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satvpr_temp_fao56

Compute saturation vapor pressure using temperature as described in FAO 56.

Available in version 6.4.0 and later.

Prototype

load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/crop.ncl"  ; This library is automatically loaded
                                                      ; from NCL V6.5.0 onward.
                                                      ; No need for user to explicitly load.

	function satvpr_temp_fao56 (
		temp       : numeric,  
		iounit [2] : integer   
	)

Arguments

temp

A scalar or array of any dimensionality containing temperatures.

Return value

An array with the same dimensionality and type as temp containing saturation vapor pressure.

Description

Compute saturation vapor pressure as described in the Food and Agriculture Organization (FAO) Irrigation and Drainage Paper 56 entitled: Crop evapotranspiration - Guidelines for computing crop water requirement. Specifically, see equation 11 of Chapter 3.

This is Tetens' Formula: an empirical expression for saturation vapour pressure with respect to liquid water that includes the variation of latent heat with temperature.

Tetens, O. 1930. Uber einige meteorologische Begriffe. Z. Geophys.. 6. 297–309

Examples

A table which displays saturation vapor pressures for different temperatures is here.

Example 1: Replicate example 3 in Chapter 3. Use different temperature units to illustrate that the answers are identical.

  tminC   = 15.0                         ; degC
  tmaxC   = 24.5
                                        ; iounit=(/0,2/) , degC, kPa
  es_minC = satvpr_temp_fao56(tminC, (/0,2/))   ; 1.705 kPa 
  es_maxC = satvpr_temp_fao56(tmaxC, (/0,2/))   ; 3.075 kPa 
 
; ---
  tminK   = tminC+273.16                  ; degK
  tmaxK   = tmaxC+273.16
                                        ; iounit(0)=1 , degK
  es_minK = satvpr_temp_fao56(tminK, (/1,2/))   ; 1.705 kPa 
  es_maxK = satvpr_temp_fao56(tmaxK, (/1,2/))   ; 3.075 kPa

; ---
  tminF   = 1.8*tminC + 32                ; deg Farenheit
  tmaxF   = 1.8*tminC + 32
                                        ; iounit(0)=2 , deg Farenheit
  es_minF = satvpr_temp_fao56(tminF, (/2,2/))   ; 1.70535 
  es_maxF = satvpr_temp_fao56(tmaxF, (/2,2/))   ; 3.07465

A sample output for 'es_minC' is


   Variable: esminC
   Type: float
   Total Size: 4 bytes
               1 values
   Number of Dimensions: 1
   Dimensions and sizes:	[1]
   Coordinates: 
   Number Of Attributes: 4
     long_name :	saturation vapor pressure
     units :	kPa
     url :	http://www.fao.org/docrep/X0490E/x0490e07.htm
     info :	FAO 56; EQN 11

Example 2: Consider T(time,lat,lon) with sizes (3,73,144) and the temperatures have units of degK. Here iounit(0)=1.


  satvpr  = satvpr_temp_fao56(T, (/1,2/))  
  printVarSummary(satvpr)

The output for 'satvpr' is