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refevt_turc_rh

Use the Turc ETo formulation with a correction for low relative humidity to derive reference evapotranspiration.

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 refevt_turc_rh (
		tmean      : numeric,  
		radsol     : numeric,  
		rh         : numeric,  
		iounit [3] : integer   
	)

Arguments

tmean

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

radsol

A variable containing solar radiation. It must have the same dimension sizes as the tmean. See the iounit argument.

A variable containing relative humidity (%). It must have the same dimension sizes as the tmean.

iounit

An integer array of length 3 indicating the units of input arguments tmean, radsol 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 ; input radsol are in mm/day
iounit(1)=1 ; input radsol are in MJ/(m2-day)
iounit(1)=2 ; input radsol are in W/m2
iounit(1)=3 ; input radsol are in cal/(cm2-day)

iounit(2)=0 ; output units => mm/day
iounit(2)=1 ; output units => MJ/(m2-day)
iounit(2)=2 ; output units => W/m2
iounit(2)=3 ; output units => cal/(cm2-day)

Return value

An array the same size as tmean containing the estimated evapotranspiration.

Description

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

This version of the 'Turc formulation' function has an adjustment factor for relative humidities less that 50%. For very dry conditions this can result is substantial increases in evapotranspiration. Specifically, this factor has the form:

   rh_adjust_factor = (1+(50-rh)/70.0)

Otherwise (rh>=50%) the results are the same as refevt_turc.

Reference:
   C.-Y. Xu, V. P. Singh (2000) 
   Evaluation and generalization of radiation-based methods for calculating evaporation
   Hydrol. Process .  14 , 339-349 
   http://folk.uio.no/chongyux/papers_SCI/HYP_4.pdf

  Turc L. 1961
  Estimation of irrigation water requirements, potential evapotranspiration: 
    a simple climatic formula evolved up to date.
  Annals of Agronomy 12: 13-49

Examples

Example 1: