| Variable | Unit | Formula | |
| Tmax | ˚C | ||
| Tmin | ˚C | Taverage = (Tmax + Tmin)/2 | ˚C |
| Taverage | ˚C | Δ | |
| Altitude | m | γ | |
| U2 | m/s | (1 + 0.34 U2) | |
| Δ/(Δ + γ (1 + 0.34 U2)) | |||
| γ/(Δ + γ (1 + 0.34 U2)) | |||
| (900/(Taverage + 273)) × U2 | |||
| Vapor Pressure Deficit (VPD) | |||
| Tmax | ˚C | eo (Tmax) | KPa |
| Tmin | ˚C | eo (Tmin) | KPa |
| Saturation vapor pressure: es = [eo (Tmax) + (eo (Tmin)]/2 | KPa | ||
| ea derived from air moisture | |||
| RHmax | % | e˚ (Tmin) × RHmax/100 | KPa |
| RHmin | % | e˚ (Tmax) × RHmin/100 | KPa |
| ea: average value | KPa | ||
| Saturation vapour pressure deficit (es − ea) KPa | |||
| Radiation | |||
| Latitude | |||
| Day | Ra | MJ∙m–2∙day–1 | |
| Month | N | Hour | |
| n | Hour | n/N | |
| if Rs is not given: Rs = (0.25 + 0.5 n/N) × Ra | MJ∙m–2∙day–1 | ||
| Rso = [0.75 + 2 (altitude)/100000] × Ra | MJ∙m–2∙day–1 | ||
| Rs/Rso | MJ∙m–2∙day–1 | ||
| Rns = 0.77 Rs | MJ∙m–2∙day–1 | ||
| Tmax | MJ∙m–2∙day–1 | Δ Tmax. K4 | MJ.m–2.day–1 |
| Tmin | MJ∙m–2∙day–1 | Δ Tmin. K4 | MJ.m–2.day–1 |
| (Δ Tmax, K4 + Δ Tmin. K4)/2 | MJ∙m–2∙day–1 | ||
| ea | KPa | (0.34 – 0.14 ea1/2) | |
| Rs/Rso | (1.35 Rs/Rso – 0.35) | ||
| Rnl = (Δ Tmax. K4 + Δ Tmin. K4)/2 × (0.34 – 0.14 ea1/2) × (1.35 Rs/Rso – 0.35) | MJ∙m–2∙day–1 | ||
| Rn = Rns – Rnl | MJ∙m–2∙day–1 | ||
| Tmois | MJ∙m–2∙day–1 | G | MJ∙m–2∙day–1 |
| Tmois–1 | MJ∙m–2∙day–1 | G month = 0.14 (T month – T month – 1) | MJ∙m–2∙day–1 |
| Rn – G | MJ∙m–2∙day–1 | ||
| 0.408 (Rn – G) | MJ∙m–2∙day–1 | ||
| Reference evapotranspiration | |||
| Δ/(Δ + γ) (1 + 0.34 U2) × 0.408 (Rn – G) | mm/day | ||
| [γ/(Δ + γ) (1 + 0.34 U2)] × (900/(Taverage + 273)) × U2 × (es-ea) | mm/day | ||
|
| mm/day | ||
