Esat.slope {bigleaf}

Saturation Vapor Pressure (Esat) and Slope of the Esat Curve

Description

Calculates saturation vapor pressure (Esat) over water and the corresponding slope of the saturation vapor pressure curve.

Usage

Esat.slope(
  Tair,
  formula = c("Sonntag_1990", "Alduchov_1996", "Allen_1998"),
  constants = bigleaf.constants()
)

Arguments

Tair

Air temperature (deg C)

formula

Formula to be used. Either "Sonntag_1990" (Default), "Alduchov_1996", or "Allen_1998".

constants

Pa2kPa - conversion pascal (Pa) to kilopascal (kPa)

Details

Esat (kPa) is calculated using the Magnus equation:

Esat = a * exp((b * Tair) / (c + Tair)) / 1000

where the coefficients a, b, c take different values depending on the formula used. The default values are from Sonntag 1990 (a=611.2, b=17.62, c=243.12). This version of the Magnus equation is recommended by the WMO (WMO 2008; p1.4-29). Alternatively, parameter values determined by Alduchov & Eskridge 1996 or Allen et al. 1998 can be used (see references). The slope of the Esat curve (\Delta) is calculated as the first derivative of the function:

\Delta = dEsat / dTair

which is solved using D.

Value

A dataframe with the following columns:

Esat

Saturation vapor pressure (kPa)

Delta

Slope of the saturation vapor pressure curve (kPa K-1)

References

Sonntag D. 1990: Important new values of the physical constants of 1986, vapor pressure formulations based on the ITS-90 and psychrometric formulae. Zeitschrift fuer Meteorologie 70, 340-344.

World Meteorological Organization 2008: Guide to Meteorological Instruments and Methods of Observation (WMO-No.8). World Meteorological Organization, Geneva. 7th Edition.

Alduchov, O. A. & Eskridge, R. E., 1996: Improved Magnus form approximation of saturation vapor pressure. Journal of Applied Meteorology, 35, 601-609

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.

Examples

Esat.slope(seq(0,45,5))[,"Esat"]  # Esat in kPa
Esat.slope(seq(0,45,5))[,"Delta"] # the corresponding slope of the Esat curve (Delta) in kPa K-1