Esat.slope: Saturation Vapor Pressure (Esat) and Slope of the Esat Curve

In lhmet-forks/bigleaf: Physical and Physiological Ecosystem Properties from Eddy Covariance Data

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 (Δ) is calculated as the first derivative of the function:

Δ = 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
       

lhmet-forks/bigleaf documentation built on Aug. 4, 2020, 12:08 a.m.