Esat.slope {bigleaf} R Documentation

## 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



[Package bigleaf version 0.8.2 Index]