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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

; from NCL V6.5.0 onward.
; No need for user to explicitly load.

function refevt_turc_rh (
tmean      : 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.

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

rh

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:

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:

jday   = 196               ; 15 July
lat    = 45.72             ; Lyon, France
tmin   = 14.8              ; degC
tmax   = 26.6

ra_0   = radext_fao56(jday, lat, 0)   ;  16.5463  MJ/(m2-day)

nsun   = 9.2                                                ; 'observed' hrs/sun day
sunhrx = daylight_fao56(jday, lat)                          ; max daylight/sun; 15.2 hr per day
rs_0   = radsol_fao56(ra_0, sunhrx, nsun, (/0,0/), False)   ;    9.15  mm/day

evturc = refevt_turc( tmean, rs_0, (/0,0,0/) )              ;    4.53 mm/day

rh = 75
evturc_75 = refevt_turc_rh( tmean, rs_0, rh, (/0,0,0/) )    ;    4.53 mm/day (no change; rh >50%)

rh = 25
evturc_25 = refevt_turc_rh( tmean, rs_0, rh, (/0,0,0/) )    ;    6.14 mm/day