Penman's Equation Referenced to Grass
Excel spread sheet to do the calculations
| 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. tsammis@nmsu.edu Last modified 01/15/96