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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
  2. FAO Radiation (Makkink)
    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
    Rs = solar radiation (mm/day)
    ADOT = 1.2
    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 (%)
    Rs = solar radiation (mm/day)
    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:
    Rs = solar radiation (mm/day)
    Ra = extraterrestrial radiation (mm/day), Table 10, p.25-FA024
    nN = percentage of sunshine data required
    Fnn = sunshine function
    Fed = vapor pressure function
    T = mean temperature (°C)
    Ft = temperature function
    Rn1 = net longwave radiation (mm/day)
    a = actual vapor pressure at dewpoint (mbar)
    c) es = press (T) , p. 129-ASCE report
    - use same pressure equation as for ASCE Penman method
    where:
    T = mean temperature (°C)

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