NCL Home > Documentation > Functions > Meteorology, Crop

# 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

```  Variable: actvpr
Type: float
Total Size: 126144 bytes
31536 values
Number of Dimensions: 3
Dimensions and sizes:	[time | 3] x [lat | 73] x [lon | 144]
Coordinates:
time: [1..3]
lat: [-90..90]
lon: [ 0..357.5]
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
```