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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 [2] : 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
     Generalized watershed loading functions for stream flow nutrients.
     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:	[1]
     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)