Penman's Equation Referenced to Grass
Excel spread sheet to do the calculations 

PENMAN'S Equations for Potential Evaptranspiration
List of Symbols Used
Name Value Units Description
albedo reflection coefficient, 0.07+0.053*LAI
avpma mb maximum air vapor pressure
avpmi mb minimum air vapor pressure
cp 0.242 cal/C/cm^3 specific heat of air
delta mb/C slope of the saturated vapor pressure curve
elev m elevation
eop mm/day penman potential evaporation
gamma mb/C psychrometric constant
hl cal/g latent heat of vaporization
potet cm/day penman potential evaporation
pr mb air pressure.
rl -64.0 cal/cm^2/day long wave radiation
rn cal/cm^2/day net radiation
svpma mb maximum saturated vapor pressure
svpmi mb minimum saturated vapor pressure
tempav C average temperature
vpa1 mb actual vapor pressure at mean temperature
vps1 mb saturated vapor pressure at mean temperature
windk km/day wind speed

Equation for pressure when elevation is in meters:
      pr = 1013.0 - 0.1055 * elev

      windk' = Win_Vel*1.6093

correct wind speed at 3.74m height to wind speed at 2.0m height
      windk = windk' * (2.0/3.74)^0.20

      tempav = (Max_Temp + Min_Temp)/2.0
      ap = 7.5*Max_Temp/(Max_Temp+237.3)
      svpmi = 6.108*10.**ap
      avpmi = svpmi*Min_Humidity/100.
      ap = 7.5*Min_Temp/(Min_Temp+237.3)
      svpma = 6.108*10.**ap
      avpma = svpma*Max_Humidity/100.
      vps1 = (svpmi+svpma)/2.0
      vpa1 = (avpmi+avpma)/2.0
      albedo = 0.21
      hl = 595.0 - 0.51*tempav

to calculate net radiation use a modified version of the penman
equation and a wind factor referenced to water
      rn = 0.95*(1.0-albedo)*Solar_Radiation + rl
      delta = 33.8639*(0.05904*(0.00739*tempav+0.8072)**7 - 0.0000342)
      gamma = cp*pr/(0.622*hl)

Penman equation calculates the potential evaporation from
surface of well watered short grass.
      ea = 15.36*(1.0+0.0062*windk)*(vps1-vpa1)
      eop = ((delta*rn+gamma*ea)/(delta+gamma)) / hl*10.0

Reference  Me. Jensen, R. D. Burman, and R. G. Allen 1990. Evapotranspiration and Irrigation water Requiremtnts , 
ASCE manuals and Reports on Engineering Practice NO.70 American Society of Civil Engineers  NY 1-332
Sammis, T.W., C.L. Mapel, D.G. Lugg, R.R. Lansford, J.T. McGuckin. 1985. 
Evapotranspiration crop coefficients predicted using growing-degree-days.
Transactions of the ASAE, 28(3) 773-780.
Last modified 01/15/96