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satvpr_tdew_fao56

Compute actual saturation vapor pressure 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_tdew_fao56 (
		tdew       : numeric,  
		iounit [2] : integer   
	)

Arguments

tdew

An array of any dimensionality containing dew point temperatures.

iounit

A scalar integer indicating the units of the input temperature:

iounit(0)=0 ; degrees C (degC);
iounit(0)=1 ; degrees K (degK)
iounit(0)=2 ; degrees Farenheit.

If iounit(0)=1 or 2, the will be converted to degC internally prior to the computation. The input t are unaltered.

iounit(1)=0 means the output are in hPa
iounit(1)=1 means the output are in Pa
iounit(1)=2 means the output are in kPa

Return value

An array with the same dimensionality and type as tdew containing actual saturation vapor pressure (kPa).

Description

Compute 'actual' saturation vapor pressure (kPa) 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 14 of Chapter 3.

The dewtemp_trh function could be used to calculate the dew point temperatures if they are not readily available.

NOTE: The dew point temperature is synonymous with the wet bulb temperature.

Examples

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

Example 1: Replicate example 3 in Chapter 3.

  tdewC   = 15.0                         ; degC
  tdewK   = 15.0+273.16                  ; degK
  tdewF   = 15.0*1.8+32                  ; deg Fahrenheit

  actvprC = satvpr_tdew_fao56(tdewC, (/0,2/))   ; 1.70535 
  actvprK = satvpr_tdew_fao56(tdewK, (/1,2/))   ; 1.70535 
  actvprF = satvpr_tdew_fao56(tdewF, (/2,2/))   ; 1.70535 

  printVarSummary(actvprC)

The output for 'actvprC' is

   Variable: actvprC
   Type: float
   Total Size: 4 bytes
               1 values
   Number of Dimensions: 1
   Dimensions and sizes:	[1]
   Coordinates: 
   Number Of Attributes: 4
     long_name :	actual saturation vapor pressure
     units :	kPa
     url :	http://www.fao.org/docrep/X0490E/x0490e07.htm
     info :	FAO 56; EQN 14

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

  actvpr  = satvpr_tdew_fao56(TDEW, (/1,2/))  
  printVarSummary(actvpr)

The output for 'actvpr' is