 NCL Home > Documentation > Functions > Meteorology, Crop

# satvpr_slope_fao56

Compute the slope of the saturation vapor pressure curve 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_slope_fao56 (
temp       : numeric,
iounit  : integer
)
```

## Arguments

temp

An array of any dimensionality containing temperatures.

iounit

An integer array of length 2:

• 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.

If iounit(0)=1 or 2, the temp will be converted to degC internally prior to the computation. The input temp 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 temp containing the slopes of the saturation vapor pressure curve.

## Description

Compute the slope of the saturation vapor pressure (kPa/degC) 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 13 of Chapter 3.

## Examples

A table which displays slopes of saturation vapor pressure for different temperatures is here.

Example 1: Let t contain a temperature value (degC).

```
t       = 19.75                      ; degC; temperature
; iounit(0)=0 degC
; iounit(1)=2 kPa/C
slpvpr  = satvpr_slope_fao56(t, (/0,2/))   ; 0.142793 kPa/C

printVarSummary(slpvpr)

```
The output for 'slpvpr' is
```
Variable: slpvpr
Type: float
Total Size: 4 bytes
1 values
Number of Dimensions: 1
Dimensions and sizes:	
Coordinates:
Number Of Attributes: 4
long_name :	slope of saturation vapor pressure curve
units :	kPa/degC
url :	http://www.fao.org/docrep/X0490E/x0490e07.htm
info :	FAO 56; EQN 14

```

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

```
slpvpr  = satvpr_slope_fao56(T, (/0,2/))
printVarSummary(slpvpr)

```
The output for 'satvpr' is
```
Variable: slpvpr
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 :	slope saturation vapor pressure curve
units :	kPa/C
url :	http://www.fao.org/docrep/X0490E/x0490e07.htm
info :	FAO 56; EQN 13

```