The Penman-Monteith equations (available here) are the standard for calculating reference evapotranspiration. These equations, however, are known to underestimate ET in the southwest by up to 30%. Therefore, the NMCC uses the following equations for calculating ETo:

pr=1013-.1055*elevation, where elevation is in meters
windk=24*windspeed*(2/3.74)^.2, where windspeed is in meters per second
tempav=(max temp+min temp)/2, where temperatures are in degrees C
apa=7.5*max temp/(max temp+237.3)
svpma=6.108*10^apa
avpma=svpma*max humidity/100, where humidity is in %
api=7.5*min temp/(min temp+237.3)
svpmi=6.108*10^api
avpmi=svpmi*min humidity/100
vpsl=(svpma+svpmi)/2
vpal=(avpma+avpmi)/2
hl=595-.51*tempav
rn=.95*(1-.21)*solar radiation-64, where solar radiation is the daily accumulated solar radiation in Langleys
delta=33.8639*(.05904*(.00739*tempav+.8072)^7-.0000342)
gamma=.242*pr/(.622*hl)
ea=15.36*(1+.0062*windk)*(vpsl-vpal)
ETo=((delta*rn+gamma*ea)/(delta+gamma))/hl

An Excel spreadsheet comparing the method used by the NMCC to the Penman-Monteith equations is available here.