R/calc_vp.R

In stineb/ingestr: Environmental point data ingest

#' Calculate vapor pressure from relative humidity
#'
#' Follows Abtew and Meleese (2013), Ch. 5 Vapor Pressure Calculation Methods
#'
#' @param qair Air specific humidity (g g-1)
#' @param patm Atmospehric pressure (Pa)
#' @param elv Elevation above sea level (m) (Used only if \code{patm} is missing 
#' for calculating it based on standard sea level pressure)
#'
#' @return vapor pressure (Pa)
#' @export
#' 
calc_vp <- function(
  qair,
  patm = NA,
  elv = NA
) {
  
  # Ref:      Eq. 5.1, Abtew and Meleese (2013), Ch. 5 Vapor Pressure 
  #           Calculation Methods, in Evaporation and Evapotranspiration: 
  #           Measurements and Estimations, Springer, London.
  #             vpd = 0.611*exp[ (17.27 tc)/(tc + 237.3) ] - ea
  #             where:
  #                 tc = average daily air temperature, deg C
  #                 eact  = actual vapor pressure, Pa
  

  # calculate atmopheric pressure (Pa) assuming standard 
  # conditions at sea level (elv=0)
  # if (is.na(elv) && is.na(patm)){
  #   
  #   warning(
  #     "
  #     calc_vp(): Either patm or elv must be provided 
  #     if eact is not given.
  #     ")
  #   vp <- NA
  #   
  # } else {

    patm <- ifelse(is.na(patm),
                   calc_patm(elv),
                   patm)
    
    # Calculate VP.
    vp <- calc_vp_inst(qair = qair, patm = patm)
  # }
  return( vp )
  
}

#' Instantaneous vapour pressure
#'
#' Follows Abtew and Meleese (2013), Ch. 5 Vapor Pressure Calculation Methods
#'
#' @param qair Air specific humidity (g g-1)
#' @param patm Atmospheric pressure (Pa)
#'
#' @return Instantaneous vapour pressure
#' @export

calc_vp_inst <- function(
  qair,
  patm
){
  
  # Ref:      Eq. 5.1, Abtew and Meleese (2013), Ch. 5 Vapor Pressure 
  #           Calculation Methods, in Evaporation and Evapotranspiration: 
  #           Measurements and Estimations, Springer, London.
  #             vpd = 0.611*exp[ (17.27 tc)/(tc + 237.3) ] - ea
  #             where:
  #                 tc = average daily air temperature, deg C
  #                 ea  = actual vapor pressure, Pa
  
  kR  = 8.3143   # universal gas constant, J/mol/K (Allen, 1973)
  kMv = 18.02    # molecular weight of water vapor, g/mol (Tsilingiris, 2008)
  kMa = 28.963   # molecular weight of dry air, g/mol (Tsilingiris, 2008)
  
  # calculate the mass mixing ratio of water vapor to dry air (dimensionless)
  wair <- qair / (1 - qair)
  
  # calculate water vapor pressure 
  rv <- kR / kMv
  rd <- kR / kMa
  eact = patm * wair * rv / (rd + wair * rv)
  
  return( eact )
}