WRDS Library [Home] Digital Library Publications Videos Card Catalog

### Appendix G

The following defines the methodology for determining the various parameters for the ET formulas presented in Chapter 4.

1. FAO Blaney Criddle:
ET0 = Ab + Bb (p(0.46T + 8))
a) Ab = 0.0043 RHmin - SS-1.41, p. 110-FA024
where:
RHmin = minimum relative humidity (%)
SS = mean monthly percent of possible sunshine

b) Bb = B1 + B2
B1 = a1 + a2 RHmin + a3 SS1
B2 = a4 Ud + a5 RHmin SS + a6 RHmin Ud
where:
Ud = daytime wind (m/s)
a1 = 0.81917
a2 = - 0.0040922
a3 = 1.0705
a4 = 0.065649
a5 = 0.0059684
a6 = - 0.0005967
c) p = found in table 6.4 p. 82 - ASCE report
ET0 = Ar + Br (W Rse)
a) Ar = -0.3
b) Br = B1 + B2
B1 = a1 + a2 RHmean = a3 Ud
B2 = a4 RHmean Ud + a5 RHmean 2 + a6 Ud 2
where:
RHmean = mean relative humidity (%)
Ud = daytime wind (m/s)
a1 = 1.0656
a2 = - 0.0012795
a3 = 0.044953
a4 = -2.0033 x 10-4
a5 = -3.1508 x 10-5
a6 = -0.0011026
c) W = D /D+g
D = 33.8639(0.05904(0.00738 T + 0.8072) 7-0.0000342)
g = 0.240 P/(0.622l)
P = 1013.0-0.1093 E
l = 595.0-0.51 T
where:
T = mean temperature (°C)
E = elevation (m)
P = atmospheric pressure (mbar), p. 122-ASCE report
l = function of mean temperature, p. 122-ASCE report
g = psychrometric constant, p. 123-ASCE report
D = slope of saturation vapor pressure-temp. curve at temperature of interest, p. 123-ASCE report
d) Rse = solar radiation in equivalent evaporation (mm/day)
3. ASCE Penman
ETr =(D/(D+g)) (Rn+G) + (g/(D+g)) (15.36)(1.0+0.0062U2) (es-ea)
a) D,g = described in previous equation
b) Rn = (1-0.23)Rs (58.5)-RB
RB = (ADOT(Rs (58.5)/RSO ) + BDOT)RBO
RBO = EDOT(SIG)(T+273.) 4
EDOT = -0.02 + 0.261 EXP(-0.000777(T 2))
where:
RB = net longwave radiation (ly/dy), p.26-ASCE report
RBO = net outgoing clear day longwave radiation (ly/dy), p.26-ASCE report
EDOT = net emissivity, p.27-ASCE report
BDOT = -0.2, p. 27-ASCE report
RSO = mean solar radiation for cloudless skies (ly/dy), Table 3.1, p. 22-ASCE report
SIG = Stefan-Boltzmann Constant (ly/day/k4
= 11.71xl0-8, p.26-ASCE report
T = mean temperature (°C)
c) G = ((Ti-1-Ti+1)/Dt) * 100, p.32-ASCE report
Ti = mean temperature for time period i (°C)
Dt = time (days) between the midpoints of the two periods
d) es = (emax + emin )/2
Press (va1) = 33.8639((0.00738 va1 + 0.8072) 8-0.000019 x 1.8 val + 48 + 0.001316), p. 129-ASCE report
where:
emax = Press (Tmax), (mbar)
emin = Press (Tmin), (mbar)
Tmax = maximum temperature (°C)
Tmin = minimum temperature (°C)
e) ea= Press (Tdp), (mbar)
where:
Tdp = average daily dewpoint temperature (°C)

4. FAO Penman:
ETO = Cp(WRn + (1-W)(.27)(l+U/100)(es-ea)

a) Cp = a0 + a1RHmax + a2Rs - a3Ud + a4DNR + a5UdDNR + A6RHmaxRsUd - a7RHmaxRsDNR,

where:
a0 = 0.6817006
a1 = 0.0027864
a2 = 0.0181767
a3 = 0.0682501
a4 = 0.0126514
a5 = 0.0097297
a6 = 0.43025xl0-4
a7 = 0.92118xl0-7
DNR = Ud/Un
Ud = daytime wind (m/s)
Un = night wind (m/s)
RHmax = maximum relative humidity (%)
b) Rn = 0.75Rs-Rn1, p.19-FAO 24
Rn1 = FtFedFnn
Ft = sTk 4/58.5, p.27-FA024
Tk = T+273
s = 1.17lxl0-7
Fed = 0.34-0.044(ea)1/2, p.27-FA024
Fnn = 0.1+0.9 nN 1, p.27-FA024
nN = (Rs-Ra0.25)/(0.5Ra)

where: