Calculates saturation vapor pressure (Esat) over water and the corresponding slope of the saturation vapor pressure curve.
Esat.slope(
Tair,
formula = c("Sonntag_1990", "Alduchov_1996", "Allen_1998"),
constants = bigleaf.constants()
)A dataframe with the following columns:
Saturation vapor pressure (kPa)
Slope of the saturation vapor pressure curve (kPa K-1)
Air temperature (deg C)
Formula to be used. Either "Sonntag_1990" (Default),
"Alduchov_1996", or "Allen_1998".
Pa2kPa - conversion pascal (Pa) to kilopascal (kPa)
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.
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.
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
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