Evapotranspiration (ET) & Watering Schedule Calculator
Estimate daily reference evapotranspiration (ET₀) using the Hargreaves-Samani equation and generate a weekly watering schedule based on your crop, soil type, and local climate conditions.
📍 Location & Climate
🌱 Crop & Soil
Formulas Used
1. Extraterrestrial Radiation (Ra) — FAO-56:
Ra = (24·60/π) · Gsc · dr · [ωs·sin(φ)·sin(δ) + cos(φ)·cos(δ)·sin(ωs)]
- Gsc = 0.0820 MJ/m²/min (solar constant)
- dr = 1 + 0.033·cos(2πJ/365) — inverse relative Earth-Sun distance
- δ = 0.409·sin(2πJ/365 − 1.39) — solar declination (rad)
- ωs = arccos(−tan(φ)·tan(δ)) — sunset hour angle (rad)
- J = day of year (mid-month value used)
2. Reference ET₀ — Hargreaves-Samani (1985):
ET₀ = 0.0023 · Ra · (Tmean + 17.8) · (Tmax − Tmin)0.5
3. Humidity & Wind Correction (Samani 2000):
CF = 1 + 0.00185·(50 − RH) + 0.0041·(u − 2) → ET₀corrected = ET₀ · CF
4. Crop Evapotranspiration:
ETc = Kc · ET₀
5. Soil Water Balance:
- TAW (mm) = FC × root depth (cm) × 10
- RAW (mm) = p × TAW (p = allowable depletion fraction)
- Net irrigation depth = RAW (mm)
- Gross irrigation = Net / irrigation efficiency
- Irrigation interval (days) = RAW / ETc
- Volume (L) = gross depth (mm) × area (m²)
Assumptions & References
- ET₀ is calculated using the Hargreaves-Samani (1985) temperature-based method, suitable when only temperature data is available.
- Extraterrestrial radiation (Ra) follows the FAO Irrigation and Drainage Paper No. 56 (Allen et al., 1998) methodology.
- Humidity and wind correction follows Samani (2000) adjustment for arid/humid deviations from the reference condition (RH=50%, u=2 m/s).
- Crop coefficients (Kc) are mid-season values from FAO-56 Table 12.
- Field capacity (FC) values are typical volumetric water content at −33 kPa matric potential.
- Allowable depletion (p) defaults to 50% of TAW, appropriate for most crops (FAO-56 Table 22).
- The Penman-Monteith equation (FAO-56) is the gold standard but requires solar radiation, humidity, and wind data; Hargreaves-Samani is used here as a practical temperature-only alternative with ±15–20% accuracy.
- Rainfall, groundwater contribution, and mulching effects are not included — subtract effective rainfall from gross irrigation needs.
- Schedule assumes uniform soil moisture at field capacity at the start of the week.