Method of ET₀ computation

Climatic variable

Formula

Blaney-Criddle modified (1950) by Doorenbos and Pruitt (1977)

T_average, n, N

ET_o = (8 + 0.46 T_average) × n/N/i

Hargreaves-Samani (1985)

T_average, R_a, T_max, T_min, R_a

Original: ET_o = 0.0023 R_a ΔT^0,5 × (T_average + 17.8)

Modified: ET_o = 0.0035 R_a ΔT^0,5 × (T_average + 12.54)

Christiansen-Hargreaves (1969)

R_s or R_a, U₂, T_average, RH_average, E

Original: ET_o = 0.492 R_s x C_TT × C_WT x C_HT

Modified: ET_o = 0.324 R_a x C_TT x C_WT × C_HT x C_ST × C_R

(1) Penman Original (1963)

(2) Penman Monteith (1998)

T_average, e_o, U₂, n, N, R_a, RH_average,

ET_o = [Δ/(Δ + γ) × (R_n + G)] + [γ/(Δ + γ) × 15.36 × (1 + 0,0062 × U₂ × (e^o − e))]

Ivanov (1954)

T_max, T_min, T_m, e_o, RH_average, RH_max, RH_min

ET_o = 0.0018 x (T + 25)² × (100 − e/e^o 100)

Eagleman (1967)

T_max, T_min, T_average, e_o, RH_average, RH_max, RH_min

ET_o = 0.035 x e^o × (100 − RH_average)^0.5

Stephens and Stewart (1965)

T_average, R_a, n, N

ET_o = (0.014 T_average – 0.37) x R_s /1500/0.039 with R_s = (0.25 + 0.5 n/N) × R_a

Turc (1961-1965).

T_average, RH_average, R_a, n, N

RH > 50%: ET_o = 0.40 (R_s + 50) T_average/(T_average + 15)

RH < 50%: ET_o = 0.40 (R_s + 50) T_average/(T_average + 15) (1 + 50 − RH_average)/70 With R_s = (0.25 + 0.5 n/N) × R_a