
psychro_fao56
Computes the psychrometric constant [kPa/C] 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 psychro_fao56 ( p : numeric, iounit [2] : integer )
Arguments
pA scalar or array of any dimensionality containing pressure. See iounit(0) argument.
iounitAn integer array of size 2 indicating the units of the input pressure and returned psychrometric constant:
- iounit(0)=0 ; (hPa)
- iounit(0)=1 ; (Pa)
- iounit(0)=2 ; (kPa)
If iounit=0 or 1, the input p will be converted internally to the appropriate units prior to the computation. The input p 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 p containing the psychrometric constant.
Description
Compute psychrometric constant [kPa/C], 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 7 of Chapter 3 or equation 3-2 in Annex 3.
From FAO 56:
A table listing the psychometric constant for different altitudes is Table 2.2.
See Also
Crop & Evapotranspiration functions
Examples
Example 1: Replicate Example 2 in Chapter 3. Use two different methods to estimate the pressure (small difference).
P0 = 101.3 ; kPa z0 = 0.0 ; m t = 20.0 ; degC z = 1800. ; m prs = prsatm_tz_fao56(t, z, P0, z0, 0) ; FAO 56; Annex 3: EQN 3-2 print("prs ="+prs) ; 81.78 kPa prs7 = 101.3*((293-0.0065*z)/293)^5.26 ; EQN 7 print("prs7 ="+prs7) ; 81.76 kPa psy = psychro_fao56(prs, (/2,2/) ) ; FAO 56: Chapter 3: EQN 8 print(psy)The output for 'psy' is
Variable: psy Type: float Total Size: 4 bytes 1 values Number of Dimensions: 1 Dimensions and sizes: [1] Coordinates: Number Of Attributes: 4 long_name : psychrometric constant units : kPa/C url : http://www.fao.org/docrep/X0490E/x0490e07.htm info : FAO 56; EQN 8; psychro_fao56 (0) 0.05434634