water requirements-FAO Irrigation and drainage paper 56. FAO, Rome, 300(9).

Slope of the saturation vapor pressure curve

Description

Slope of the saturation vapor pressure curve [kPa/degreeC].

Usage

deltaVP(Tmean = NULL, Tmax = NULL, Tmin = NULL)

Arguments

Details

valid for daily, hourly and shorter periods

Value

slope of the saturation vapor pressure curve [kPa/degreeC]

Note

eq. 13 of reference

References

Allen, R. G., Pereira, L. S., Raes, D., & Smith, M. (1998). Crop evapotranspiration-Guidelines for computing crop water requirements-FAO Irrigation and drainage paper 56. FAO, Rome, 300(9).

Examples

deltaVP(Tmax = 34.8, Tmin = 25.6)
deltaVP(Tmean = 30.2)

Dayligth hours (N)

Description

Dayligth hours in dependence to latitude.

Usage

dlh(x, lat.rad = NULL, lat.deg = NULL)

Arguments

Details

x:

must be provided as number (1-366) or as a common date-time object (e.g, POSIXct, POSIXlt or Date objects). All formats for which is.timepoint from the lubridate package returns TRUE can be used

Note

eq. 34 of reference

References

Allen, R. G., Pereira, L. S., Raes, D., & Smith, M. (1998). Crop evapotranspiration-Guidelines for computing crop water requirements-FAO Irrigation and drainage paper 56. FAO, Rome, 300(9).

Examples

dlh(x = 105, lat.deg = 13.73)
dlh(x = 105, lat.rad = 0.283)
dlh(x = as.Date('2018-04-15'), lat.deg = 13.73)

Inverse relative distance Earth-Sun

Description

Calculate inverse relative distance Earth-Sun for daily and shorther periods.

Usage

dr(x)

Arguments

Details

x:

must be provided as number (1-366) or as a common date-time object (e.g, POSIXct, POSIXlt or Date objects). All formats for which is.timepoint from the lubridate package returns TRUE can be used

Note

eq. 23 of reference

References

Allen, R. G., Pereira, L. S., Raes, D., & Smith, M. (1998). Crop evapotranspiration-Guidelines for computing crop water requirements-FAO Irrigation and drainage paper 56. FAO, Rome, 300(9).

Saturation Vapor Pressure

Description

Mean saturation vapour pressure [kPa] for short time intervals less than a day. Calculated with air temperature.

Usage

e0(Temp)

Arguments

Value

saturation vapour pressure at air temperature [kPa/degreeC])

Note

eq. 11 of the reference

for day, week, decade or month, the mean saturation vapour pressure should be computed with satVP

References

Allen, R. G., Pereira, L. S., Raes, D., & Smith, M. (1998). Crop evapotranspiration-Guidelines for computing crop water requirements-FAO Irrigation and drainage paper 56. FAO, Rome, 300(9).

See Also

satVP, VP

Estimate soil heat flux (G)

Description

Estimate soil heat flux (G) for periods shorter than a day.

Usage

estG(
  x,
  Rs,
  Tmean,
  Rhmean,
  lat.rad = NULL,
  lat.deg = NULL,
  long.deg = NULL,
  elev = 1,
  tl,
  control = list(albedo = 0.23, Lz = 345, est.ratio.Rs.Rso = NA)
)

Arguments

Details

during daylight periods G is estimated to be Rn x 0.1 During nighttime G = Rn x 0.5. Day is defined for extraterrestrial radiation > 0.

x:

must be provided as a common date-time object (e.g, POSIXct, POSIXlt or Date objects). All formats for which is.timepoint from the lubridate package returns TRUE can be used

control:

albedo:
default 0.23 for the hypothetical grass and alfalfa reference crops used in
the FAO-56 PM equations

Lz:
longitude of the centre of the local time zone (degrees west of Greenwich)
- 0 for Greenwich
- 345 for Germany
- 330 for Cairo (Egypt)
- 255 for Bangkok (Thailand)
- 75, 90, 105 and 120 for Eastern, Central, Rocky Mountain and Pacific time zones (United States)
Lz is only needed if calculation period is shorter 1 day.

est.ratio.Rs.Rso:
Rs/Rso is used to represent cloud cover. For hourly periods during the nighttime, the ratio Rs/Rso is set equal to the Rs/Rso calculated for a time period occurring 2-3 hours before sunset. If single values during nighttime are calculated Rs/Rso ration 2-3 hours before sunset can not be calculated and an approximation is needed. Following Allen (1999) one can assume Rs/Rso = 0.4 to 0.6 during nighttime periods in humid and subhumid climates and Rs/Rso = 0.7 to 0.8 in arid and semiarid climates. A value of Rs/Rso = 0.3 presumes total cloud cover.

Note

eq. 45 and 46 of reference

References

Allen, R. G., Pereira, L. S., Raes, D., & Smith, M. (1998). Crop evapotranspiration-Guidelines for computing crop water requirements-FAO Irrigation and drainage paper 56. FAO, Rome, 300(9).

Examples

estG(x = as.POSIXct(c('2018-10-01 14:30', '2018-10-01 15:00')), Tmean = 38, Rhmean = 52, Rs = 2.450,
     elev = 8, lat.deg = 16.21, long.deg = 343.75, control = list(Lz = 15))

estG(x = as.POSIXct('2018-10-01 02:30'), Tmean = 28, Rhmean = 90, tl = 1, Rs = 0, elev = 8,
     lat.deg = 16.2, long.deg = 343.75, control = list(Lz = 15, est.ratio.Rs.Rso = 0.8))

estG(x = as.POSIXct('2018-10-01 14:30'), Tmean = 38, Rhmean = 52, tl = 1, Rs = 2.450, elev = 8,
     lat.deg = 16.21, long.deg = 343.75, control = list(Lz = 15))

Estimate atmospheric pressure (P)

Description

Values for atmospheric pressure as a function of altitude.

Usage

estP(elev, control = list(Tko = 20))

Arguments

Details

Control variables:

Tko: reference temperature [degreeC] at elevation z0. Often assumed to be 20 degreeC

z0: elevation at reference level [m]

a1: constant lapse rate moist air (0.0065 [K/m])

g: gravitational acceleration (9.807 [m/s2])

R: specific gas constant (287 [J/(kg K)])

Value

atmospheric pressure [kPa]

Note

eq. 3-2 of Reference

References

Allen, R. G., Pereira, L. S., Raes, D., & Smith, M. (1998). Crop evapotranspiration-Guidelines for computing crop water requirements-FAO Irrigation and drainage paper 56. FAO, Rome, 300(9).

Examples

estP(elev = 25, control = list(Tko = 20))

Estimate solar radiation (Rs)

Description

Rs is calculated from relative sunshine duration and extraterrestrial radiation with the Angstrom formula.

Usage

estRs(
  x,
  n,
  lat.rad = NULL,
  lat.deg = NULL,
  tl,
  control = list(as = 0.25, bs = 0.5)
)

Arguments

Details

control:

as: regression constant, expressing fraction of extraterrestrial radiation reaching earth on overcast days (n = 0) (default = 0.25)

bs: as + bs fraction of extraterrestrial radiation reaching earth on clear days (n = N) (default = 0.5)

x:

must be provided as.numeric (1-366) or as a common date-time object (e.g, POSIXct, POSIXlt or Date objects). All formats for which is.timepoint from the lubridate package returns TRUE can be used

Value

solar or shortwave radiation (Rs) [MJ/(m2 day)]

Note

eq. 35 of reference

References

Allen, R. G., Pereira, L. S., Raes, D., & Smith, M. (1998). Crop evapotranspiration-Guidelines for computing crop water requirements-FAO Irrigation and drainage paper 56. FAO, Rome, 300(9).

Examples

estRs(x = 135, n = 7, lat.rad = NULL, lat.deg = -22.9)

Prepare date

Description

Checks if x is date-time object or day of the year (doy) and returns doy.

Usage

prep.date(x)

Arguments

Value

Day of the year

psychrometric constant

Description

The psychrometric constant [kPa/degreeC]

Usage

psyc_cons(elev, P = NULL, control = list())

Arguments

Details

control:

Tko: reference temperature [degreeC] at elevation z0. Only needed if atmospheric pressure is missing. Often assumed to be 20 degreeC

elev: station elevation above sea level [m]. Needed if P = NULL for estP

lambda: latent heat of vaporization [MJ/kg]

eps: ratio molecular weight of water vapor/dry air = 0.622

cp: specific heat of moist air = 1.013 x 10-3 [MJ/(kg degreeC)]

Value

psychrometric constant [kPa/degreeC]

Note

eq. 8 of reference

References

Allen, R. G., Pereira, L. S., Raes, D., & Smith, M. (1998). Crop evapotranspiration-Guidelines for computing crop water requirements-FAO Irrigation and drainage paper 56. FAO, Rome, 300(9).

Examples

psyc_cons(elev = 2, P = 101.3)

Saturation Vapor Pressure

Description

Saturation vapor pressure [kPa].

Usage

satVP(
  Tmax = NULL,
  Tmin = NULL,
  Tmean = NULL,
  interval = "day",
  print.warning = T
)

Arguments

Details

interval:

hour (eq. 11 of reference)

interval:

day, week or month (eq. 12 of reference or eq. 11 if only Tmean is provided)

Value

Saturation Vapor Pressure [kPa]

References

Allen, R. G., Pereira, L. S., Raes, D., & Smith, M. (1998). Crop evapotranspiration-Guidelines for computing crop water requirements-FAO Irrigation and drainage paper 56. FAO, Rome, 300(9).

See Also

e0, VP

Examples

satVP(Tmax = 24.5, Tmin = 15, interval = 'day')
satVP(Tmax = 24.5, Tmin = 15, interval = 'week')
satVP(Tmax = 24.5, Tmin = 15, interval = 'month')

satVP(Tmax = 24.5, Tmin = 15, interval = 'hour')
satVP(Tmean = 19.75, interval = 'hour')

Solar time angle

Description

Solar time angle at midpoint of the period (for periods < 1 day).

Usage

w(x, long.deg, control = list(Lz = 345))

Arguments

Details

control:

Lz:
longitude of the centre of the local time zone (degrees west of Greenwich)
- 0 for Greenwich
- 345 for Germany
- 330 for Cairo (Egypt)
- 255 for Bangkok (Thailand)
- 75, 90, 105 and 120 for the Eastern, Central, Rocky Mountain and Pacific time zones (United States)

Note

eq. 31 of reference

References

Allen, R. G., Pereira, L. S., Raes, D., & Smith, M. (1998). Crop evapotranspiration-Guidelines for computing crop water requirements-FAO Irrigation and drainage paper 56. FAO, Rome, 300(9).

Sunset hour angle

Description

Sunset hour angle for given latitude and solar declination (SolarDec).

Usage

ws(x, lat.rad)

Arguments

Details

x:

must be provided as number (1-366) or as a common date-time object (e.g, POSIXct, POSIXlt or Date objects). All formats for which is.timepoint from the lubridate package returns TRUE can be used

Value

Sunset hour angle

Note

eq. 25 of reference

References

Allen, R. G., Pereira, L. S., Raes, D., & Smith, M. (1998). Crop evapotranspiration-Guidelines for computing crop water requirements-FAO Irrigation and drainage paper 56. FAO, Rome, 300(9).

Examples

ws(x = 246, lat.rad = -0.35)