PENMAN MONTEITH
(FAO-56)
Climate Access Database
Tmx, Tmn, RH, SS, SR, Wnd
SWa = 0.25
Alpha = 0.23
LWa = 0.9
LWb = 0.1
LWbl = -0.139
CropHeight = 12
TempHumid = 190
CropHgtWnd = 200
AerDynResist=Log((CropHgtWnd - 0.667CropHeight)/(0.123CropHeight)) Log((TempHumid - 0.667CropHeight)/(0.0123CropHeight))/(0.41)2
AeroTCoeff = 0.622*3.486*86400/(AerDynResist*1.01)
rc = 70 'For Grass, 86 for Alfalfa
2
1
1 to 12
i
AvgT = Average(Tmx,Tmn)
Wind = Wnd/86.4
EaTn = 0.61078*Exp(17.27*Tmn/(Tmn + 237.3))
EaTx = 0.61078*e(17.27*Tmx/(Tmx + 237.3))
EaTxTn = Average(EaTx, EaTn)
Edew = RH/(50/EaTx + 50/EaTn)
RHmean = Edew/EaTx*100
RHdel = Edew/EaTx/2 + Edew/EaTn/2
ETxTn = 2049*EaTx/(Tmx + 237.3)2 + 2049*EaTn/(Tmn + 237.3)2
Patm = 101.3*((293 - 0.0065*Elev)/293)5.253
Lambda = 2.501 - 0.002361*AvgT
Gamma = 0.0016286*Patm/Lambda
Ra = AerDynResist/Wind
GammaP = Gamma*(1 + rc)/Ra
3
2
DlGm = ETxTn/(ETxTn + GammaP)
GmDgm = Gamma/(ETxTn + GammaP)
Aeroterm = GmDgm*Wind*AeroTCoeff*(EaTxTn - Edew)/(AvgT + 273)
Dayno = 30.5*(i - 14.6)
Soldeclin = Sin(((2Dayno/365) - 1.39))*0.4093
xx = Sin(Soldeclin)*Sin(radiant)
yy = Cos(Soldeclin)*Cos(radiant)
temp1 = -Tan(Soldeclin*Tan(radiant))
Omega = tan-1(-temp1/√(-temp1*temp1 + 1)) + 2*tan-1(1)
Dr = (1 + 0.033*Cos((2π/365)*Dayno))
RAa = 37.586*Dr*(Omega*xx + Sin(Omega)*yy)
N1 = 24/*Omega
NbyN = SS/N1
Rns = (1 - Alpha)*RAa*(SWa + SWb*NbyN)
4
3
FNbyN = (LWb + LWa*NbyN)
Sigma = 0.00000000245*((Tmx + 273)4 + (Tmn + 273.16)4)
Emissivity = LWal + LWbl √Edew
Rbo = (2.45E-9)*(LWal + LWbl*Edew)*((Tmx + 273.16)4 + (Tmn + 273.16)4)
LWR = Rbo*(LWb + LWa*NbyN)
RnsRl = Rns - LWR
i
AvgT0 = AvgT12
i
1 to 12
G = 0.14*(AvgT - AvgT-1)
RnG = RnsRl – G
RadTm = DlGm*RnsRl/Lambda
RadTmG = DlGm*RnG/Lambda
5
4
ETcomb = RadTm + Aeroterm
ETG = RadTmG + Aeroterm
ETo = (ETcomb - ETG)/ETcomb
i