VP {MeTo}

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

Tmax

maximum temperature during 24-hour period (for daily values)

Tmin

minimum temperature during 24-hour period (for daily values)

Rhmax

maximum relative humidity [precent] (for daily values)

Rhmin

minimum relative humidity [precent] (for daily values)

interval

hour, day, week or month

Tmean

Mean air temperature [degreeC] (for periods shorter 1 day)

Rhmean

Mean air humidity [percent] (for periods shorter 1 day or if Rhmax and Rhmin are missing)

Twet

wet bulb temperature (for calculation with psychrometric data)

Tdry

dry bulb tamperature (for calculation with psychrometric data)

apsy

coefficient depending on the type of ventilation of the wet bulb [kPa/(degreeC)] (for calculation with psychrometric data)

P

atmospheric pressure [kPa]

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)