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actvpr_rhmean_fao56

Compute actual vapor pressure via equation 19 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 actvpr_rhmean_fao56 (
		tmin       : numeric,  
		tmax       : numeric,  
		rhmean     : numeric,  
		iounit [2] : integer   
	)

Arguments

tmin

An array of any dimensionality containing minimum temperatures.

tmax

An array of any dimensionality containing maximum temperatures. Must be the same size and shape as tmin and rhmean.

rhmean

An array of any dimensionality containing relative humidity values. Must be the same size and shape as tmin and tmax.

iounit

A integer array indicating the units of the input tmin and tmax and returned actvpr variable.

iounit(0)=0 means the input are in degrees C (degC);
iounit(0)=1 means the input are in degrees K (degK);
iounit(0)=2 means the input are in degrees F.
iounit(1)=0 returns units hPa
iounit(1)=1 returns units Pa
iounit(1)=2 returns units kPa

If iounit(0)=1 or 2, the tmin and tmax will be converted to degC internally prior to the computations. The input tmin and tmax are unaltered.

Return value

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

Description

Compute actual 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 19 of Chapter 3.

Actual evapotranspiration (here, 'actvpr') is the quantity of water that is actually removed from a surface due to the processes of evaporation and transpiration.

Examples

Example 1: Replicate example 5 in Chapter 3. Here tunit=0.

  tmin   = 18.0               ; degC
  tmax   = 25.0  
  rhmin  = 54.0               ; %
  rhmax  = 82.0
  rhmean = (rhmin+rhmax)*0.5
                                        ; tunit=0 , degC
  actvpr_rhmean = actvpr_rhmean_fao56(tmin, tmax, rhmean, (/0,2/))   ; 1.7788 

  printVarSummary(actvpr_rhmean)

The output for 'actvpr_rhmean' is

     Variable: actvpr_rhmean
     Type: float
     Total Size: 4 bytes
                 1 values
     Number of Dimensions: 1
     Dimensions and sizes:   [1]
     Coordinates: 
     Number Of Attributes: 4
       long_rhmean_name :    actual vapor pressure
       units :       kPa
       url : http://www.fao.org/docrep/X0490E/x0490e07.htm
       info :        FAO 56; EQN 19: min/max t; mean rh

Example 1a: Same as Example 1 but the input tmin and tmax are in degK. Here tunit=1.

  tmin   = 18.0+273.16                  ; degK
  tmax   = 25.0+273.16
                                        ; tunit=1 , degK
  actvpr_rhmean = actvpr_rhmean_fao56(tmin, tmax, rhmean, (/1,2/))   ; 1.7788

Example 1b: Same as Example 1 but the input tmin and tmax are in degrees farenheit. Here tunit=2.

  tmin   = 1.8*18.0 + 32                ; deg Farenheit
  tmax   = 1.8*25.0 + 32
                                        ; tunit=2 , deg Farenheit
  actvpr_rhmean = actvpr_rhmean_fao56(tmin, tmax, rhmean, (/2,2/))   ; 1.7788

Example 2: Consider TMIN(time,lat,lon), TMAX(time,lat,lon) and RHMEAN(time,lat,lon) with sizes (3,73,144) and the temperatures have units of degK. Hence, tunit=1.


  ACTVPR  = actvpr_rhmean_fao56(TMIN, TMAX, RHMEAN, 1)  
  printVarSummary(ACTVPR)

The output for 'ACTVPR' is