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# refevt_hamon

Use the Hamon formulation to derive reference evapotranspiration. Available in version 6.5.0 and later.

## Prototype

```load "\$NCARG_ROOT/lib/ncarg/nclscripts/csm/crop.ncl"

function refevt_hamon (
tmean      : numeric,
daylight   : numeric,
iounit  : integer
)
```

## Arguments

tmean

Scalar or array containing mean temperatures. The units may be degrees Celcius, Kelvin or Farenheit. See the iounit argument.

daylight

A variable containing the number of effective hours of daylight. It must have the same dimension sizes as the tmean. See the iounit argument.

iounit

An integer array of length 2 indicating the units of input arguments tmean, es and the units of the returned argument.

• iounit(0)=0 ; input tmean are in degrees C (degC).
• iounit(0)=1 ; input tmean are in degrees K (degK).
• iounit(0)=2 ; input tmean are in degrees F.
• iounit(1)=0 ; output units => mm/day
• iounit(1)=1 ; output units => MJ/(m2-day)
• iounit(1)=2 ; output units => W/m2
• iounit(1)=3 ; output units => cal/(cm2-day)

## Return value

An array the same size as tmean containing the estimated evapotranspiration with units specified by iounit(1).

## Description

Estimates evapotranspiration via the simple "Hamon ETo" equation. This function is not described in the FAO-56 document. It is included because many refereed publications use this simple Hamon formulation described in the reference:

```     ETo = 2.1*es*daylight^2/(tmean+273.2) ; see iounit

Reference:
Haith, D. A. and Shoemaker, L. L., 1987
Water Resources Bulletin, 23:471-478.
DOI: 10.1111/j.1752-1688.1987.tb00825.x

```
The daylight_fao56 returns the maximum number of hours of sunlight. Using this would yield maximal evapotranspiration estimates. However, if (say) their was 30% cloudiness, then the effective number of hours of sunlight would be:
```
sunhrx = daylight_fao56(jday, lat)          ; max daylight/sun hr per day
sun_effective = (1-0.3)*sunhrx

```

## Examples

Example 1: As noted, this function is not from FAO_56. Hence, there is no simple 'unit test' to which NCL can refer. Here, we compare refevt_hamon to that from refevt_hargreaves_fao56. The parameters are the same as Example 20, Chapter 4. for Lyon, France in mid July.

```  jday   = 196                           ; 15 July
lat    = 45.72                         ; Lyon, France
tmin   = 14.8                          ; degC
tmax   = 26.6
tmean  = 0.5*(tmin+tmax)               ; iounit(0)=0  [degrees Celcius)

sunhrx = daylight_fao56(jday, lat)     ; max daylight/sun hr per day
print(sunhrx )                         ; print with meta data

evt_hamon = refevt_hamon( tmean, sunhrx, (/0,0,0/) ) ;   4.015 mm/day
print(evt_hamon)

```
The output is:
```
Variable: evt_hamon
Type: float
Total Size: 4 bytes
1 values
Number of Dimensions: 1
Dimensions and sizes:	
Coordinates:
Number Of Attributes: 2
long_name :	reference evapotranspiration: Hamon
units :	mm/day
(0)	4.015419

```
NOTE: For comparison, Example 1 for the refevt_hargreaves_fao56 function returns an estimate of 5.03 mm/day.

Example 2: Same as above but with 30% cloudiness.

```
suneff = (1-0.3)*sunhrx         ; 30% cloudiness; effective hours
evt_hamon_eff = refevt_hamon( tmean, suneff, (/0,0,0/) ) ;   1.967 mm/day
evt_hamon_eff@cloudiness = 0.3  ; add information
print(evt_hamon_eff)

```
The output is:
```
Variable: evt_hamon_eff
Type: float
Total Size: 4 bytes
1 values
Number of Dimensions: 1
Dimensions and sizes:	
Coordinates:
Number Of Attributes: 3
cloudiness :	0.3
long_name :	reference evapotranspiration: Hamon
units :	mm/day
(0)	1.967555
```